Detection of pulsed radio emission from bursting Magnetar Swift J1818.0-1607 below 750 MHz with the uGMRT

ATel #13580; Bhal Chandra Joshi (NCRA-TIFR, Pune, India), Manjari Bagchi(IMSc-HBNI, Chennai, India)
on 23 Mar 2020; 10:55 UT
Credential Certification: Bhal Chandra Joshi (bcj@ncra.tifr.res.in)

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A short burst on March 12, 2020 (GCN circular 27373) led to discovery of Swift J1818.0-1607. Pulsations with a spin period of 1.36 sec were subsequently detected at high energy with NICER (ATel #13551) and INTEGRAL and NUSTAR (ATEL #13569). Since then radio pulsations were reported at frequencies above 1 GHz (ATEL #13553, #13554, #13559, #13560, #13562, #13575 and #13577). No emission was reported at 350 MHz observations with Sardinia Radio Telescope (ATEL #13577). Continued monitoring indicates a large value of spin-derivative suggesting a star with high magnetic field. The current best estimate of Dispersion measure is about 700 pc-cm^-3 implying the pulse to be completely scatter-broadened at frequencies below 1 GHz.

We observed this source for about 30 minutes with the upgraded GMRT (Gupta et al. 2018) using a phased array of 20 antennas at Band 4 between 550-750 MHz in a Target of Opportunity observations on March 21, 2020. We recorded the data with 4096 channels across 200 MHz bandwidth with a sampling time of 0.6 ms. A highly scatter broadened pulse was detected with a signal-to-noise ratio (S/N) of 60 in these observations along with detection of a point source at the assumed coordinates of the source (RAJ 18:18:00.12, DecJ -16:07:52.8 : ATEL #13551). Our best estimate for the barycentric period is 1.36339(1) s, while that for the DM is 713(2). The latter estimate is likely to be affected by in-band scatter-broadening of the pulse, but is broadly consistent with the values reported elsewhere. The average pulse shows broad structures of about 25 MHz width in the spectra. A plot of the detection can be accessed on the web. Our preliminary estimate of the flux density at 650 MHz is 3.4(4) mJy. Extrapolating from the reported flux density at 1.4 GHz of about 0.7 mJy assuming a spectral index of -1.8, the expected flux density is 2.8 mJy at 650 MHz. Our higher estimates suggests a steeper spectral index for the source.

Further observations at even lower frequencies (300-500 MHz) as well as the same band are scheduled or planned. We will continue to monitor the source at these frequencies using the uGMRT.

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

Radio Detection of Swift J1818.0-1607 @ 650 MHz