We analysed the RXTE archival data of 1E 1048.1-5937 covering a time span of
more than one year. The spin down rate of this source decreases by 30 percent
during the observation. We could not resolve the X-ray flux variations because
of contamination by Eta Carinae. We find that the level of pulse frequency
fluctuations of 1E 1048.1-5937 is consistent with typical noise levels of
accretion powered pulsars. Recent RXTE observations of 1E 2259+586 have shown a
constant spin down with a very low upper limit on timing noise. We used the
RXTE archival X-ray observations of 1E 2259+586 to show that the intrinsic
X-ray luminosity times series is also stable, with an rms fractional variation
of less than 15 percent. The source could have been in a quiet phase of
accretion with a constant X-ray luminosity and spin down rate.Comment: MNRAS in pres

Alpar, A.

Baykal, A.

Stark, M. J.

Strohmayer, T.

Swank, J.

Monthly Notices of the Royal Astronomical Society

English

arXiv

We analysed the Rossi X-ray Timing Explorer (RXTE) archival data of 1E 1048.1-5937 covering a time-span of more than one year. The spin-down rate of this source decreases by similar to 30 per cent during the observation. We could not resolve the X-ray flux variations because of contamination by eta Carinae. We find that the level of pulse frequency fluctuations of 1E 1048.1-5937 is consistent with typical noise levels of accretion-powered pulsars. Recent RXTE observations of 1E 2259+586 have shown a constant spin-down with a very low upper limit on timing noise. We used the RXTE archival X-ray observations of 1E 2259+586 to show that the intrinsic X-ray luminosity times-series is also stable, with an rms fractional variation of less than 15 per cent. The source could have been in a quiet phase of accretion with a constant X-ray luminosity and spin-down rate

Baykal, Altan

Strohmayer, T

Swank, J

Alpar, MA

Stark, MJ

OpenMETU (Middle East Technical University)

