We apply a statistical method to derive very precise locations for soft gamma
repeaters using data from the interplanetary network. We demonstrate the
validity of the method by deriving a 600 arcsec^2 error ellipse for SGR1900+14
whose center agrees well with the VLA source position. We then apply it to
SGR1806-20, for which we obtain a 230 arcsec^2 error ellipse, the smallest
burst error box to date. We find that the most likely position of the source
has a small but significant displacement from that of the non-thermal core of
the radio supernova remnant G10.0-0.3, which was previously thought to be the
position of the repeater. We propose a different model to explain the changing
supernova remnant morphology and the positions of the luminous blue variable
and the bursting source.Comment: 12 pages and 2 color figures, accepted for publication in
  Astrophysical Journal Letter

C. Kouveliotou

D. D. Frederiks

Duncan R.

E. Mazets

J. van Paradijs

K. Hurley

S. Golenetskii

T. Cline

English

arXiv

Hurley, K.

Kouveliotou, C.

Cline, T.

Mazets, E.

Golenetskii, S.

Frederiks, D.D.

van Paradijs, J.A.

International Migration, Integration and Social Cohesion online publications

UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl)UvA-DARE (Digital Academic Repository)Where is SGR 1806-20?Hurley, K.; Kouveliotou, C.; Cline, T.; Mazets, E.; Golenetskii, S.; Frederiks, D.D.; vanParadijs, J.A.DOI10.1086/312256Publication date1999Published inAstrophysical JournalLink to publicationCitation for published version (APA):Hurley, K., Kouveliotou, C., Cline, T., Mazets, E., Golenetskii, S., Frederiks, D. D., & vanParadijs, J. A. (1999). Where is SGR 1806-20? Astrophysical Journal, 523, L37-L40.https://doi.org/10.1086/312256General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s)and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an opencontent license (like Creative Commons).Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, pleaselet the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the materialinaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letterto: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. Youwill be contacted as soon as possible.Download date:26 Jul 2022L37The Astrophysical Journal, 523:L37–L40, 1999 September 20q 1999. The American Astronomical Society. All rights reserved. Printed in U.S.A.WHERE IS SGR 1806220?K. Hurley,1 C. Kouveliotou,2 T. Cline,3 E. Mazets,4 S. Golenetskii,4D. D. Frederiks,4 and J. van Paradijs5,6Received 1999 June 1; accepted 1999 July 8; published 1999 August 23ABSTRACTWe apply a statistical method to derive very precise locations for soft gamma repeaters using data from theinterplanetary network. We demonstrate the validity of the method by deriving a 600 arcsec2 error ellipse forSGR 1900114 whose center agrees well with the VLA source position. We then apply it to SGR 1806220, forwhich we obtain a 230 arcsec2 error ellipse, the smallest burst error box to date. We find that the most likelyposition of the source has a small but significant displacement from that of the nonthermal core of the radiosupernova remnant G10.0-0.3, which was previously thought to be the position of the repeater. We propose adifferent model to explain the changing supernova remnant morphology and the positions of the luminous bluevariable and the bursting source.Subject headings: gamma rays: bursts — stars: neutron — supernova remnants — X-rays: stars1. INTRODUCTIONThe four known soft gamma repeaters (SGRs) are neutronstars in or near radio or optical supernova remnants. SGR1806220 was discovered in 1986 (Laros et al. 1986) and un-derwent a period of intense activity in 1987 (Laros et al. 1987;Kouveliotou et al. 1987), which led to its localization to an∼400 arcmin2 error ellipse (Atteia et al. 1987). Based on thisposition, Kulkarni & Frail (1993) suggested that the SGR wasassociated with the Galactic radio supernova remnant (SNR)G10.0-0.3. This was confirmed when the Advanced Satellitefor Cosmology and Astrophysics (ASCA) observed and imagedthe source in outburst, leading to an ∼19 radius error circle(Murakami et al. 1994). ROSAT observations of the quiescentX-ray source associated with SGR 1806220 confirmed theASCA data (Cooke 1993; Cooke et al. 1993). It is believed thatthe SGRs are “magnetars,” i.e., single neutron stars in whichthe magnetic field energy dominates all other sources of energy,including rotation (Duncan & Thompson 1992). In the case ofSGR 1806220, evidence for this model comes from obser-vations of the period and period derivative of the quiescentsoft X-ray emission (Kouveliotou et al. 1998).Studies of the radio nebula show evidence for changes inits morphology on ∼1 yr timescales and suggest that the neutronstar may be located at the nonthermal core of the radio emission(Frail, Vasisht, & Kulkarni 1997). The position of the core alsocoincides with that of an unusual star, which is identified as aluminous blue variable (LBV) by van Kerkwijk et al. (1995).This appears to be the only case so far of an SGR with anoptical stellar counterpart, and the connection between this ob-ject and the SGR has been unclear up to now.SGR 1806220 has remained active over the past several1 University of California at Berkeley, Space Sciences Laboratory, Berkeley,CA 94720-7450; khurley@sunspot.ssl.berkeley.edu.2 Universities Space Research Association at NASA Marshall Space FlightCenter, ES-84, Huntsville, AL 35812.3 NASA Goddard Space Flight Center, Greenbelt, MD 20771.4 AF Ioffe Physical-Technical Institute, Russian Academy of Science, Pol-itekhnicheskaya, 26, St. Petersburg, 194021, Russia.5 University of Alabama in Huntsville, Huntsville, AL 35899.6 Astronomical Institute “Anton Pannekoek,” University of Amsterdam,Kruislaan 403, Amsterdam, SJ, NL-1098, Netherlands.years, and many bursts have been detected by the InterplanetaryNetwork (IPN), consisting primarily in this case of BATSE,Ulysses, and KONUS-Wind. However, only eight events havebeen intense enough to trigger both Ulysses and a near-Earthspacecraft, resulting in high time resolution data (the otherbursts were recorded