4U 1755-338 still active in the soft state after almost 2.5 years in outburst
ATel #15618; Francesco Carotenuto (University of Oxford), Stephane Corbel (Universite de Paris Cite, CEA-Saclay)
on 15 Sep 2022; 16:02 UT
Credential Certification: Francesco Carotenuto (francesco.carotenuto@cea.fr)
Subjects: X-ray, Black Hole, Transient
The accreting black hole candidate 4U 1755-338 is a recurrent X-ray transient that displayed parsec-scale "fossil" X-ray jets (Angelini & White 2003). After 24 years of quiescence, it entered into outburst in April 2020 (ATel #13606, #13665) and it is still active as of today.
Since the beginning of the outburst, we monitored the source with several exposures from Swift/XRT and daily MAXI observations (e.g. ATel #14411). Throughout this time, the source has been in a typical soft X-ray state, displaying an absorbed disk blackbody spectrum and yielding a roughly constant unabsorbed flux of 1 E-09 erg/cm^2/s in the 1-10 keV range. From the MAXI monitor, the source remained active in this state since early 2020, possibly suggesting the beginning of a long period of activity, similar to the one that followed its discovery in 1970 (e.g. Giacconi et al. 1974, Roberts et al. 1996).
In order to check its current accretion state, we performed a single Swift/XRT observation of 4U 1755-338 on 2022 September 13 (MJD 59835, target ID = 13311), with a total exposure time of 850 s. The source was still detected with a high count rate of 50 c/s, which is similar to its level in previous observations. Its spectrum can be adequately fitted with an absorbed disk blackbody model, yielding a temperature kT = 1.31 ± 0.02 keV and a hydrogen column density of 0.54 ± 0.02 E+22 cm^-2. We obtain an unabsorbed X-ray flux of 2.39 ± 0.03 E-09 erg/cm^2/s in the 1-10 keV energy range.
We confirm that the source remained in the high soft state for more than two years. Interestingly, the flux has been increasing with respect to April 2020, and it is now almost 2.5 times brighter than at the beginning of the outburst. This likely indicates that 4U 1755-338 is undergoing a long-duration soft state, as in its original discovery outburst.
We thank the Swift team for scheduling and performing the observation.