arXiv:astro-ph/0009480v1  29 Sep 2000Differences between the Two Anomalous X-Ray Pulsars:Variations in the Spin Down Rate of 1E 1048.1-5937 and AnExtended Interval of Quiet Spin Down in 1E 2259+586Altan Baykal 1, Tod Strohmayer 2, Jean Swank 2, M. Ali Alpar 3, Michael J. Stark 41 Physics Department, Middle East Technical University, Ankara 06531, Turkey2 Laboratory for High Energy Astrophysics NASA/GSFC Greenbelt, Maryland 20771 USA3 Department of Physics, Sabancı University, Istanbul, Turkey4 Department of Physics, Marietta College, Marietta, OH 45750, USAReceived ; accepted– 2 –ABSTRACTWe analysed the RXTE archival data of 1E 1048.1-5937 covering a time span ofmore than one year. The spin down rate of this source decreases by ∼30% during theobservation. We could not resolve the X-ray flux variations because of contaminationby Eta Carinae. We find that the level of pulse frequency fluctuations of 1E 1048.1-5937is consistent with typical noise levels of accretion powered pulsars (Baykal & Ögelman1993, Bildsten et al., 1997). Recent RXTE observations of 1E 2259+586 have shown aconstant spin down with a very low upper limit on timing noise (Kaspi et al., 1999).We used the RXTE archival X-ray observations of 1E 2259+586 to show that theintrinsic X-ray luminosity times series is also stable, with an rms fractional variationof less than 15%. The source could have been in a quiet phase of accretion with aconstant X-ray luminosity and spin down rate.Subject headings: accretion, Low Mass X-Ray Binaries, magnetars, 1E1048.1-5937, 1E 2259+586– 3 –1. IntroductionThe two sources considered in this paper belong to a small group of X-ray pulsars whichare called anomalous X-ray pulsars (hereafter AXPs), with periods in the ∼5–12 s range. Thisnarrow pulse period distribution of the AXPs, is significantly different from that of High MassX-Ray Binaries (HMXRBs) where the pulse periods span a range from 69 ms to 25 min. TheirX-ray spectra have steep power law indices ∼ 3-4 in addition to soft blackbody components withkT∼0.5 keV in some of the sources (Stella et al., 1998). They lack observed optical counterparts.Their X-ray luminosities are at the order of 1035-1036 erg s−1 and spin down rates are relativelyconstant.The first proposed model for AXPs was accretion from low-mass X-ray companions at loweraccretion rates (Lx = GMṀ/R ∼ 1 × 1035erg s−1) with magnetic fields of B∼ 1011 Gauss(Mereghetti & Stella 1995). In this scenario, the observed pulse periods of AXPs can be explainedas rotation periods of neutron stars close to the equilibrium periods for accretion from a disk.However, orbital signatures such as periodic delays in pulse arrival times or periodic flux changeshave not been observed in the AXPs. This has led several researchers to alternative interpretationsbased on the single pulsar hypothesis. Corbet et al. (1995) suggested the possibility of accretionfrom a molecular cloud. Alternatively, AXPs could be isolated stars which are accreting from adisk formed as remnants of the common envelope evolution of HMXRBs (van Paradijs et al. 1995,Ghosh et al. 1997). On the other hand Thompson & Duncan (1993) proposed that these sourcesare highly magnetized (∼1014-1015 Gauss) isolated neutron stars (magnetars) which are slowingdown due to electromagnetic dipole radiation. According to the magnetar theory there should beseveral unseen very large glitches in pulse period histories of 1E 1048.1-5937 and 1E 2259+586(Heyl & Hernquist 1999). These glitches should be at least a factor of hundred larger than theradio pulsar glitches in δν/ν (Thompson & Duncan 1996, Heyl & Hernquist 1999).1E 1048.1-5937 was discovered by the Einstein satellite during the observations of theCarina nebula (Seward, Charles & Smale 1986). 