with lower time resolution by one or moreinstruments). It is these triggered events that lead to the mostprecise determination of the source position by triangulation.2. OBSERVATIONSDetails of the eight triggered bursts are given in Table 1. Ineach case, triangulation using Ulysses and either BATSE orKONUS results in a single annulus with a width of ∼230–280,which defines the possible arrival direction for the burst. Twosuch annuli define an error box, if the angular separation be-tween their centers is sufficient to prevent the annuli fromintersecting at grazing incidence. Over the ∼2 yr period ana-lyzed here, the Ulysses-Earth vector moved sufficiently to de-fine a nondegenerate error box. With three or more annuli, theproblem of defining the source location becomes overdeter-mined, and we can use a statistical method to derive the mostprobable source location. This consists of defining a x2 that isa function of an assumed source position in right ascensionand declination and of the parameters describing the eight an-nuli. Let a and d be the right ascension and declination of theassumed source position, and let ai, di, and vi be the rightascension, declination, and radius of the ith annulus. Then theangular distance di between the two is given by21d = v 2 cos [sin (d) sin (d ) 1 cos (d) cos (d ) cos (a 2 a )].i i i i i(1)If the 1 j uncertainty in the annulus width is , thenji2di2x = . (2)O 2ji iThe assumed source position is varied to obtain a minimumL38 WHERE IS SGR 1806220? Vol. 523TABLE 1IPN-triggered Observations of SGR 1806220DateUT(s)Annulus CenterRadius, v(deg)3 jHalf-Width(deg)a(2000)(deg)d(2000)(deg)1996 Nov 19 . . . . . . 19826 358.8945 230.9318 76.9907 0.00311996 Dec 30 . . . . . . 65965 359.0677 232.4563 76.7108 0.00351997 Jan 24 . . . . . . . 28043 355.4360 234.4572 73.2710 0.00381997 Apr 14 . . . . . . 39219 334.4596 234.6283 56.1701 0.00391997 Aug 27 . . . . . . 06045 341.3113 219.3428 64.5125 0.00261997 Sep 02 . . . . . . 39347 342.2306 218.6910 65.5168 0.00261998 Aug 05 . . . . . . 37673 335.3991 206.2919 62.7583 0.00261999 Feb 05 . . . . . . 31452 341.2378 109.8529 74.3287 0.0035Fig. 1.—Six IPN annuli (black lines) and the 1, 2, and 3 j equivalentconfidence contours (red annuli) for SGR 1900114. The best-fit position andthe position of the radio source are also indicated.TABLE 2IPN-triggered Observations of SGR 1900114DateUT(s)Annulus CenterRadius, v(deg)3 jHalf-Width(deg)a(2000)(deg)d(2000)(deg)1998 May 26 . . . . . . 80649 329.5855 211.4558 47.3345 0.00371998 May 30 . . . . . . 84457 329.6579 211.2371 47.3091 0.00361998 Jul 19 . . . . . . . . 30383 333.3281 207.7778 49.3927 0.00311998 Sep 01 . . . . . . . 58382 339.0121 203.7061 53.6270 0.00291998 Oct 22 . . . . . . . 56447 345.2596 101.5599 58.6313 0.00291998 Oct 28 . . . . . . . 83020 345.8533 102.2211 59.1029 0.0029x2; 1, 2, and 3 j equivalent confidence contours in a and d arefound by increasing by 2.3, 6.2, and 11.8.2xminWe have tested this method on six IPN annuli for SGR1900114 (Table 2), whose precise (subarcsecond) location isknown from VLA observations of a particle outburst (Frail,Kulkarni, & Bloom 1999) following the giant flare of 1998August 27 (Hurley et al. 1999a). The result is shown in Fig-ure 1. The 3 j error ellipse has an area of ∼600 arcsec2, andthe best-fitting position for the SGR, at a(2000) = 19h07m14s.3,d(2000) = 97199190, has a of 1.05 for 4 degrees of freedom2x(dof) (six annuli minus the two fitting parameters a and d). Itlies ∼00.6 from the VLA position.The results of applying the method to SGR 1806220 areshown in Figure 2. The best-fit position is at a(2000) =18h08m39s.4, d(2000) = 220724938.60, with a of 3.35 for2x6 dof (eight annuli minus two fitting parameters). It lies ∼150from the center of the nonthermal core and well outside it. The3 j error ellipse has an area of ∼230 arcsec2, making it thesmallest burst error box determined to date (the 324 arcsec2error box of the 1979 March 5 burst was, until now, “the mostprecisely determined gamma-ray source error box in existence”[Cline et al. 1982]). The position of the nonthermal core hasa total of 101.2x3. ACCURACY OF THE METHODSince each individual annulus gives, in effect, an underde-termined source position, it is possible in principle that un-known systematic errors might affect the location accuracy. Forexample, timing errors of 96–206 ms in the Ulysses data couldshift the positions of the annuli by different amounts and makethem all consistent with that of the nonthermal core. Apartfrom the unlikely combination of errors that this would require(i.e., each annulus would have to be subject to a different errorin such a way as to make the erroneous best-fit position havean acceptable ), there are several independent confirmations2xof the accuracy of the triangulation method. The first is theexcellent agreement between the VLA and triangulated posi-tions of SGR 1900114. The second is the agreement betweenIPN positions and the positions of gamma-ray bursts (GRBs)with optical counterparts (e.g., Hurley et al. 1997). The thirdand most stringent, however, is the confirmation of the Ulyssesspacecraft timing and ephemeris by end-to-end timing tests, inwhich commands are sent to the GRB experiment at preciselyknown times and the times of their execution on board thespacecraft are recorded and compared with the expected times.Because of command buffering on the spacecraft, there arerandom delays in the execution of these commands, and thetiming is verified to different accuracies during different tests.However, the tests before, during, and after the eight bursts inTable 1 took place on 1996 October 1, 1997 February 19, 1997August 25, 1998 February 18, 1998 August 21, and 1999 March7 and indicated that the timing errors at those times could notexceed 19, 21, 39, 29, 112, and 1 ms, respectively. For com-No. 1, 1999 HURLEY ET AL. L39Fig. 2.