Swift and AstroSat monitoring of GX339-4 during its 2019 activity

ATel #13264; J. A. Paice (Univ. Southampton, IUCAA), P. Gandhi (Southampton), T. Belloni (INAF), A. Rao, M. Pahari (Southampton), B. Maqbool (IUCAA), A. Beri (IISER Mohali), D. Bhattacharya (IUCAA), D. Buckley (SAAO), P. Casella (INAF), P. A. Charles (Southampton), R. Fender (Oxford), F. Lewis (Cardiff), R. Misra (IUCAA), S. Motta (Oxford), D. M. Russell (NYU Abu Dhabi)
on 5 Nov 2019; 10:53 UT
Credential Certification: Poshak Gandhi (p.gandhi@soton.ac.uk)

Tweet

GX339-4 is a well known Galactic black hole X-ray transient that often displays enhanced accretion activity, most recently undergoing a weak hard-state-only outburst beginning in December 2018 (ATel #12287), peaking in early 2019, and followed by a decline towards mid-2019. In August 2019, the source was found to be rebrightening (ATel #13024) and observations on September 17 found it back at the peak flux level seen in early 2019 (ATel #13113). Continued XRT monitoring has shown a mild rise in brightness, whilst remaining in the hard state. The last XRT observation was carried out on 2019 October 28 (see link below), and showed an observed flux F(0.3-10)=1.2(+/-0.1)x10^{-9} erg/s/cm^2 for a photon-index=1.3+/-0.2 and N(H)=5.5(+/-1.3)x10^{21} cm^{-2}.

We also report on recent AstroSat (Singh et al. 2014, Proc. SPIE, 9144, 91441S) observations of GX339-4. We triggered an anticipated ToO observation, which was carried out between 2019 September 22, 17:19 UT to September 23, 04:58 UT, producing a total exposure duration of 35 ks with the LAXPC (see Yadav et al. 2016, Proc. SPIE, 9905, 99051D). The source was found to be bright, averaging around 275 ct/s (LAXPC20 gross count rate) over the 3-80 keV energy range, against a background count rate of 139 ct/s, and with a fairly high preliminary fractional r.m.s measurement of 54% over 0.001-20 Hz (3-30 keV). Using WebPIMMS for AstroSat (for a single LAXPC detector), and assuming a power law of 1.6, gives the following fluxes:

3-80 keV: 1.7e-09 erg/cm2/s
3-30 keV: 9.6e-10 erg/cm2/s
2-10 keV: 4.5e-10 erg/cm2/s.

The X-ray lightcurve exhibited frequent rises of a factor of 3 or more above the running mean on ~10s timescales. The PSD can be characterised by three Lorentzians, and the general timing properties are similar to those seen before in the hard state (e.g. Belloni et al. 2005, A&A, 440, 1, 207), albeit with a high r.m.s. See the links below for these plots.

According to ongoing monitoring with Swift/BAT and MAXI, GX 339-4 continues to show activity, with the X-ray flux remaining roughly constant over the past few weeks. There is also mild evidence of further brightening during early November 2019 as seen with Swift/BAT, though this remains to be confirmed. The present extended hard state offers the opportunity for sustained multiwavelength follow-up.

We gratefully acknowledge the AstroSat and Swift teams for their help with the scheduling. This work is supported by STFC and a UGC-UKIERI Phase 3 Thematic Partnership.

Swift and AstroSat monitoring of GX339-4 in 2019