1E 2259+586 is located at the center of theradio/X-ray supernova remnant G109.1-1.0 (Fahlman & Gregory 1981). Both sources lack bright– 4 –optical counterparts (Mereghetti, Caraveo & Bignami 1992, Coe & Jones 1992). If they arebinary systems, their companion stars should be either white dwarfs or helium-burning stars withM<0.8Mo (Mereghetti, Israel, Stella, 1998, Baykal et al. 1998).The torque changes of 1E 1048.1-5937 and 1E 2259+586 were studied by Mereghetti (1995),Corbet & Mihara (1997) and Baykal & Swank (1996). Both sources showed pulse frequencychanges which can support the accretion hypothesis. In this work, we present two new pulsefrequency measurements from long observations in the archival RXTE data base. In the ∼400 daytime span of the observation, we found the spin-down rate of 1E 1048.1–5937 to change by 30 %.Recent pulse timing analysis of 1E 2259+586 (Kaspi et al., 1999) has shown that the source hadconstant spin down over a 2.6 yr time span, with very low timing noise. We extracted archivaldata of 1E 2259+586 and constructed a bolometric X-ray luminosity time series. We found thatthe X-ray luminosity is almost constant while the spin-down rate is constant (Kaspi et al., 1999).2. Data AnalysisThe archival observations of 1E 1048.1-5937 and 1E 2259+586 are listed in Table 1. Theresults presented here are based on data collected with the Proportional Counter Array (PCA,Jahoda et al., 1996). The PCA instrument consists of an array of 5 proportional counters operatingin the 2-60 keV energy range, with a total effective area of approximately 7000 cm2 and a field ofview of ∼ 1◦ FWHM.Background light curves and the pulse height amplitudes were generated using the backgroundestimator models based on the rate of very large events (VLE), spacecraft activation and cosmicX-ray emission with the standard PCA analysis tools (ftools) and were subtracted from the sourcelight curve obtained from the first Good Xenon layer of event data. The background subtractedlight curves were corrected with respect to the barycenter of the solar system. From the longarchival data string, pulse periods for 1E 1048.1-5937 were found by folding the time series onstatistically independent trial periods (Leahy et al. 1983). Master pulses were constructed from– 5 –these observations by folding the data on the period giving the maximum χ2. The master pulseswere arranged in 55 phase bins and represented by their Fourier harmonics (Deeter & Boynton1985) and cross-correlated with the harmonic representation of average pulse profiles from eachobservation. The pulse arrival times are obtained from the cross-correlation analysis. The lineartrend of pulse arrival times is a direct measure of the pulse frequency during the observation,δφ = φo + δν(t− to) (1)where δφ is the pulse phase offset deduced from the pulse timing analysis, to is the mid-time ofthe observation, φo is the phase offset at to, δν is the deviation from the mean pulse frequency (oradditive correction to the pulse frequency). The pulse period measurments of 1E 1048.1-5937 fromarchival RXTE observations are presented in Table 2.During the RXTE observations the pulse frequency derivative of 1E 1048.1-5937 decreasedapproximately by 30 % (see Table 2). However these changes cannot be correlated with X-rayflux changes since Eta Carinae lies 45’ a way from 1E 1048.1-5937. The strong flux changes ofEta Carinae (Corcoran et al., 1997) contaminate the FOV of 1E 1048.1-5937 since the FWHM ofRXTE/PCA ∼ 1◦. It should also be pointed out that in the archival RXTE observations, there arefound short, ∼ 2000 sec, observations separated by months; due to the pulse frequency derivativechanges we have not phase connected them in order to avoid any cycle count ambiguity. Similarly,two successive Ginga observations of 1E 1048.1-5937 which were separated by 10 days cannot becombined in phase because of the cycle count ambiguity (Corbet & Day 1990).Einstein, EXOSAT and Ginga observations of 1E 1048.1–5937 provided 5 pulse frequencymeasurements over 10 years, which were consistent with a constant spin-down rate of ν̇ ∼ 3.8×10−13Hz s−1 (Mereghetti 1995). This is ∼38 times higher than that of 1E 2259+586 (Baykal et al.,1998). Observations with ROSAT in 1992-1993 indicated that the spin down rate almost doubledfrom its value in 1988 (Mereghetti 1995). The mean flux decreased by a factor of 3 compared tothe value measured with EXOSAT in mid of ∼ 1985 (Corbet & Mihara 1997). These variationsare consistent with accretion-powered X-ray emission.