Low-frequency radio detection of MAXI J1348-630 with the MWA

ATel #12520; Jaiverdhan Chauhan, James Miller-Jones, Gemma Anderson(ICRAR/Curtin), Tom Russell (UvA), Paul Hancock, Arash Bahramian, Stefan Duchesne, Andrew Williams (ICRAR/Curtin)
on 20 Feb 2019; 06:42 UT
Credential Certification: Gemma Anderson (gemma.anderson@curtin.edu.au)

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Since 2019-02-06, the Murchison Widefield Array (MWA) has regularly monitored the outburst of the black hole candidate X-ray binary MAXI J1348-630 at 154 MHz and 216 MHz, which was first detected by MAXI on 2019-01-26 (ATel #12425). The ongoing outburst has been detected across the electromagnetic spectrum with observations from INTEGRAL, Swift, NICER, various optical facilities, ATCA and MeerKAT (e.g. ATels #12434, #12439, #12441, #12447, #12456, #12480, #12497).

We report on the preliminary analysis of the data taken on 2019-02-09 from 19:07:02 to 21:05:02 UTC, using 128 MWA tiles in the extended configuration, with 30 MHz of bandwidth. These data were taken within a few hours of both MeerKAT (ATel #12497) and ATCA monitoring. MWA detect the source at high significance, with radio flux densities of 301+/- 21 mJy and 362+/-22 mJy at 154 MHz and 216 MHz, respectively. A spectral fit to the radio spectrum on 2019-02-09, which includes fluxes from MWA, MeerKAT (1.3 GHz) and ATCA (5-21 GHz), resulted in a spectrum peaking at 360 MHz at the synchrotron self-absorption turnover, with a power-law index of alpha = 2.45 +/- 0.03 and alpha = -0.82 +/- 0.03 pre- and post-peak, respectively. The MWA observations have constrained the spectrum in the low-frequency regime.

Ongoing MAXI and XRT monitoring show that MAXI J1348-630 had recently made its first excursion to the soft X-ray spectral state around 2019-02-09 (MAXI light curve: http://maxi.riken.jp/star_data/J1348-632/J1348-632.html ). This suggests that our observations may have covered the radio flare known to be associated with the hard-to-soft X-ray spectral state transition.

We thank MWA operations staff for rapidly responding to our target of opportunity request.