—Eight IPN annuli (black lines) and the 1, 2, and 3 j equivalentconfidence contours (red annuli) for SGR 1806220. The best-fit position andthe position of the nonthermal core are indicated. The ASCA error circle isjust visible in the lower left-hand and upper left-hand corners; its radius is 19,quoted as a systematic error, with no confidence limit given (Murakami et al.1994). The ROSAT PSPC error circle is at the center; its radius is 110, withno confidence limit quoted (Cooke et al. 1993). We have reanalyzed the ROSATdata and confirm the presence of a weak source at this position, but we areunable to establish confidence limits for its position. The 3.6 cm radio contoursof G10.0-0.3 are also shown, from Vasisht, Frail, & Kulkarni (1995).parison, the 3 j uncertainties in these triangulations have beentaken to be 125 ms. This includes both the statistical errors,and a conservative estimate of unknown timing and spacecraftephemeris errors. The low values for the two SGR positions2xare probably due in part to this estimate. Thus, the most likelyexplanation of our results is indeed that SGR 1806220 is notin the nonthermal core of G10.0-0.3, as has been assumedup to now. In this respect, SGR 1806220 resembles SGR1627241, which also displays a significant displacement fromthe core of its radio SNR (Hurley et al. 1999b).4. DISCUSSIONThe association of the possible LBV of van Kerkwijk et al.(1995) with the SNR is compelling; they estimate that thereare only several hundred stars this luminous in the Galaxy, andthis one lies within 10 of the radio peak. Indeed, in assuminga distance to the object of 6 kpc, they may have underesti-mated the star’s luminosity; a better distance estimate is now14.5 kpc (Corbel et al. 1997), giving a luminosity of 6 #. The fact that this object has not yet been observed to610 L,vary is not an argument against the LBV identification: Hum-phreys & Davidson (1994) note that LBVs do not always appearblue or variable. They are simply very luminous, unstable, hotsupergiants that undergo irregular eruptions. In a giant eruption,they may radiate as much luminous energy as a supernova andeject a solar mass of material. But this does not explain theSGR bursts, the changing radio morphology, or the displace-ment between the radio core and the source of the bursts.We propose that the LBV drives the morphological changes.LBVs are characterized by sporadic mass-loss rates of up to∼1024 M, yr21 (Humphreys & Davidson 1994) and more.Moreover, these flows may be bipolar or jetlike, as in the caseof h Car or P Cygni (Meaburn, Lopez, & O’Connor 1999).The measurements of van Kerkwijk et al. (1995) of the possibleLBV in G10.0-0.3 indicate an outflow velocity of 500 km s21.Coupled with a mass-loss rate of 1024 M, yr21, this gives atotal wind energy of ergs yr21, or a factor of ∼30442.5 # 10greater than the rate of energy deposition into the radio nebulaby the neutron star in the model of Frail et al. (1997). Thus,the LBV is easily capable of supplying the energy to explainthe changing radio morphology. In the case of h Car, the LBVnot only changes the morphology of its radio nebula dramat-ically but it also powers the (apparently nonthermal) radio neb-ula (Duncan et al. 1995; Duncan, White, & Lim 1997).We believe that the magnetar model is the best current ex-planation for the bursts. It is possible that the SGR is notassociated with the radio nebula and that we are simply ob-serving a chance alignment of the two. But if the two are indeedassociated, the SGR and the LBV may once have formed abinary system, which became unbound following the supernovaexplosion. In the magnetar model, the neutron star may beborn with a kick velocity greater than 1000 km s21 (Dun-can & Thomson 1992). If we assume that the distance toSGR 1806220 is 14.5 kpc (Corbel et al. 1997), that its age is10,000 yr, and that the neutron star originated at the positionof the LBV/nonthermal core, its approximate transverse ve-locity is a rather modest 100 km s21. (This estimate is subjectto large uncertainties because of the unknown age of the SNR;also, the actual space velocity could be much larger.) Thiscertainly does not strain the magnetar model, but it does raiseanother interesting question. Why did the SGR progenitor forma neutron star rather than a black hole, given that it must havebeen very massive to end its life earlier than the LBV? In anycase, SGR 1806 now appears to be similar to the other SGRsin that there is no associated radio emission at its position,except for the brief radio flare from SGR 1900114 (Frail etal. 1999).K. H. is grateful to JPL for Ulysses support under contract958056 and to NASA for Compton Gamma-Ray Observatorysupport under grant NAG5-3811. We thank the referee, M. vanKerkwijk, for helpful comments and P. Li for his analysis ofthe ROSAT data.REFERENCESAtteia, J.-L., et al. 1987, ApJ, 320, L105Cline, T., et al. 1982, ApJ, 255, L45Cooke, B. A. 1993, Nature, 366, 413Cooke, B. A., Tanaka, Y., Kulkarni, S., Matthews, K., Neugebauer, G., Reid,N., Soifer, T., & Vasisht, G. 1993, IAU Circ. 5883Corbel, S., Wallyn, P., Dame, T. M., Durouchoux, P., Mahoney, W. A., Vilhu,O., & Grindlay, J. E. 1997, ApJ, 478, 624Duncan, R., & Thompson, C. 1992, ApJ, 392, L9Duncan, R., White, S., & Lim, J. 1997, MNRAS, 290, 680Duncan, R., White, S., Lim, J., Nelson, G., Drake, S., & Kundu, M. 1995,ApJ, 441, L73Frail, D., Kulkarni, S., & Bloom, J. 1999, Nature, 398, 127Frail, D. A., Vasisht, G., & Kulkarni, S. R. 1997, ApJ, 480, L129Humphreys, R., & Davidson, K. 1994, PASP, 106, 1025L40 WHERE IS SGR 1806220? Vol. 523Hurley, K., et al. 1999a, Nature, 397, 41———. 1997, ApJ, 485, L1Hurley, K., Kouveliotou, C., Woods, P., Mazets, E., Golenetskii, S., Frederiks,D. D., Cline, T., & van Paradijs, J. 1999b, ApJ, 519, L143Kouveliotou, C., et al. 1987, ApJ, 322, L21———. 1998, Nature, 393, 235Kulkarni, S., & Frail, D. 1993, Nature, 365, 33Laros, J. G., Fenimore, E. E., Fikani, M. M., Klebesadel, R. W., & Baret, C.1986, Nature, 322, 152Laros, J. G., et al. 1987, ApJ, 320, L111Meaburn, J., Lopez, J., & O’Connor, J. 1999, ApJ, 516, L29Murakami, T., Tanaka, Y., Kulkarni, S. R., Ogasaka, Y., Sonobe, T., Ogawara,Y., Aoki, T., & Yoshida, A. 1994, Nature, 368, 127van Kerkwijk, M. H., Kulkarni, S. R., Matthews, K., & Neugebauer, G. 1995,ApJ, 444, L33Vasisht, G., Frail, D., & Kulkarni, S. 1995, ApJ, 440, L65

