Crawford, F.

Hessels, J.W.T.

Kaspi, V.M.

English

We observed four southern AXPs in 1999 near 1400 MHz with the Parkes 64-m radio telescope to search for periodic radio emission. No Fourier candidates were discovered in the initial analysis, but the recent radio activity observed for the AXP XTE J1810-197 has prompted us to revisit these data to search for single radio pulses and bursts. The data were searched for both persistent and bursting radio emission at a wide range of dispersion measures, but no detections of either kind were made. These results further weaken the proposed link between rotating radio transient sources and magnetars. However, continued radio searches of these and other AXPs at different epochs are warranted given the transient nature of the radio emission seen from XTE J1810-197, which until very recently was the only known radio-emitting AXP

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UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl)UvA-DARE (Digital Academic Repository)A search for single radio pulses and bursts from southern AXPsCrawford, F.; Hessels, J.W.T.; Kaspi, V.M.DOI10.1063/1.2900156Publication date2008Document VersionAuthor accepted manuscriptPublished in40 Years of Pulsars: Millisecond Pulsars, Magnetars and MoreLink to publicationCitation for published version (APA):Crawford, F., Hessels, J. W. T., & Kaspi, V. M. (2008). A search for single radio pulses andbursts from southern AXPs. In C. Bassa, Z. Wang, A. Cumming, & V. M. Kaspi (Eds.), 40Years of Pulsars: Millisecond Pulsars, Magnetars and More: McGill University, Montréal,Canada, 12-17 August 2007 (pp. 259-261). (AIP Conference Proceedings; Vol. 983).American Institute of Physics. https://doi.org/10.1063/1.2900156General 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:10 Mar 2023arXiv:0710.2628v1  [astro-ph]  13 Oct 2007A Search for Single Radio Pulses and Bursts from SouthernAXPsF. Crawford∗, J. W. T. Hessels† and V. M. Kaspi∗∗∗Department of Physics and Astronomy, Franklin & Marshall Colege, Lancaster, PA 17604, USA†Astronomical Institute “Anton Pannekoek,” University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, TheNetherlands∗∗Department of Physics, McGill University, Montreal, QC H3A2T8, CanadaAbstract. We observed four southern AXPs in 1999 near 1400 MHz with the Parkes 64-m radio telescope to search forperiodic radio emission. No Fourier candidates were discovered in the initial analysis, but the recent radio activity observedfor the AXP XTE J1810−197 has prompted us to revisit these data to search for singleradio pulses and bursts. The datawere searched for both persistent and bursting radio emission at a wide range of dispersion measures, but no detections ofeither kind were made. These results further weaken the proposed link between rotating radio transient sources and magnet rs.However, continued radio searches of these and other AXPs atdifferent epochs are warranted given the transient nature of theradio emission seen from XTE J1810−197, which until very recently was the only known radio-emitting AXP.Keywords: neutron stars, AXPs, magnetarsPACS: 97.60.Jd, 98.70.QyINTRODUCTION AND MOTIVATIONThe detection of pulsed radio emission from the anoma-lous X-ray pulsar (AXP) XTE J1810−197 in 2006 [1],and more recently from a second AXP, 1E 1547.0−5408[2], has renewed interest in searching for radio emissionfrom these objects. In both of these cases, the radio ac-tivity is believed to be connected to the X-ray variabil-ity of the sources and is transient in nature (or at leasthighly variable). Given this transient behavior and thatboth persistent periodic emission and single pulses weredetected from both AXPs, renewed searches of archivalradio search data of AXPs at different epochs may revealpreviously undetected radio signals from these sources.OBSERVATIONS AND ANALYSISThree southern AXPs and one AXP candidate were ob-served in July and August 1999 with the Parkes 64-mradio telescope. The four targets observed were:• 1E 1048.1−5937• AX J1845−0258 (AXP candidate only)• 1E 1841−045• 1RXS J170849.0−400910All observations were conducted with the center beamof the multibeam receiver [3] at a center frequency of1374 MHz. 288 MHz of bandwidth was split into 96frequency channels. This is the same observing setup aswas used for the Parkes Multibeam Survey [4] and theAXP search observations reported by [5]. None of thefour targets was in a state of X-ray outburst at the time ofobservation.The data were processed using the PRESTO1 suite ofpulsar analysis tools [6, 7]. First, the raw data were ex-cised of