4U 1755-338 is still in a soft/thermally dominated state after 10 months
ATel #14411; Francesco Carotenuto, Stephane Corbel (Universite de Paris, CEA-Saclay)
on 19 Feb 2021; 17:00 UT
Credential Certification: S. CORBEL (stephane.corbel@cea.fr)
Subjects: X-ray, Black Hole, Transient
Referred to by ATel #: 15618
The accreting black hole candidate 4U 1755-338 entered into outburst in 2020 April (ATel #13606, ATel #13665), after a period of 24 years in quiescence. The source is known for displaying parsec-scale "fossil" X-ray jets (Angelini & White 2003). We monitored the new outburst with Swift/XRT between 2020 April and June and we checked its accretion state with daily MAXI observations. Throughout this time, the source has been in a stable soft X-ray state typical of black hole X-ray binaries, with an approximately constant unabsorbed X-ray flux of 1E-09 erg/cm2/s in the 1-10 keV energy range.
The source has been Sun-constrained for Swift between 2020 November 5 and 2021 February 4. Once the period of Sun-constraint ended, we performed a single Swift/XRT observation of 4U 1755-338 on 2021 February 17 (MJD 59262.985, target ID = 13311), with a total exposure time of 600 s. The source is still detected with a high count rate of 45 c/s, which is similar to its level in previous observations. Its spectrum is well-fitted with an absorbed disk blackbody model, yielding a temperature kT = 1.32 ± 0.02 keV and a hydrogen column density of 0.43 ± 0.02 E+22 cm^-2. We obtain an unabsorbed X-ray flux of 1.93 ± 0.03 E-09 erg/cm^2/s in the 1-10 keV energy range.
The disk temperature, and consequently the flux, shows a slightly increasing trend with respect to April 2020. We confirm that the source remained approximately stable in the high-soft state for at least 10 months, which is something unusual for transient black hole low mass X-ray binaries.
We thank the Swift team for scheduling and performing the observation.