Where is SGR 1806-20?

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Where is SGR 1806−20?

UvA-DARE

UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl)UvA-DARE (Digital Academic Repository)Where is SGR 1806-20?Hurley, K.; Kouveliotou, C.; Cline, T.; Mazets, E.; Golenetskii, S.; Frederiks, D.D.; vanParadijs, J.A.DOI10.1086/312256Publication date1999Published inAstrophysical JournalLink to publicationCitation for published version (APA):Hurley, K., Kouveliotou, C., Cline, T., Mazets, E., Golenetskii, S., Frederiks, D. D., & vanParadijs, J. A. (1999). Where is SGR 1806-20? Astrophysical Journal, 523, L37-L40.https://doi.org/10.1086/312256General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s)and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an opencontent license (like Creative Commons).Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, pleaselet the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the materialinaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letterto: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. Youwill be contacted as soon as possible.Download date:31 Aug 2023L37The Astrophysical Journal, 523:L37–L40, 1999 September 20q 1999. The American Astronomical Society. All rights reserved. Printed in U.S.A.WHERE IS SGR 1806220?K. Hurley,1 C. Kouveliotou,2 T. Cline,3 E. Mazets,4 S. Golenetskii,4D. D. Frederiks,4 and J. van Paradijs5,6Received 1999 June 1; accepted 1999 July 8; published 1999 August 23ABSTRACTWe apply a statistical method to derive very precise locations for soft gamma repeaters using data from theinterplanetary network. We demonstrate the validity of the method by deriving a 600 arcsec2 error ellipse forSGR 1900114 whose center agrees well with the VLA source position. We then apply it to SGR 1806220, forwhich we obtain a 230 arcsec2 error ellipse, the smallest burst error box to date. We find that the most likelyposition of the source has a small but significant displacement from that of the nonthermal core of the radiosupernova remnant G10.0-0.3, which was previously thought to be the position of the repeater. We propose adifferent model to explain the changing supernova remnant morphology and the positions of the luminous bluevariable and the bursting source.Subject headings: gamma rays: bursts — stars: neutron — supernova remnants — X-rays: stars1. INTRODUCTIONThe four known soft gamma repeaters (SGRs) are neutronstars in or near radio or optical supernova remnants. SGR1806220 was discovered in 1986 (Laros et al. 1986) and un-derwent a period of intense activity in 1987 (Laros et al. 1987;Kouveliotou et al. 1987), which led to its localization to an∼400 arcmin2 error ellipse (Atteia et al. 1987). Based on thisposition, Kulkarni & Frail (1993) suggested that the SGR wasassociated with the Galactic radio supernova remnant (SNR)G10.0-0.3. This was confirmed when the Advanced Satellitefor Cosmology and Astrophysics (ASCA) observed and imagedthe source in outburst, leading to an ∼19 radius error circle(Murakami et al. 1994). ROSAT observations of the quiescentX-ray source associated with SGR 1806220 confirmed theASCA data (Cooke 1993; Cooke et al. 1993). It is believed thatthe SGRs are “magnetars,” i.e., single neutron stars in whichthe magnetic field energy dominates all other sources of energy,including rotation (Duncan & Thompson 1992). In the case ofSGR 1806220, evidence for this model comes from obser-vations of the period and period derivative of the quiescentsoft X-ray emission (Kouveliotou et al. 1998).Studies of the radio nebula show evidence for changes inits morphology on ∼1 yr timescales and suggest that the neutronstar may be located at the nonthermal core of the radio emission(Frail, Vasisht, & Kulkarni 1997). The position of the core alsocoincides with that of an unusual star, which is identified as aluminous blue variable (LBV) by van Kerkwijk et al. (1995).This appears to be the only case so far of an SGR with anoptical stellar counterpart, and the connection between this ob-ject and the SGR has been unclear up to now.SGR 1806220 has remained active over the past several1 University of California at Berkeley, Space Sciences Laboratory, Berkeley,CA 94720-7450; khurley@sunspot.ssl.berkeley.edu.2 Universities Space Research Association at NASA Marshall Space FlightCenter, ES-84, Huntsville, AL 35812.3 NASA Goddard Space Flight Center, Greenbelt, MD 20771.4 AF Ioffe Physical-Technical Institute, Russian Academy of Science, Pol-itekhnicheskaya, 26, St. Petersburg, 194021, Russia.5 University of Alabama in Huntsville, Huntsville, AL 35899.6 Astronomical Institute “Anton Pannekoek,” University of Amsterdam,Kruislaan 403, Amsterdam, SJ, NL-1098, Netherlands.years, and many bursts have been detected by the InterplanetaryNetwork (IPN), consisting primarily in this case of BATSE,Ulysses, and KONUS-Wind. However, only eight events havebeen intense enough to trigger both Ulysses and a near-Earthspacecraft, resulting in high time resolution data (the otherbursts were recorded with lower time resolution by one or moreinstruments). It is these triggered events that lead to the mostprecise determination of the source position by triangulation.2. OBSERVATIONSDetails of the eight triggered bursts are given in Table 1. Ineach case, triangulation using Ulysses and either BATSE orKONUS results in a single annulus with a width of ∼230–280,which defines the possible arrival direction for the burst. Twosuch annuli define an error box, if the angular separation be-tween their centers is sufficient to prevent the annuli fromintersecting at grazing incidence. Over the ∼2 yr period ana-lyzed here, the Ulysses-Earth vector moved sufficiently to de-fine a nondegenerate error box. With three or more