radio frequency interference (RFI). This is par-ticularly important given the slow rotation rates of theAXPs and the pernicious effect of RFI at low modulationfrequencies. Typically, 15-25% of the frequency chan-nels and∼ 5% of the integration time were discarded inthis process. Note that this data excision increases oursensitivity limits by∼ 10% and is not included in the es-timates presented in Table 1. Standard Fourier searchesof the data were previously reported [8], but no signalswere confirmed in that analysis.In both the folding search and single pulse analysisreported here, the data were dedispersed at a wide rangeof dispersion measures (DMs). The DMs ranged from 0to 4000-8000 pc cm−3, depending on the spin period ofthe AXP. The dedispersed time series were searched forboth persistent periodic emission and single pulses andbursts.To detect periodic emission from these sources, thedata were divided into 9 MHz subbands, and each sub-band was dedispersed at a trial DM of 500 pc cm−3.Each subband was then folded at the known neutron starspin period using the X-ray timing ephemeris (whereavailable). In the case of AX J1845−0258, where no1 http://www.cv.nrao.edu/∼sransom/prestoephemeris was available, the discovery period was used.The full range of trial DMs was created by shifting thesubbands with respect to each other in pulse phase. Wealso folded the data allowing for a search in period±5-10 ms from the predicted period (depending on the spinperiod). Additionally, separate folds were made for eachtarget using overlapping separate shorter segments of thefull observation; each of these segments was 20% of thefull observation length and started at intervals of 10% ofthe data length. This accounted for possible scintillation,strong transient RFI near the AXP spin period, and pulsestrength variability on time-scales shorter than the obser-vation length.For the single pulse search, the raw data were againdedispersed at a set of trial DMs ranging from 0 to 4000-8000 pc cm−3. Using the 0.25 ms native sampling of thedata, each dedispersed time series was searched for can-didate signals having a signal-to-noise ratio greater than6.5. This threshold was chosen to avoid confusion withthe RFI background. To maintain sensitivity to pulses ofwidth greater than 0.25 ms, a matched filter was em-ployed using a boxcar function of varying width (rang-ing from 1 to 30 samples). To enhance sensitivity to evenlonger pulses and bursts, the dedispersed time series wasdownsampled by combining the original samples intocontiguous blocks of 2, 4, 8, 16, and 32 samples, andthe same boxcar filtering was then used. This providedsensitivity to bursts with durations up to 240 ms.RESULTS AND CONCLUSIONSWe found no convincing radio signals in either the fold-ing or single pulse searches. The derived upper limits onthe radio emission from our AXP targets are presented inTable 1. These are the most stringent radio upper limitsto date for these sources. The estimated 1400 MHz lu-minosity limits on the periodic radio emission (<∼ 1 mJykpc2) are 2-3 times lower than those established for XTEJ1810−197 prior to outburst. However, it is still conceiv-able that weak radio pulses are being emitted, but thatthey are below our detection threshold.The luminosity limits presented here for the periodicemission are lower by about two orders of magnitudethan the 1400 MHz luminosity of the periodic pulsed ra-dio emission from XTE J1810−197 soon after the radioemission was first detected (∼ 80 mJy kpc2) [1]. Thuswe would expect to be able to easily detect comparablystrong radio emission if it were beamed toward us.Our luminosity limits on single radio pulses from oursources range from 22 to 69 Jy kpc2 in the most con-servative case, which is below the 1400 MHz luminosityof >∼ 100 Jy kpc2 derived from the pulse strengths re-ported for XTE J1810−197 in its radio discovery paper[1]. Since single pulses were detected from almost everyrotation of XTE J1810−197, it is likely that we wouldhave detected a large number of comparable pulses dur-ing our observations if such pulses were beamed towardus.Our non-detection of single pulses further weakensthe hypothesis that rotating radio transients (RRATs) [9]and magnetars are linked. This has been weakened bytwo other recent results. First, the X-ray detection ofthe RRAT J1819−1458 shows that its emission is moretypical of middle-aged pulsars than it is of magnetars[10]. Second, the nearby, rotation-powered pulsar PSRB0656+54 would probably have been identified as anRRAT if it were farther away [11].We conclude from our results that any