– 6 –In order to deduce the pulse frequency changes of 1E 1048.1-5937, the residuals of pulsefrequencies are extracted from their linear trends and the residuals are presented in terms of theirsigma values. Fig. 1 (lower) clearly shows that the pulse frequency fluctuations are significant atthe order of several σ levels. The residual pulse frequencies between ∼ 48600 – MJD ∼ 50800MJD yield a noise strength on the order of S ≈ (2π)2 < δν2 > /T ≈ (2π)2 < δφ2 > /T 3 ∼ 10−17rad2 sec−3, where < δν2 > and < δφ2 > are the normalized variances of residual pulse frequenciesand pulse arrival times and T is the total time span (see Cordes 1980 for further definitions ofnoise strength). This value is comparable with typical accretion powered pulsar noise strengthBaykal & Ögelman (1993).Recent RXTE observations of 1E 2259+586 have shown a constant spin down rate with a lowupper limit on timing noise (Kaspi et al., 1999). The residuals of the pulse arrival times gives anupper limit to the noise strength at Tobservation ∼ 2.6 years, S≈ (2π)2 < δφ2 > /T 3observation∼ 10−24rad2 sec−3. This value is 5 decades lower than the value which is deduced from 15 years of pulseperiod history of 1E 2259+586 (Baykal & Swank 1996, Baykal & Ögelman 1993) and 2 orderslower than that of the Crab pulsar (Boynton et al., 1972) or that of the LMXRB pulsar 4U 1626-67(Chakrabarty 1997). This upper limit is indeed very low for an accretion-powered X-ray pulsar. Ifthe pulse period changes are due to variations in the accretion process, the X-ray luminosity wouldbe a constant (Ghosh & Lamb 1979). In order to check the variations in its X-ray luminosity, wederived the X-ray luminosities for all observations from the X-ray spectra.The X-ray background spectrum was calculated using the background estimator models basedupon the rate of very large events (VLE), spacecraft activation and cosmic X-ray emission as usedto calculate background light curves. The X-ray spectra are fitted with a power law spectrumwith a photon index 4.78 and column density NH=(2.2± 0.8)× 1022 cm−2, parameters consistentwith those obtained from ASCA and SAX measurements in the 2-10 keV range (Corbet et al.,1995, Parmar et al., 1998). The resultant background subtracted X-ray luminosity time series inthe energy range 2-10 keV is presented in Fig. 2. The variation in the bolometric X-ray luminosityis less than %15. This is consistent with low timing noise and secular spin down according to– 7 –accretion models.In Ginga observations 1E 2259+586 had flux levels a factor of two higher than average(Iwasawa et al. 1992). This implied that, if due to accretion, the rate onto the source was variableand fluctuations in the spin down rate would be expected. Indeed for this era high noise levelsS ∼ 10−19rad2sec−3 are found (Baykal & Swank 1996).3. DiscussionIn the pulse timing analysis of 1E 1048.1-5937, we found pulse frequency changes on the timescale of 400 days. The source spin down rate has doubled since 1992 (Mereghetti 1995, Corbet &Mihara 1997). The pulse frequency measurements since 1992 show significant fluctuations fromthe average spin down trend. The level of pulse frequency fluctuations of 1E 1048.1-5937 are foundto be consistent with typical noise levels of accretion powered pulsars (Baykal & Ögelman 1993).Two Soft Gamma ray Repeaters (SGR) SGR 1806-20, SGR 1900+14 were recently identifiedas magnetars (Kouveliotou et al. 1998; 1999). The similarity of SGR and AXP pulse periods andsecular spin down trends raised the question whether all AXPs are actually magnetars. Accordingto the magnetar theory pulse frequency fluctuations may be caused by processes such as: variableemission of Alfven waves or particles from magnetars (Thompson & Blaes 1998), radiativeprecession of magnetars (Melatos 1999), or discontinous spin up events (glitches) like those seenin radio pulsars (Thompson & Duncan 1996, see also Kaspi et al., 2000). Heyl & Hernquist (1999)investigated the pulse frequency histories of 1E 1048.1-5937 and 1E 2259+586 and proposed thatthe irregularities of these sources are explained by several large glitches with sizes δν ∼ 2 × 10−4s−1 and ∼ 5× 10−6 s−1, respectively. While the values of δν are only about 20 times those of theVela and Crab pulsars, respectively, the values of δν/ν are several orders of magnitude larger thanradio pulsar glitches and they occur much more frequently than scaling from the statistics of largeradio pulsar glitches would lead us to expect (Alpar & Baykal 1994).In summary, both sources have shown pulse frequency fluctuations on the order of a few– 8 –decades (Mereghetti 1995, Baykal & Swank 1996). The level of torque changes of 1E 2259+586over a time scale of 15 years is consistent with accretion-powered X-ray binaries (Baykal & Swank1996). The steady level of the bolometric X-ray luminosity during the quiet spin down epoch(Kaspi et al., 1999) is consistent with accretion models. Alternatively it may turn out to be madeout of discrete glitches over a background of relatively quiet spin down, as in isolated pulsarsand magnetars (Heyl & Hernquist 1999). Continued phase coherent X-ray timing of this sourceshould prove extremely important in finally deciding its nature. The correlation of the spin-downrate of 1E 1048.1-5937 with the X-ray flux supports the possibility that it is an accreting source(Mereghetti 1995, Corbet & Mihara 1997). Our work indicates that the source has high timingnoise. To see the exact nature of correlations between X-ray flux and pulse frequency derivatives,an even more extensive broad band X-ray observation should be carried out.Acknowledgements :We thank the referee for helpful comments.– 9 –REFERENCESAlpar M.A., Baykal A., 1994, MNRAS, 269, 849Baykal A., Ögelman, H, 1993, A&A, 267, 119Baykal A., Swank J., 1996, ApJ, 460, 470Baykal A., Swank J.H., Strohmayer T., Stark M.J., 1998, 336, 173Bildsten L., et al., 1997, ApJS, 113, 367Boynton P.E., et al, ApJ, 1972, 175, 217Chakrabarty D., et al., 1997, ApJ, 474, 414Corbet R.H.D., Day C.S.R., 1990, MNRAS, 259, 191Cordes J.M., 1980, ApJ, 237, 216Coe M.J., Jones L.R., 1992, MNRAS, 259, 191Corbet R.H.D., Smale A.P., Ozaki M., Koyama K., Iwasawa K., 1995, ApJ, 443, 786Corbet R.H.D., Mihara T., 1997, ApJ, 475, L127Corcoran, M.F., et al., 1997, Nature, 390, 587Deeter J.E., Boynton P.E., 1985, in Proc. Inuyama Workshop on Timing Studies of X-Ray Sources,ed. S. Hayakawa & F. Nagase (Nagoya: Nagoya Univ.), 29Fahlman G.G., Gregory P.C., 1981, Nat, 293, 202Ghosh P., Angelini L., White N.E., 1997, ApJ, 478, 713Ghosh P., Lamb F.K., 1979, ApJ., 232, 259– 10 –Jahoda, K., Swank, J., Giles, A.B., Stark, M.J., Strohmayer, T., Zhang, W., 1996, Proc. SPIE,2808, 59Heyl, S.H., Hernquist L., 1999, MNRAS, 304, L37Iwasawa K., Koyama, K., Halpern, J.P. 1992, PASJ, 44, 9Kaspi V.M, Chakrabarty D., Steinberger J., 1999 ApJL 525, L33Kaspi V.M, Lackey J.R, Chakrabarty, D, 2000, astro-ph/0005326, accepted for ApJ LettersKouveliotou C., et al., 1998, Nature, 393, 325Kouveliotou C., et al., 1999, ApJ, 510, L115Melatos A., 1999, ApJ., 519, L77Mereghetti S., Caraveo P., Bignami G.F., 1992, A&A, 263, 172Mereghetti S., 1995, ApJ, 455, 598Mereghetti S., Stella L., 1995, ApJ, 442, L17Mereghetti S., Israel G.L., Stella L., 1998, MNRAS, 269, 689Leahy, D.A., et al., 1983, ApJ., 266, 160Parmar, A.N., et al., 1998, A&A, 330, 175Seward F., Charles P.A., Smale A.P., 1986, ApJ, 305, 814Stella L., Israel G.L., Mereghetti S., 1998, Adv. Space Res, vol 22, No. 7, pp 1025Thompson C., Blaes O., 1998, Phys. Rev D, 57, 3219Thompson C., Duncan R.C., 1993, ApJ, 408, 194Thompson C., Duncan R.C., 1996, ApJ, 473, 322– 11 –van Paradijs., J, Taam, R.E., van den Heuvel, E.P.J., 1995, A&A, 299, L41Figure CaptionFig. 1 Pulse frequency time series of 1E 1048.1-5937. The solid lines (upper panel) are fitsto the measurements before and after Sep 1988, respectively. Residuals from these linear trendsare shown in terms of sigma values (lower panel). Measurements with Einstein, EXOSAT, andGinga (x) fit a trend well, while the later measurements with ROSAT and ASCA (open circles)and RXTE (filled circles) fit a trend of higher spin-down, but with well measured deviations.Fig. 2 X-ray luminosity time series of 1E 2259+586.This manuscript was prepared with the AAS LATEX macros v4.0.– 12 –Table 1: RXTE observations of 1E 1048.1-5937 and 1E 2259+586Time of Observation Exposuremm/dd/yy secSource Name 1E 1048.1-593729/07/96-31/07/96 6258608/03/97 2116508/10/97-09/10/97 14971Source Name 1E 2259+58629/09/96-01/10/96 7475825/12/96 92825/01/97 100422/02/97 107625/02/97-26/03/97 10023718/04/97 72710/05/97 98818/06/97 92217/07/97 84712/08/97 72919/09/97 103816/10/97 83214/11/97 88413/08/98-02/12/98 121833– 13 –Table 2: RXTE Pulse Period Measurements of 1E 1048.1-5937Epoch(MJD) Pulse Period (sec)50294.67 6.449769±0.000004 Mereghetti et al., (1998)50515.69 6.450198±0.000018 This work50729.71 6.450486±0.000018 This workTime Span (MJD) Derivative of Pulse Frequency Hz sec−150294.67-50729.71 −(4.74 ± 0.18) × 10−1350294.67-50515.69 −(5.40 ± 0.38) × 10−1350515.69-50729.71 −(3.72 ± 0.52) × 10−13– 14 –1– 15 –