annuli, theproblem of defining the source location becomes overdeter-mined, and we can use a statistical method to derive the mostprobable source location. This consists of defining a x2 that isa function of an assumed source position in right ascensionand declination and of the parameters describing the eight an-nuli. Let a and d be the right ascension and declination of theassumed source position, and let ai, di, and vi be the rightascension, declination, and radius of the ith annulus. Then theangular distance di between the two is given by21d = v 2 cos [sin (d) sin (d ) 1 cos (d) cos (d ) cos (a 2 a )].i i i i i(1)If the 1 j uncertainty in the annulus width is , thenji2di2x = . (2)O 2ji iThe assumed source position is varied to obtain a minimumL38 WHERE IS SGR 1806220? Vol. 523TABLE 1IPN-triggered Observations of SGR 1806220DateUT(s)Annulus CenterRadius, v(deg)3 jHalf-Width(deg)a(2000)(deg)d(2000)(deg)1996 Nov 19 . . . . . . 19826 358.8945 230.9318 76.9907 0.00311996 Dec 30 . . . . . . 65965 359.0677 232.4563 76.7108 0.00351997 Jan 24 . . . . . . . 28043 355.4360 234.4572 73.2710 0.00381997 Apr 14 . . . . . . 39219 334.4596 234.6283 56.1701 0.00391997 Aug 27 . . . . . . 06045 341.3113 219.3428 64.5125 0.00261997 Sep 02 . . . . . . 39347 342.2306 218.6910 65.5168 0.00261998 Aug 05 . . . . . . 37673 335.3991 206.2919 62.7583 0.00261999 Feb 05 . . . . . . 31452 341.2378 109.8529 74.3287 0.0035Fig. 1.—Six IPN annuli (black lines) and the 1, 2, and 3 j equivalentconfidence contours (red annuli) for SGR 1900114. The best-fit position andthe position of the radio source are also indicated.TABLE 2IPN-triggered Observations of SGR 1900114DateUT(s)Annulus CenterRadius, v(deg)3 jHalf-Width(deg)a(2000)(deg)d(2000)(deg)1998 May 26 . . . . . . 80649 329.5855 211.4558 47.3345 0.00371998 May 30 . . . . . . 84457 329.6579 211.2371 47.3091 0.00361998 Jul 19 . . . . . . . . 30383 333.3281 207.7778 49.3927 0.00311998 Sep 01 . . . . . . . 58382 339.0121 203.7061 53.6270 0.00291998 Oct 22 . . . . . . . 56447 345.2596 101.5599 58.6313 0.00291998 Oct 28 . . . . . . . 83020 345.8533 102.2211 59.1029 0.0029x2; 1, 2, and 3 j equivalent confidence contours in a and d arefound by increasing by 2.3, 6.2, and 11.8.2xminWe have tested this method on six IPN annuli for SGR1900114 (Table 2), whose precise (subarcsecond) location isknown from VLA observations of a particle outburst (Frail,Kulkarni, & Bloom 1999) following the giant flare of 1998August 27 (Hurley et al. 1999a). The result is shown in Fig-ure 1. The 3 j error ellipse has an area of ∼600 arcsec2, andthe best-fitting position for the SGR, at a(2000) = 19h07m14s.3,d(2000) = 97199190, has a of 1.05 for 4 degrees of freedom2x(dof) (six annuli minus the two fitting parameters a and d). Itlies ∼00.6 from the VLA position.The results of applying the method to SGR 1806220 areshown in Figure 2. The best-fit position is at a(2000) =18h08m39s.4, d(2000) = 220724938.60, with a of 3.35 for2x6 dof (eight annuli minus two fitting parameters). It lies ∼150from the center of the nonthermal core and well outside it. The3 j error ellipse has an area of ∼230 arcsec2, making it thesmallest burst error box determined to date (the 324 arcsec2error box of the 1979 March 5 burst was, until now, “the mostprecisely determined gamma-ray source error box in existence”[Cline et al. 1982]). The position of the nonthermal core hasa total of 101.2x3. ACCURACY OF THE METHODSince each individual annulus gives, in effect, an underde-termined source position, it is possible in principle that un-known systematic errors might affect the location accuracy. Forexample, timing errors of 96–206 ms in the Ulysses data couldshift the positions of the annuli by different amounts and makethem all consistent with that of the nonthermal core. Apartfrom the unlikely combination of errors that this would require(i.e., each annulus would have to be subject to a different errorin such a way as to make the erroneous best-fit position havean acceptable ), there are several independent confirmations2xof the accuracy of the triangulation method. The first is theexcellent agreement between the VLA and triangulated posi-tions of SGR 1900114. The second is the agreement betweenIPN positions and the positions of gamma-ray bursts (GRBs)with optical counterparts (e.g., Hurley et al. 1997). The thirdand most stringent, however, is the confirmation of the Ulyssesspacecraft timing and ephemeris by end-to-end timing tests, inwhich commands are sent to the GRB experiment at preciselyknown times and the times of their execution on board thespacecraft are recorded and compared with the expected times.Because of command buffering on the spacecraft, there arerandom delays in the execution of these commands, and thetiming is verified to different accuracies during different tests.However, the tests before, during, and after the eight bursts inTable 1 took place on 1996 October 1, 1997 February 19, 1997August 25, 1998 February 18, 1998 August 21, and 1999 March7 and indicated that the timing errors at those times could notexceed 19, 21, 39, 29, 112, and 1 ms, respectively. For com-No. 1, 1999 HURLEY ET AL. L39Fig. 2.