periodic orbursting radio emission from the four target AXPs iseither very weak (below our detection thresholds), notbeamed toward us, or non-existent or sporadic at theepoch of observation. This last possibility is suggestedby the connection between the X-ray and radio activ-ity observed for the two known radio-emitting magnetarsto date. Continued radio searches of AXPs are thereforewarranted given the apparent transient nature of the radioemission. Further details of this work and a more com-plete discussion of the results are presented in a recentjournal article [12].REFERENCES1. F. Camilo, S. M. Ransom, J. P. Halpern, J. Reynolds, D. J.Helfand, N. Zimmerman, and J. Sarkissian,Nature442,892–895 (2006).2. F. Camilo, S. M. Ransom, J. P. Halpern, and J. Reynolds,Astrophysical Journal Letters666, L93–L96 (2007).3. L. Staveley-Smith, W. E. Wilson, T. S. Bird, M. J. Disney,R. D. Ekers, K. C. Freeman, R. F. Haynes, M. W. Sinclair,R. A. Vaile, R. L. Webster, and A. E. Wright,Publicationsof the Astronomical Society of Australia13, 243–248(1996).4. R. N. Manchester, A. G. Lyne, F. Camilo, J. F. Bell, V. M.Kaspi, N. D’Amico, N. P. F. McKay, F. Crawford, I. H.Stairs, A. Possenti, M. Kramer, and D. C. Sheppard,Monthly Notices of the Royal Astronomical Society328,17–35 (2001).5. M. Burgay, N. Rea, G. L. Israel, A. Possenti, L. Burderi,T. di Salvo, N. D’Amico, and L. Stella,Monthly Noticesof the Royal Astronomical Society372, 410–416 (2006).6. S. M. Ransom,New Search Techniques for BinaryPulsars, Ph.D. thesis, Harvard University (2001).7. S. M. Ransom, S. S. Eikenberry, and J. Middleditch,Astronomical Journal124, 1788–1809 (2002).8. F. Crawford, M. J. Pivovaroff, V. M. Kaspi, and R. N.Manchester, “A Sensitive Targeted Search Campaign atParkes to Find Young Radio Pulsars at 20 cm,” inNeutronStars in Supernova Remnants, edited by P. O. Slane, andB. M. Gaensler, 2002, vol. 271 ofAstronomical Society ofthe Pacific Conference Series, pp. 37–40.9. M. A. McLaughlin, A. G. Lyne, D. R. Lorimer,M. Kramer, A. J. Faulkner, R. N. Manchester, J. M.TABLE 1. Radio Search Parameters and Results1E 1048.1−5937 AX J1845−0258∗ 1E 1841−045 1RXS J170849.0−400910Spin period (s) 6.45 6.97 11.77 11.00Ephemeris reference [13] [14]† [15] [16]Galactic longitude, latitude (deg) 288.26,−0.52 29.52, 0.07 27.39,−0.01 346.47, 0.03Tsky (K)∗∗ 9.1 12.3 13.2 16.3Observation MJD 51378 51391 51382 51379Observation date 1999 Jul 19 1999 Aug 1 1999 Jul 23 1999 Jul 20S1400 (mJy)‡ <∼ 0.02 <∼ 0.02 <∼ 0.02 <∼ 0.02S1400 single (mJy)§ <∼ 875-50 <∼ 975-60 <∼ 1000-60 <∼ 1085-65Distance (kpc)¶ ∼ 5? ∼ 8? ∼ 7 ∼ 8?L1400 (mJy kpc2)‖ <∼ 0.5 <∼ 1.3 <∼ 1.0 <∼ 1.3L1400 single (Jy kpc2)†† <∼ 22-1.3 <∼ 62-3.7 <∼ 49-2.9 <∼ 69-4.1∗ AXP candidate only† No period derivative available∗∗ 1374 MHz sky temperature estimated from [17] assuming a spectral index of−2.6‡ 1400 MHz flux density limit on pulsed emission estimated using the modified radiometer equation and an assumed duty cycle of 2.7%§ Range of single-pulse 1400 MHz flux limits for pulse time-scales 0.25-240 ms¶ Taken from [5]. Question marks indicate significant uncertainty in the value‖ 1400 MHz luminosity limit on pulsed emission, assuming a 1 srbeaming fraction†† Range of 1400 MHz luminosity limits on single pulses for pulse time-scales 0.25-240 msCordes, F. Camilo, A. Possenti, I. H. Stairs, G. Hobbs,N. D’Amico, M. Burgay, and J. T. O’Brien,Nature439,817–820 (2006).10. S. P. Reynolds, K. J. Borkowski, B. M. Gaensler, N. Rea,M. McLaughlin, A. Possenti, G. Israel, M. Burgay,F. Camilo, S. Chatterjee, M. Kramer, A. Lyne, andI. Stairs,Astrophysical Journal Letters639, L71–L74(2006).11. P. Weltevrede, B. W. Stappers, J. M. Rankin, and G. A. E.Wright, Astrophysical Journal Letters645, L149–L152(2006).12. F. Crawford, J. W. T. Hessels, and V. M. Kaspi,Astrophysical Journal662, 1183–1187 (2007).13. V. M. Kaspi, F. P. Gavriil, D. Chakrabarty, J. R. Lackey,and M. P. Muno,Astrophysical Journal558, 253–262(2001).14. K. Torii, K. Kinugasa, K. Katayama, H. Tsunemi, andS. Yamauchi,Astrophysical Journal503, 843–847 (1998).15. E. V. Gotthelf, G. Vasisht, and T. Dotani,AstrophysicalJournal Letters522, L49–L52 (1999).16. F. P. Gavriil, and V. M. Kaspi,Astrophysical Journal567,1067–1076 (2002).17. C. G. T. Haslam, C. J. Salter, H. Stoffel, and W. E. Wilson,Astronomy and Astrophysics Supplement Series47, 1, 2,4–51, 53–142 (1982).

A search for single radio pulses and bursts from southern AXPs

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A search for single radio pulses and bursts from southern AXPs

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