Differences between the two anomalous X-ray pulsars: variations in the spin-down rate of 1E 1048.1-5937 and an extended interval of quiet spin-down in 1E 2259+586

Strohmayer, Tod

Swank, Jean

Alpar, M. Ali

Stark, Michael J.

Sabanci University Research Database

Mon. Not. R. Astron. Soc. 319, 205±208 (2000)Differences between the two anomalous X-ray pulsars: variations in thespin-down rate of 1E 1048.125937 and an extended interval of quiet spin-down in 1E 22591586Altan Baykal,1w Tod Strohmayer,2 Jean Swank,2 M. Ali Alpar3 and Michael J. Stark41Physics Department, Middle East Technical University, Ankara 06531, Turkey2Laboratory for High Energy Astrophysics NASA/GSFC Greenbelt, MD 20771, USA3Department of Physics, Sabancõ University, Istanbul, Turkey4Department of Physics and Astronomy, Denison University, Granville, OH 43023, USAAccepted 2000 June 29. Received 2000 June 12; in original form 1999 June 16A B S T R A C TWe analysed the Rossi X-ray Timing Explorer (RXTE) archival data of 1E 1048.125937covering a time-span of more than one year. The spin-down rate of this source decreases by,30 per cent during the observation. We could not resolve the X-ray flux variations becauseof contamination by eta Carinae. We find that the level of pulse frequency fluctuations of 1E1048.125937 is consistent with typical noise levels of accretion-powered pulsars. RecentRXTE observations of 1E 22591586 have shown a constant spin-down with a very lowupper limit on timing noise. We used the RXTE archival X-ray observations of 1E 22591586to show that the intrinsic X-ray luminosity times-series is also stable, with an rms fractionalvariation of less than 15 per cent. The source could have been in a quiet phase of accretionwith a constant X-ray luminosity and spin-down rate.Key words: accretion, accretion discs ± binaries: general ± stars: individual: 1E1048.125937 ± stars: individual: 1E 22591586 ± stars: low-mass, brown dwarfs.1 I N T R O D U C T I O NThe two sources considered in this paper belong to a small groupof X-ray pulsars which are called anomalous X-ray pulsars(hereafter AXPs), with periods in the ,5±12 s range. This narrowpulse period distribution of the AXPs, is significantly differentfrom that of high-mass X-ray binaries (HMXRBs) where the pulseperiods span a range from 69 ms to 25 min. Their X-ray spectrahave steep power-law indices ,3±4 in addition to soft blackbodycomponents with kT , 0:5 keV in some of the sources (Stella,Israel & Mereghetti 1998). They lack observed optical counter-parts. Their X-ray luminosities are at the order of 1035±1036erg s21 and spin-down rates are relatively constant.The first proposed model for AXPs was accretion from low-mass X-ray companions at lower accretion rates Lx GM _M=R , 1  1035 erg s21 with magnetic fields of B , 1011 G(Mereghetti & Stella 1995). In this scenario, the observed pulseperiods of AXPs can be explained as rotation periods of neutronstars close to the equilibrium periods for accretion from a disc.However, orbital signatures such as periodic delays in pulse arrivaltimes or periodic flux changes have not been observed in theAXPs. This has led several researchers to alternative interpreta-tions based on the single pulsar hypothesis. Corbet et al. (1995)suggested the possibility of accretion from a molecular cloud.Alternatively, AXPs could be isolated stars that are accreting froma disc formed as remnants of the common envelope evolution ofHMXRBs (van Paradijs, Taam & van den Heuvel 1995; Ghosh,Angelini & White 1997). On the other hand Thompson & Duncan(1993) proposed that these sources are highly magnetized(,1014±1015 G) isolated neutron stars (magnetars) which areslowing down owing to electromagnetic dipole radiation.According to the magnetar theory there should be several unseenvery large glitches in pulse period histories of 1E 1048.125937and 1E 22591586 (Heyl & Hernquist 1999). These glitchesshould be at least a factor of hundred larger than the radio pulsarglitches in dn /n (Thompson & Duncan 1996; Heyl & Hernquist1999).1E 1048.125937 was discovered by the Einstein satelliteduring the observations of the Carina nebula (Seward, Charles &Smale 1986). 1E 22591586 is located at the centre of the radio/X-ray supernova remnant G109.121.0 (Fahlman & Gregory1981). Both sources lack bright optical counterparts (Mereghetti,Caraveo & Bignami 1992; Coe & Jones 1992). If they are binarysystems, their companion stars should be either white dwarfs orhelium-burning stars with M , 0:8 M( (Mereghetti, Israel &Stella 1998; Baykal et al. 1998).The torque changes of 1E 1048.125937 and 1E 22591586were studied by Mereghetti (1995), Corbet & Mihara (1997) andq 2000 RASw E-mail: altan@astroa.physics.metu.edu.trBaykal & Swank (1996). Both sources showed pulse frequencychanges which can support the accretion hypothesis. In this work,we present two new pulse frequency measurements from longobservations in the archival RXTE data base. In the ,400 d time-span of the observation, we found the spin-down rate of 1E1048.125937 to change by 30 per cent. Recent pulse timinganalysis of 1E 22591586 (Kaspi, Chakrabarty & Steinberger1999) has shown that the source had constant spin-down over a2.6-yr time span, with very low timing noise. We extractedarchival data of 1E 22591586 and constructed a bolometric X-rayluminosity time-series. We found that the X-ray luminosity isalmost constant while the spin-down rate is constant (Kaspi et