WSRT Non-Detection of Radio Pulsations from the New X-ray MSP SWIFT J1756.9-2508 During its Recent Outburst

ATel #1129; J. W.T. Hessels (API, Univ. of Amsterdam), B. W. Stappers (ASTRON, Univ. of Amsterdam)
on 4 Jul 2007; 21:42 UT
Credential Certification: Jason W.T. Hessels (jhessels@science.uva.nl)

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We have searched for coherent radio pulsations from the new 182-Hz X-ray pulsar SWIFT J1756.9-2508 (ATel #1105, #1107, #1108, #1111, #1114, #1117, #1128) with the Westerbork Synthesis Radio Telescope (WSRT) in the Netherlands. We observed SWIFT J1756.9-2508 (pointing at RA = 17:56:57.2, DEC = -25:06:26.28 J2000) for 3000s (> 90% of the source's orbital period) on MJD 54266.103 (15 June 2007) with WSRT in a tied array (non-imaging) mode, using the PuMa pulsar backend. PuMa provided 25-us sampling and 0.16-MHz channels covering 60 MHz of RFI clean bandwidth centered at 1.4 GHz.

Given the unknown dispersion measure (DM) towards this source, we searched trial timeseries dedispersed from DM 0-700 pc cm^-3 in steps of 1-4 pc cm^-3 (the step size being larger at higher DMs). These timeseries were searched using a Fourier-based linear acceleration search (Ransom et al. 2002, AJ, 124, 1788) of the full data span, as well as overlapping chunks of 750s each. This was done to partially account for Doppler shifting of the signal due to orbital motion. Furthermore, these timeseries were folded at the known spin period of the pulsar, allowing for a small search in spin period and period derivative to account for orbital motion.

No pulsations were detected in these searches. Assuming the radio signal was not overly smeared by intra-binary or interstellar scattering, or orbital motion, we can place a 1.4-GHz flux upper limit of ~ 0.5 mJy on pulsed radio emission from SWIFT J1756.9-2508. These radio observations were taken shortly after the source's discovery, during which it was X-ray active. The availability of a more precise rotational ephemeris would allow deeper searches of this data for radio pulsations. Recent observations, now that this source is X-ray quiet (ATel #1117), have failed to detect a radio point source at this position (ATel #1128).