—Eight IPN annuli (black lines) and the 1, 2, and 3 j equivalentconfidence contours (red annuli) for SGR 1806220. The best-fit position andthe position of the nonthermal core are indicated. The ASCA error circle isjust visible in the lower left-hand and upper left-hand corners; its radius is 19,quoted as a systematic error, with no confidence limit given (Murakami et al.1994). The ROSAT PSPC error circle is at the center; its radius is 110, withno confidence limit quoted (Cooke et al. 1993). We have reanalyzed the ROSATdata and confirm the presence of a weak source at this position, but we areunable to establish confidence limits for its position. The 3.6 cm radio contoursof G10.0-0.3 are also shown, from Vasisht, Frail, & Kulkarni (1995).parison, the 3 j uncertainties in these triangulations have beentaken to be 125 ms. This includes both the statistical errors,and a conservative estimate of unknown timing and spacecraftephemeris errors. The low values for the two SGR positions2xare probably due in part to this estimate. Thus, the most likelyexplanation of our results is indeed that SGR 1806220 is notin the nonthermal core of G10.0-0.3, as has been assumedup to now. In this respect, SGR 1806220 resembles SGR1627241, which also displays a significant displacement fromthe core of its radio SNR (Hurley et al. 1999b).4. DISCUSSIONThe association of the possible LBV of van Kerkwijk et al.(1995) with the SNR is compelling; they estimate that thereare only several hundred stars this luminous in the Galaxy, andthis one lies within 10 of the radio peak. Indeed, in assuminga distance to the object of 6 kpc, they may have underesti-mated the star’s luminosity; a better distance estimate is now14.5 kpc (Corbel et al. 1997), giving a luminosity of 6 #. The fact that this object has not yet been observed to610 L,vary is not an argument against the LBV identification: Hum-phreys & Davidson (1994) note that LBVs do not always appearblue or variable. They are simply very luminous, unstable, hotsupergiants that undergo irregular eruptions. In a giant eruption,they may radiate as much luminous energy as a supernova andeject a solar mass of material. But this does not explain theSGR bursts, the changing radio morphology, or the displace-ment between the radio core and the source of the bursts.We propose that the LBV drives the morphological changes.LBVs are characterized by sporadic mass-loss rates of up to∼1024 M, yr21 (Humphreys & Davidson 1994) and more.Moreover, these flows may be bipolar or jetlike, as in the caseof h Car or P Cygni (Meaburn, Lopez, & O’Connor 1999).The measurements of van Kerkwijk et al. (1995) of the possibleLBV in G10.0-0.3 indicate an outflow velocity of 500 km s21.Coupled with a mass-loss rate of 1024 M, yr21, this gives atotal wind energy of ergs yr21, or a factor of ∼30442.5 # 10greater than the rate of energy deposition into the radio nebulaby the neutron star in the model of Frail et al. (1997). Thus,the LBV is easily capable of supplying the energy to explainthe changing radio morphology. In the case of h Car, the LBVnot only changes the morphology of its radio nebula dramat-ically but it also powers the (apparently nonthermal) radio neb-ula (Duncan et al. 1995; Duncan, White, & Lim 1997).We believe that the magnetar model is the best current ex-planation for the bursts. It is possible that the SGR is notassociated with the radio nebula and that we are simply ob-serving a chance alignment of the two. But if the two are indeedassociated, the SGR and the LBV may once have formed abinary system, which became unbound following the supernovaexplosion. In the magnetar model, the neutron star may beborn with a kick velocity greater than 1000 km s21 (Dun-can & Thomson 1992). If we assume that the distance toSGR 1806220 is 14.5 kpc (Corbel et al. 1997), that its age is10,000 yr, and that the neutron star originated at the positionof the LBV/nonthermal core, its approximate transverse ve-locity is a rather modest 100 km s21. (This estimate is subjectto large uncertainties because of the unknown age of the SNR;also, the actual space velocity could be much larger.) Thiscertainly does not strain the magnetar model, but it does raiseanother interesting question. Why did the SGR progenitor forma neutron star rather than a black hole, given that it must havebeen very massive to end its life earlier than the LBV? In anycase, SGR 1806 now appears to be similar to the other SGRsin that there is no associated radio emission at its position,except for the brief radio flare from SGR 1900114 (Frail etal. 1999).K. H. is grateful to JPL for Ulysses support under contract958056 and to NASA for Compton Gamma-Ray Observatorysupport under grant NAG5-3811. We thank the referee, M. vanKerkwijk, for helpful comments and P. Li for his analysis ofthe ROSAT data.REFERENCESAtteia, J.-L., et al. 1987, ApJ, 320, L105Cline, T., et al. 1982, ApJ, 255, L45Cooke, B. A. 1993, Nature, 366, 413Cooke, B. A., Tanaka, Y., Kulkarni, S., Matthews, K., Neugebauer, G., Reid,N., Soifer, T., & Vasisht, G. 1993, IAU Circ. 5883Corbel, S., Wallyn, P., Dame, T. M., Durouchoux, P., Mahoney, W. A., Vilhu,O., & Grindlay, J. E. 1997, ApJ, 478, 624Duncan, R., & Thompson, C. 1992, ApJ, 392, L9Duncan, R., White, S., & Lim, J. 1997, MNRAS, 290, 680Duncan, R., White, S., Lim, J., Nelson, G., Drake, S., & Kundu, M. 1995,ApJ, 441, L73Frail, D., Kulkarni, S., & Bloom, J. 1999, Nature, 398, 127Frail, D. A., Vasisht, G., & Kulkarni, S. R. 1997, ApJ, 480, L129Humphreys, R., & Davidson, K. 1994, PASP, 106, 1025L40 WHERE IS SGR 1806220? Vol. 523Hurley, K., et al. 1999a, Nature, 397, 41———. 1997, ApJ, 485, L1Hurley, K., Kouveliotou, C., Woods, P., Mazets, E., Golenetskii, S., Frederiks,D. D., Cline, T., & van Paradijs, J. 1999b, ApJ, 519, L143Kouveliotou, C., et al. 1987, ApJ, 322, L21———. 1998, Nature, 393, 235Kulkarni, S., & Frail, D. 1993, Nature, 365, 33Laros, J. G., Fenimore, E. E., Fikani, M. M., Klebesadel, R. W., & Baret, C.1986, Nature, 322, 152Laros, J. G., et al. 1987, ApJ, 320, L111Meaburn, J., Lopez, J., & O’Connor, J. 1999, ApJ, 516, L29Murakami, T., Tanaka, Y., Kulkarni, S. R., Ogasaka, Y., Sonobe, T., Ogawara,Y., Aoki, T., & Yoshida, A. 1994, Nature, 368, 127van Kerkwijk, M. H., Kulkarni, S. R., Matthews, K., & Neugebauer, G. 1995,ApJ, 444, L33Vasisht, G., Frail, D., & Kulkarni, S. 1995, ApJ, 440, L65

