Detection of radio pulsations from the direction of the Galactic center Soft Gamma-ray Repeater with Parkes and the GBT

ATel #5035; M. Burgay (INAF-Cagliari), R. Shannon (CASS-Marsfield), A. Possenti (INAF-Cagliari), M. Keith (CASS-Marsfield), J. Sarkissian (CASS-Parkes), G. Israel (INAF-O. A.Roma), S. Johnston (CASS-Marsfield), N. Rea (CSIC/IEEC-Barcelona), P. Esposito (INAF-IASF Milano), on behalf of a larger collaboration
on 1 May 2013; 13:09 UT
Credential Certification: Marta Burgay (burgay@ca.astro.it)

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We have observed the new soft gamma-ray repeater located in the direction of the Galactic center region (Atel #5009) from the Parkes and Green Bank radio telescopes, within our pre-approved Target of Opportunity programs of radio follow-up of transient magnetars.

With the 64-metre Parkes Telescope, observations were performed with the PDFB3 and PDFB4 backends operating in search mode. An initial 1.5 hour observation was carried out starting on 2013-04-27 at 13:53 UT, with a central frequency of 3094 MHz and bandwidth 1024 MHz. Additional observations were taken on 2013-04-28 at 18:13 UT (20 minutes long) and on 2013-04-29 at 16:38 UT (1.5 hour long) using the aforementioned setup and simultaneously observing at a central frequency of 732 MHz (bandwidth 64 MHz). Moreover, we observed at a central frequency of 1369 MHz (bandwidth 256 MHz) on 2013-04-28 at UT 13:48 for a total of 20 minutes. The system performances were verified by detecting pulsations from J1721-3532.

The 100-metre Green Bank Telescope (GBT) observations were performed with the GUPPI backend, operated in search mode at a central frequency of 2000 MHz, with a bandwidth of 800 MHz, starting on 2012-04-29 at 07:15 UT, for a duration of 25 minutes. We previously looked in search mode at the known pulsar J1745-3040 to check the system and the source displayed well in the data.

For all the observations on-source a strong signal (visible in less than 1 minutes at 1369 MHz, 2000 MHz and 3094 MHz) with a large duty cycle (approaching 100%) was found, showing a rapidly changing period with values close to the 3.76 s detected in the NUSTAR (Atel #5020), and Chandra (Atel #5032) observations. The dispersion measure (DM) appears to be very low, ~< 50 cm-3 pc, although an accurate determination of DM is very hard because of the fact that the shape of the pulses is varying in time and possibly also in frequency.

No signal at a similar period was detected in any of the off-source observations performed at both telescopes. In particular, for Parkes, besides the observations of the test pulsar, we collected data at a position (J2000) RA=17:45:00 DEC=-28:00:00, approximately one degree from the X-ray source, whereas for GBT we searched the data taken at position (J2000) RA=17:45:56 DEC=-30:40:23, about 2 degrees from the X-ray source.

The detection of a signal of similar shape from two telescopes at multiple frequencies and different sidereal times (and hence variable heights above the horizon), as well as its absence in the offset pointings, excludes an RFI nature (possibly hinted by its low DM), and suggests that the soft gamma-ray repeater might be radio bright, and located at a distance < 2kpc projected toward the Galactic Center.

Further observations are ongoing for fully characterize the properties of this signal.