Upper limits on the pulsed radio emission from SGR candidate SGR 0755-2933

ATel #8943; M. P. Surnis(NCRA-TIFR), Y. Maan(NCRA-TIFR), B. C. Joshi(NCRA-TIFR), P. K. Manoharan(RAC, NCRA-TIFR)
on 13 Apr 2016; 05:21 UT
Credential Certification: Bhal Chandra Joshi (bcj@ncra.tifr.res.in)

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SGR J0755-2933 was discovered through a sharp, soft burst on 16 March 2016 at 22:41:24 UT by the Swift Burst Alert Telescope (BAT) instrument. While the X-ray flux has been reported to decline by a factor of 16 in subsequent Swift observations, no X-ray pulsations have been as yet detected (ATEL #8868). Thus, the nature of the source is as yet unknown.

We performed radio observations with the Ooty Radio Telescope (ORT) at 327 MHz, and the Giant Metrewave Radio Telescope (GMRT) at 1390 MHz. The phase center for these observations was taken as the best-fit position determined by Swift X-Ray Telescope
RA (J2000) : 07h 55m 42.48s
DEC (J2000) : -29d 33’ 49.2” (ATEL # 8831).
The observations at the ORT were performed on 17 March 2016 (MJD 57464.56) with 8 ms sampling over a band of 16 MHz, spread over 1024 channels. The total integration time was 3 hours. The observations at the GMRT were performed on 25 March 2016 (MJD 57472.52, proposal code ddtB216) wherein, the data were recorded simultaneously in imaging and phased array (PA) mode, over a bandwidth of 33 MHz, spread over 512 channels. The PA data were recorded with a sampling time of 123 microseconds, while the imaging data were recorded with a sampling time of 16 s for a duration of 1 hour. For the imaging observations, 3C147 and 0828-375 were observed as flux density and phase calibrators, respectively.

The GMRT PA and ORT data were searched for periodic and transient signals over a dispersion measure range of 0--2000 pc/cc. We did not detect any significant pulsation with 8-sigma upper limits on flux density of 0.89 mJy and 0.12 mJy (assuming a 10% duty cycle) for periodic signals at 327 and 1390 MHz, respectively. The 8-sigma upper flux density limits for any transient signal are 2.8 Jy and 220 mJy (assuming 10 ms burst duration) at 327 and 1390 MHz, respectively. The 3-sigma upper limit on the continuum flux density from the imaging data at 1390 MHz is 0.3 mJy.

We thank the GMRT and the ORT staff members for making these observations possible. We are also thankful for a prompt allocation of Director's discretionary time for these observations.