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Hurley, K.; Kouveliotou, C.; Cline, T.; Mazets, E.; Golenetskii, S.; Frederiks, D.D.; van
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Hurley, K., Kouveliotou, C., Cline, T., Mazets, E., Golenetskii, S., Frederiks, D. D., & van Paradijs, J. A. (1999).
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WHERE IS SGR 1806220?
K. Hurley,1 C. Kouveliotou,2 T. Cline,3 E. Mazets,4 S. Golenetskii,4
D. D. Frederiks,4 and J. van Paradijs5,6
Received 1999 June 1; accepted 1999 July 8; published 1999 August 23
ABSTRACT
We apply a statistical method to derive very precise locations for soft gamma repeaters using data from the
interplanetary network. We demonstrate the validity of the method by deriving a 600 arcsec2 error ellipse for
SGR 1900114 whose center agrees well with the VLA source position. We then apply it to SGR 1806220, for
which we obtain a 230 arcsec2 error ellipse, the smallest burst error box to date. We find that the most likely
position of the source has a small but significant displacement from that of the nonthermal core of the radio
supernova remnant G10.0-0.3, which was previously thought to be the position of the repeater. We propose a
different model to explain the changing supernova remnant morphology and the positions of the luminous blue
variable and the bursting source.
Subject headings: gamma rays: bursts — stars: neutron — supernova remnants — X-rays: stars
1. INTRODUCTION
The four known soft gamma repeaters (SGRs) are neutron
stars in or near radio or optical supernova remnants. SGR
1806220 was discovered in 1986 (Laros et al. 1986) and un-
derwent a period of intense activity in 1987 (Laros et al. 1987;
Kouveliotou et al. 1987), which led to its localization to an
∼400 arcmin2 error ellipse (Atteia et al. 1987). Based on this
position, Kulkarni & Frail (1993) suggested that the SGR was
associated with the Galactic radio supernova remnant (SNR)
G10.0-0.3. This was confirmed when the Advanced Satellite
for Cosmology and Astrophysics (ASCA) observed and imaged
the source in outburst, leading to an ∼19 radius error circle
(Murakami et al. 1994). ROSAT observations of the quiescent
X-ray source associated with SGR 1806220 confirmed the
ASCA data (Cooke 1993; Cooke et al. 1993). It is believed that
the SGRs are “magnetars,” i.e., single neutron stars in which
the magnetic field energy dominates all other sources of energy,
including rotation (Duncan & Thompson 1992). In the case of
SGR 1806220, evidence for this model comes from obser-
vations of the period and period derivative of the quiescent
soft X-ray emission (Kouveliotou et al. 1998).
Studies of the radio nebula show evidence for changes in
its morphology on ∼1 yr timescales and suggest that the neutron
star may be located at the nonthermal core of the radio emission
(Frail, Vasisht, & Kulkarni 1997). The position of the core also
coincides with that of an unusual star, which is identified as a
luminous blue variable (LBV) by van Kerkwijk et al. (1995).
This appears to be the only case so far of an SGR with an
optical stellar counterpart, and the connection between this ob-
ject and the SGR has been unclear up to now.
SGR 1806220 has remained active over the past several
1 University of California at Berkeley, Space Sciences Laboratory, Berkeley,
CA 94720-7450; khurley@sunspot.ssl.berkeley.edu.
2 Universities Space Research Association at NASA Marshall Space Flight
Center, ES-84, Huntsville, AL 35812.
3 NASA Goddard Space Flight Center, Greenbelt, MD 20771.
4 AF Ioffe Physical-Technical Institute, Russian Academy of Science, Pol-
itekhnicheskaya, 26, St. Petersburg, 194021, Russia.
5 University of Alabama in Huntsville, Huntsville, AL 35899.
6 Astronomical Institute “Anton Pannekoek,” University of Amsterdam,
Kruislaan 403, Amsterdam, SJ, NL-1098, Netherlands.
years, and many bursts have been detected by the Interplanetary
Network (IPN), consisting primarily in this case of BATSE,
Ulysses, and KONUS-Wind. However, only eight events have
been intense enough to trigger both Ulysses and a near-Earth
spacecraft, resulting in high time resolution data (the other
bursts were recorded with lower time resolution by one or more
instruments). It is these triggered events that lead to the most
precise determination of the source position by triangulation.
2. OBSERVATIONS
Details of the eight triggered bursts are given in Table 1. In
each case, triangulation using Ulysses and either BATSE or
KONUS results in a single annulus with a width of ∼230–280,
which defines the possible arrival direction for the burst. Two
such annuli define an error box, if the angular separation be-
tween their centers is sufficient to prevent the annuli from
intersecting at grazing incidence. Over the ∼2 yr period ana-
lyzed here, the Ulysses-Earth vector moved sufficiently to de-
fine a nondegenerate error box. With three or more annuli, the
problem of defining the source location becomes overdeter-
mined, and we can use a statistical method to derive the most
probable source location. This consists of defining a x2 that is
a function of an assumed source position in right ascension
and declination and of the parameters describing the eight an-
nuli. Let a and d be the right ascension and declination of the
assumed source position, and let ai, di, and vi be the right
ascension, declination, and radius of the ith annulus. Then the
angular distance di between the two is given by
21d = v 2 cos [sin (d) sin (d ) 1 cos (d) cos (d ) cos (a 2 a )].i i i i i
(1)
If the 1 j uncertainty in the annulus width is , thenji
2di2x = . (2)O 2ji i
The assumed source position is varied to obtain a minimum
L38 WHERE IS SGR 1806220? Vol. 523
TABLE 1
IPN-triggered Observations of SGR 1806220
Date
UT
(s)
Annulus Center
Radius, v
(deg)
3 j
Half-Width
(deg)
a(2000)
(deg)
d(2000)
(deg)
1996 Nov 19 . . . . . . 19826 358.8945 230.9318 76.9907 0.0031
1996 Dec 30 . . . . . . 65965 359.0677 232.4563 76.7108 0.0035
1997 Jan 24 . . . . . . . 28043 355.4360 234.4572 73.2710 0.0038
1997 Apr 14 . . . . . . 39219 334.4596 234.6283 56.1701 0.0039
1997 Aug 27 . . . . . . 06045 341.3113 219.3428 64.5125 0.0026
1997 Sep 02 . . . . . . 39347 342.2306 218.6910 65.5168 0.0026
1998 Aug 05 . . . . . . 37673 335.3991 206.2919 62.7583 0.0026
1999 Feb 05 . . . . . . 31452 341.2378 109.8529 74.3287 0.0035
Fig. 1.—Six IPN annuli (black lines) and the 1, 2, and 3 j equivalent
confidence contours (red annuli) for SGR 1900114. The best-fit position and
the position of the radio source are also indicated.
TABLE 2
IPN-triggered Observations of SGR 1900114
Date
UT
(s)
Annulus Center
Radius, v
(deg)
3 j
Half-Width
(deg)
a(2000)
(deg)
d(2000)
(deg)
1998 May 26 . . . . . . 80649 329.5855 211.4558 47.3345 0.0037
1998 May 30 . . . . . . 84457 329.6579 211.2371 47.3091 0.0036
1998 Jul 19 . . . . . . . . 30383 333.3281 207.7778 49.3927 0.0031
1998 Sep 01 . . . . . . . 58382 339.0121 203.7061 53.6270 0.0029
1998 Oct 22 . . . . . . . 56447 345.2596 101.5599 58.6313 0.0029
1998 Oct 28 . . . . . . . 83020 345.8533 102.2211 59.1029 0.0029
x2; 1, 2, and 3 j equivalent confidence contours in a and d are
found by increasing by 2.3, 6.2, and 11.8.2xmin
We have tested this method on six IPN annuli for SGR
1900114 (Table 2), whose precise (subarcsecond) location is
known from VLA observations of a particle outburst (Frail,
Kulkarni, & Bloom 1999) following the giant flare of 1998
August 27 (Hurley et al. 1999a). The result is shown in Fig-
ure 1. The 3 j error ellipse has an area of ∼600 arcsec2, and
the best-fitting position for the SGR, at a(2000) = 19h07m14s.3,
d(2000) = 97199190, has a of 1.05 for 4 degrees of freedom2x
(dof) (six annuli minus the two fitting parameters a and d). It
lies ∼00.6 from the VLA position.
The results of applying the method to SGR 1806220 are
shown in Figure 2. The best-fit position is at a(2000) =
18h08m39s.4, d(2000) = 220724938.60, with a of 3.35 for2x