al.1999).2 DATA A N A LY S I SThe archival observations of 1E 1048.125937 and 1E 22591586are listed in Table 1. The results presented here are based on datacollected with the Proportional Counter Array (PCA, Jahoda et al.1996). The PCA instrument consists of an array of fiveproportional counters operating in the 2±60 keV energy range,with a total effective area of approximately 7000 cm2 and a fieldof view of ,18 FWHM.Background light curves and the pulse height amplitudes weregenerated using the background estimator models based on therate of very large events (VLE), spacecraft activation and cosmicX-ray emission with the standard PCA analysis tools (ftools) andwere subtracted from the source light curve obtained from the first`good xenon' layer of event data. The background subtracted lightcurves were corrected with respect to the barycentre of the solarsystem. From the long archival data string, pulse periods for 1E1048.125937 were found by folding the time-series on statisti-cally independent trial periods (Leahy et al. 1983). Master pulseswere constructed from these observations by folding the data onthe period giving the maximum x2. The master pulses werearranged in 55 phase bins and represented by their Fourierharmonics (Deeter & Boynton 1985) and cross-correlated with theharmonic representation of average pulse profiles from eachobservation. The pulse arrival times are obtained from the cross-correlation analysis. The linear trend of pulse arrival times is adirect measure of the pulse frequency during the observation,df  fo 1 dnt 2 to 1where df is the pulse phase offset deduced from the pulse timinganalysis, to is the mid-time of the observation, fo is the phaseoffset at to, dn is the deviation from the mean pulse frequency (oradditive correction to the pulse frequency). The pulse periodmeasurements of 1E 1048.125937 from archival RXTE observa-tions are presented in Table 2.During the RXTE observations the pulse frequency derivative of1E 1048.125937 decreased approximately by 30 per cent (seeTable 2). However these changes cannot be correlated with X-rayflux changes since eta Carinae lies 45 arcmin away from 1E1048.125937. The strong flux changes of eta Carinae (Corcoranet al. 1997) contaminate the FOV of 1E 1048.125937 since theFWHM of RXTE/PCA ,18. It should also be pointed out that inthe archival RXTE observations, there are found short, ,2000 s,observations separated by months; owing to the pulse frequencyderivative changes we have not phase connected them in order toavoid any cycle count ambiguity. Similarly, two successive Gingaobservations of 1E 1048.125937 which were separated by 10 dcannot be combined in phase because of the cycle count ambiguity(Corbet & Day 1990).Einstein, EXOSAT and Ginga observations of 1E 1048.125937provided five pulse frequency measurements over 10 yr, whichwere consistent with a constant spin-down rate of _n , 3:8 10213 Hz s21 (Mereghetti 1995). This is ,38 times higher thanthat of 1E 22591586 (Baykal et al. 1998). Observations withROSAT in 1992±1993 indicated that the spin-down rate almostdoubled from its value in 1988 (Mereghetti 1995). The mean fluxdecreased by a factor of 3 compared with the value measured withEXOSAT in approximately the middle of 1985 (Corbet & Mihara1997). These variations are consistent with accretion-poweredX-ray emission.Table 1. RXTE observations of 1E1048.125937 and 1E 2259 1 586:Time of Observation Exposuremm/dd/yy sSource Name 1E 1048.12593729/07/96-31/07/96 62 58608/03/97 21 16508/10/97±09/10/97 14 971Source Name 1E 2259158629/09/96±01/10/96 74 75825/12/96 92825/01/97 100422/02/97 107625/02/97±26/03/97 100 23718/04/97 72710/05/97 98818/06/97 92217/07/97 84712/08/97 72919/09/97 103816/10/97 83214/11/97 88413/08/98±02/12/98 121 833Table 2. RXTE Pulse Period Measurements of 1E 1048.125937.Epoch(MJD) Pulse Period (s)50294.67 6.449769^ 0.000004 Mereghetti et al. (1998)50515.69 6.450198^ 0.000018 This work50729.71 6.450486^ 0.000018 This workTime Span (MJD) Derivative of Pulse Frequency Hz s2150294.67250729.71 2(4.74^ 0.18) 1021350294.67250515.69 2(5.40^ 0.38) 1021350515.69250729.71 2(3.72^ 0.52) 10213206 A. Baykal et al.q 2000 RAS, MNRAS 319, 205±208In order to deduce the pulse frequency changes of 1E1048.125937, the residuals of pulse frequencies are extractedfrom their linear trends and the residuals are presented in terms oftheir sigma values. Fig. 1 (lower) clearly shows that the pulsefrequency fluctuations are significant at the order of several slevels. The residual pulse frequencies between ,48600 ± MJD,50800 ± MJD yield a noise strength on the order of S <2p2kdn2l=T < 2p2kdf2l=T3 , 10217 rad2 s23; where kdn2land kdf2l are the normalized variances of residual pulsefrequencies and pulse arrival times and T is the total time-span(see Cordes 1980 for further definitions of noise strength). Thisvalue is comparable with typical accretion powered pulsar noisestrength Baykal & OÈ gelman (1993).Recent RXTE observations of 1E 22591586 have shown aconstant spin-down rate with a low upper limit on timing noise(Kaspi et al. 1999). The residuals of the pulse arrival times givesan upper limit to the noise strength at Tobservation , 2:6 yr; S <2p2kdf2l=T3observation , 10224 rad2 s23: This value is five decadeslower than the value which is deduced from 15 yr of pulse periodhistory of 1E 22591586 (Baykal & Swank 1996; Baykal &OÈ gelman 1993) and two orders lower than that of the Crab pulsar(Boynton