New transient ULX in NGC 7793: upper limits on its radio flux

ATel #6828; Christian Motch (Strasbourg), Thomas Russell (ICRAR-Curtin), Roberto Soria (ICRAR-Curtin), Fabien Grise' (Strasbourg), Manfred Pakull (Strasbourg), James Miller-Jones (ICRAR-Curtin)
on 16 Dec 2014; 11:08 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Roberto Soria (rsoria@physics.usyd.edu.au)

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From our analysis of a Swift X-ray telescope observation taken on 2014 November 27, we discovered that the luminosity of an X-ray binary in the galaxy NGC 7793 had increased by two orders of magnitude with respect to an XMM-Newton observation on 2013 November 25, putting it in the class of transient ultraluminous X-ray sources (ULXs). The same source had previously been detected by Chandra in 2003 and 2011, with 0.3-10 keV luminosities varying around a few 10^38 erg/s. It was fainter than 5 x 10^37 erg/s in 2013, but reached L_X ~ 3.5 x 10^39 erg/s in the recent Swift observation. From the Chandra archival data, we determined its position as RA = 23:58:08.82, Dec = -32:34:03.6 (uncertainty of ~0".5). We requested and were granted DDT observations of this ULX from the Australia Telescope Compact Array. We observed it between 04:40 UT and 15:22 UT on 2014 December 4 (MJD 56995.4), at 5.5 and 9.0 GHz with a bandwidth of 2 GHz at each frequency. The array was in its most extended 6A configuration, providing angular resolutions of 3".8 x 1".9 and 2".7 x 1".2 at 5.5 and 9.0 GHz, respectively. The data were reduced following standard procedures. We did not detect core or extended radio emission from the ULX at either 5.5 or 9.0 GHz, with 3-sigma upper limits of 22 microJy/beam and 24 microJy/beam, respectively. By stacking the two images we reach a 3-sigma upper limit of 16.5 microJy/beam. This is 30% lower than the approximate time-averaged 5 GHz core radio flux density that would have been observed from the transient ULX in M31 (XMMU J004243.6+412519) during the first few weeks of its 2012 outburst, scaled to the distance of NGC 7793 (Middleton et al. 2013, Nature, 493, 187). The M31 ULX only reached a peak X-ray luminosity ~1.3 x 10^39 erg/s but its core radio emission was Doppler boosted towards us. There is another currently bright ULX in NGC 7793, known as P13 (Motch et al. 2014, Nature, 514, 198): we also did not detect core or extended radio emission from that source, down to 3-sigma upper limits of 24 microJy/beam and 26 microJy/beam at 5.5 and 9.0 GHz, respectively, and 18 microJy/beam for the the stacked image. Further multiband observations of the two ULXs are already planned or ongoing.