Continued Swift XRT Monitoring of GRS 1915+105
ATel #12848; M. Balakrishnan, B. Tetarenko, L. Corrales, M. Reynolds, J. M. Miller (Univ. of Michigan)
on 7 Jun 2019; 19:22 UT
Credential Certification: Jon Miller (jonmm@umich.edu)
Subjects: X-ray, Black Hole, Transient
We report on continued monitoring of GRS 1915+105 in soft X-rays with Swift XRT. The microquasar was in an unusual, low flux state for most of May (ATELs #12742, #12743, #12755, #12761,#12765, #12769) with short X-ray flares (ATELs #12761, #12787, #12793, #12805). We have used a power-law + neutral Compton reflection model to fit all observations from MJD 58610 to MJD 58639 with a fixed galactic column density of 4.0 x 10^22 cm^-2. A full treatment of these observations will be reported in a forthcoming paper. We offer this update in light of more restricted MAXI monitoring during a short period of pointing restrictions.
In the last few days, the flux observed from GRS 1915+105 has slightly increased. On June 5th (MJD 58639.9), the 1-10 keV flux was (5.7 +- 0.6) x 10^-10 erg/cm^2/s, with a partial covering fraction of 0.93 +- 0.04 and column density of (28 +- 4.5) x 10^22 cm^-2. The unabsorbed flux was (3.1 +- 0.6) x 10^-9 erg/cm^2/s (2.7 x 10^37 erg/s). The photon index for the underlying power law spectrum is constrained to 1.97 (+0.966736,-0.188418). (The errors are reported with 90% confidence.)
These results can be contrasted with similar fits to an XRT spectrum obtained on May 16 (ATEL #12771). That spectrum has an observed flux of F ~ 1.4 x10^-10 erg/cm^2/s, but an unobscured flux of 2.3 x10^-9 erg/cm^2/s after removing Compton-thick obscuration (N_H = 1.8 x 10^24 cm^-2). The more recent observations of GRS 1915+105 generally suggest less extreme obscuration, and a higher observed flux, further indicating that the current low flux state is partly due to internal obscuration.
More observations are encouraged in order to determine whether the source is truly rebrightening. We thank Brad Cenko, Jamie Kennea, and the Neil Gehrels Swift Observatory for the continued frequent observations of GRS 1915+105.