et al. 1972) or that of the LMXRB pulsar 4U 1626267(Chakrabarty et al. 1997). This upper limit is indeed very low foran accretion-powered X-ray pulsar. If the pulse period changes areas a result of the variations in the accretion process, the X-rayluminosity would be a constant (Ghosh & Lamb 1979). In order tocheck the variations in its X-ray luminosity, we derived the X-rayluminosities for all observations from the X-ray spectra.The X-ray background spectrum was calculated using thebackground estimator models based upon the rate of very largeevents (VLE), spacecraft activation and cosmic X-ray emission asused to calculate background light curves. The X-ray spectra arefitted with a power-law spectrum with a photon index 4.78 andcolumn density NH  2:2 ^ 0:8  1022 cm22; parameters con-sistent with those obtained from ASCA and SAX measurements inthe 2±10 keV range (Corbet et al. 1995; Parmar et al. 1998). Theresultant background subtracted X-ray luminosity time-series inthe energy range 2±10 keV is presented in Fig. 2. The variation inFigure 1. Pulse frequency time-series of 1E 1048.125937. The solid lines(upper panel) are fits to the measurements before and after 1988September, respectively. Residuals from these linear trends are shown interms of sigma values (lower panel). Measurements with Einstein,EXOSAT and Ginga (x) fit a trend well, while the later measurementswith ROSAT and ASCA (open circles) and RXTE (filled circles) fit a trendof higher spin-down, but with well-measured deviations.Figure 2. X-ray luminosity time-series of 1E 22591586.Two anomalous X-ray pulsars 207q 2000 RAS, MNRAS 319, 205±208the bolometric X-ray luminosity is less than 15 per cent. This isconsistent with low timing noise and secular spin-down accordingto accretion models.In Ginga observations 1E 22591586 had flux levels a factor oftwo higher than average (Iwasawa, Koyama & Halpern 1992).This implied that, if because of accretion, the rate on to the sourcewas variable and fluctuations in the spin-down rate would beexpected. Indeed for this era high noise levels S , 10219 rad2 s23are found (Baykal & Swank 1996).3 D I S C U S S I O NIn the pulse timing analysis of 1E 1048.125937, we found pulsefrequency changes on the time-scale of 400 d. The source spin-down rate has doubled since 1992 (Mereghetti 1995; Corbet &Mihara 1997). The pulse frequency measurements since 1992show significant fluctuations from the average spin-down trend.The level of pulse frequency fluctuations of 1E 1048.125937 arefound to be consistent with typical noise levels of accretionpowered pulsars (Baykal & OÈ gelman 1993).Two soft gamma-ray repeaters (SGR) SGR 1806220, SGR1900114 were recently identified as magnetars (Kouveliotou et al.1998, 1999). The similarity of SGR and AXP pulse periods andsecular spin-down trends raised the question whether all AXPs areactually magnetars. According to the magnetar theory pulsefrequency fluctuations may be caused by processes such asvariable emission of Alfven waves or particles from magnetars(Thompson & Blaes 1998), radiative precession of magnetars(Melatos 1999), or discontinous spin-up events (glitches) likethose seen in radio pulsars (Thompson & Duncan 1996, see alsoKaspi, Lackey & Chakrabarty 2000). Heyl & Hernquist (1999)investigated the pulse frequency histories of 1E 1048.125937 and1E 22591586 and proposed that the irregularities of these sourcesare explained by several large glitches with sizes dn , 2 1024 s21 and ,5  1026 s21; respectively. While the values of dnare only about 20 times those of the Vela and Crab pulsars,respectively, the values of dn /n are several orders of magnitudelarger than radio pulsar glitches and they occur much morefrequently than scaling from the statistics of large radio pulsarglitches would lead us to expect (Alpar & Baykal 1994).In summary, both sources have shown pulse frequencyfluctuations on the order of a few decades (Mereghetti 1995;Baykal & Swank 1996). The level of torque changes of 1E22591586 over a time scale of 15 yr is consistent with accretion-powered X-ray binaries (Baykal & Swank 1996). The steady levelof the bolometric X-ray luminosity during the quiet spin-downepoch (Kaspi et al. 1999) is consistent with accretion models.Alternatively it may turn out to be made out of discrete glitchesover a background of relatively quiet spin-down, as in isolatedpulsars and magnetars (Heyl & Hernquist 1999). Continued phasecoherent X-ray timing of this source should prove extremelyimportant in finally deciding its nature. The correlation of thespin-down rate of 1E 1048.125937 with the X-ray flux supportsthe possibility that it is an accreting source (Mereghetti 1995;Corbet & Mihara 1997). Our work indicates that the source hashigh timing noise. To see the exact nature of correlations betweenX-ray flux and pulse frequency derivatives, an even moreextensive broad band X-ray observation should be carried out.AC K N OW L E D G M E N TWe thank the referee for helpful comments.R E F E R E N C E SAlpar M. 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Differences between the Two Anomalous X-Ray Pulsars: Variations in the
  Spin Down Rate of 1E 1048.1-5937 and An Extended Interval of Quiet Spin Down
  in 1E 2259+586

Differences between the Two Anomalous X-Ray Pulsars: Variations in the Spin Down Rate of 1E 1048.1-5937 and An Extended Interval of Quiet Spin Down in 1E 2259+586

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