6 dof (eight annuli minus two fitting parameters). It lies ∼150
from the center of the nonthermal core and well outside it. The
3 j error ellipse has an area of ∼230 arcsec2, making it the
smallest burst error box determined to date (the 324 arcsec2
error box of the 1979 March 5 burst was, until now, “the most
precisely determined gamma-ray source error box in existence”
[Cline et al. 1982]). The position of the nonthermal core has
a total of 101.2x
3. ACCURACY OF THE METHOD
Since each individual annulus gives, in effect, an underde-
termined source position, it is possible in principle that un-
known systematic errors might affect the location accuracy. For
example, timing errors of 96–206 ms in the Ulysses data could
shift the positions of the annuli by different amounts and make
them all consistent with that of the nonthermal core. Apart
from the unlikely combination of errors that this would require
(i.e., each annulus would have to be subject to a different error
in such a way as to make the erroneous best-fit position have
an acceptable ), there are several independent confirmations2x
of the accuracy of the triangulation method. The first is the
excellent agreement between the VLA and triangulated posi-
tions of SGR 1900114. The second is the agreement between
IPN positions and the positions of gamma-ray bursts (GRBs)
with optical counterparts (e.g., Hurley et al. 1997). The third
and most stringent, however, is the confirmation of the Ulysses
spacecraft timing and ephemeris by end-to-end timing tests, in
which commands are sent to the GRB experiment at precisely
known times and the times of their execution on board the
spacecraft are recorded and compared with the expected times.
Because of command buffering on the spacecraft, there are
random delays in the execution of these commands, and the
timing is verified to different accuracies during different tests.
However, the tests before, during, and after the eight bursts in
Table 1 took place on 1996 October 1, 1997 February 19, 1997
August 25, 1998 February 18, 1998 August 21, and 1999 March
7 and indicated that the timing errors at those times could not
exceed 19, 21, 39, 29, 112, and 1 ms, respectively. For com-
No. 1, 1999 HURLEY ET AL. L39
Fig. 2.—Eight IPN annuli (black lines) and the 1, 2, and 3 j equivalent
confidence contours (red annuli) for SGR 1806220. The best-fit position and
the position of the nonthermal core are indicated. The ASCA error circle is
just visible in the lower left-hand and upper left-hand corners; its radius is 19,
quoted as a systematic error, with no confidence limit given (Murakami et al.
1994). The ROSAT PSPC error circle is at the center; its radius is 110, with
no confidence limit quoted (Cooke et al. 1993). We have reanalyzed the ROSAT
data and confirm the presence of a weak source at this position, but we are
unable to establish confidence limits for its position. The 3.6 cm radio contours
of G10.0-0.3 are also shown, from Vasisht, Frail, & Kulkarni (1995).
parison, the 3 j uncertainties in these triangulations have been
taken to be 125 ms. This includes both the statistical errors,
and a conservative estimate of unknown timing and spacecraft
ephemeris errors. The low values for the two SGR positions2x
are probably due in part to this estimate. Thus, the most likely
explanation of our results is indeed that SGR 1806220 is not
in the nonthermal core of G10.0-0.3, as has been assumed
up to now. In this respect, SGR 1806220 resembles SGR
1627241, which also displays a significant displacement from
the core of its radio SNR (Hurley et al. 1999b).
4. DISCUSSION
The association of the possible LBV of van Kerkwijk et al.
(1995) with the SNR is compelling; they estimate that there
are only several hundred stars this luminous in the Galaxy, and
this one lies within 10 of the radio peak. Indeed, in assuming
a distance to the object of 6 kpc, they may have underesti-
mated the star’s luminosity; a better distance estimate is now
14.5 kpc (Corbel et al. 1997), giving a luminosity of 6 #
. The fact that this object has not yet been observed to610 L,
vary is not an argument against the LBV identification: Hum-
phreys & Davidson (1994) note that LBVs do not always appear
blue or variable. They are simply very luminous, unstable, hot
supergiants that undergo irregular eruptions. In a giant eruption,
they may radiate as much luminous energy as a supernova and
eject a solar mass of material. But this does not explain the
SGR bursts, the changing radio morphology, or the displace-
ment between the radio core and the source of the bursts.
We propose that the LBV drives the morphological changes.
LBVs are characterized by sporadic mass-loss rates of up to
∼1024 M, yr21 (Humphreys & Davidson 1994) and more.
Moreover, these flows may be bipolar or jetlike, as in the case
of h Car or P Cygni (Meaburn, Lopez, & O’Connor 1999).
The measurements of van Kerkwijk et al. (1995) of the possible
LBV in G10.0-0.3 indicate an outflow velocity of 500 km s21.
Coupled with a mass-loss rate of 1024 M, yr
21, this gives a
total wind energy of ergs yr21, or a factor of ∼30442.5 # 10
greater than the rate of energy deposition into the radio nebula
by the neutron star in the model of Frail et al. (1997). Thus,
the LBV is easily capable of supplying the energy to explain
the changing radio morphology. In the case of h Car, the LBV
not only changes the morphology of its radio nebula dramat-
ically but it also powers the (apparently nonthermal) radio neb-
ula (Duncan et al. 1995; Duncan, White, & Lim 1997).
We believe that the magnetar model is the best current ex-
planation for the bursts. It is possible that the SGR is not
associated with the radio nebula and that we are simply ob-
serving a chance alignment of the two. But if the two are indeed
associated, the SGR and the LBV may once have formed a
binary system, which became unbound following the supernova
explosion. In the magnetar model, the neutron star may be
born with a kick velocity greater than 1000 km s21 (Dun-
can & Thomson 1992). If we assume that the distance to
SGR 1806220 is 14.5 kpc (Corbel et al. 1997), that its age is
10,000 yr, and that the neutron star originated at the position
of the LBV/nonthermal core, its approximate transverse ve-
locity is a rather modest 100 km s21. (This estimate is subject
to large uncertainties because of the unknown age of the SNR;
also, the actual space velocity could be much larger.) This
certainly does not strain the magnetar model, but it does raise
another interesting question. Why did the SGR progenitor form
a neutron star rather than a black hole, given that it must have
been very massive to end its life earlier than the LBV? In any
case, SGR 1806 now appears to be similar to the other SGRs
in that there is no associated radio emission at its position,
except for the brief radio flare from SGR 1900114 (Frail et
al. 1999).
K. H. is grateful to JPL for Ulysses support under contract
958056 and to NASA for Compton Gamma-Ray Observatory
support under grant NAG5-3811. We thank the referee, M. van
Kerkwijk, for helpful comments and P. Li for his analysis of
the ROSAT data.
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Cline, T., et al. 1982, ApJ, 255, L45
Cooke, B. A. 1993, Nature, 366, 413
Cooke, B. A., Tanaka, Y., Kulkarni, S., Matthews, K., Neugebauer, G., Reid,
N., Soifer, T., & Vasisht, G. 1993, IAU Circ. 5883
Corbel, S., Wallyn, P., Dame, T. M., Durouchoux, P., Mahoney, W. A., Vilhu,
O., & Grindlay, J. E. 1997, ApJ, 478, 624
Duncan, R., & Thompson, C. 1992, ApJ, 392, L9
Duncan, R., White, S., & Lim, J. 1997, MNRAS, 290, 680
Duncan, R., White, S., Lim, J., Nelson, G., Drake, S., & Kundu, M. 1995,
ApJ, 441, L73
Frail, D., Kulkarni, S., & Bloom, J. 1999, Nature, 398, 127
Frail, D. A., Vasisht, G., & Kulkarni, S. R. 1997, ApJ, 480, L129
Humphreys, R., & Davidson, K. 1994, PASP, 106, 1025
L40 WHERE IS SGR 1806220? Vol. 523
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