INTEGRAL and Neil Gehrels Swift observations of GRS 1915+105 during its recent low luminosity hard state may indicate the source is going towards quiescence

The black hole microquasar GRS 1915+105 has been in a very low luminosity state for almost a year (ATel #11828) both at soft and hard X-rays as seen with the MAXI (http://maxi.riken.jp/star_data/J1915+109/J1915+109.html) and Swift/BAT (https://swift.gsfc.nasa.gov/results/transients/GRS1915p105/) monitorings.

Recently, a further sudden decrease of the flux has been reported from NICER and Chandra data (ATels #12742 and #12743). We have been observing GRS 1915+105 with INTEGRAL as part of a long-term high-energy monitoring program (see eg. Rodriguez et al. 2008). Since 2015 this program has been made with simultaneous Neil Gehrels Swift snapshots to cover the soft X-rays.

Observations taken in October 2018 showed the source to be in a hard flavour of its various variability states (Belloni et al. 2000), with a rather steady and low X-ray flux. The Swift/XRT (1-9 keV)+ INTEGRAL/ISGRI (30-300keV) spectra from MJD 58420 (October 29) are well fitted with a phenomenological model consisting of an absorbed cut-off power law (tbabs*cutoffpo), with N_H=6.3 (+-0.2) e22 cm-2, Gamma=1.58 (+-0.03) E_cut=96 (-10 +12) keV, and an unabsorbed 2-200 keV flux of 5.9e-9 erg/cm-2/s.

This showed already an unusual hard state for GRS 1915+105. The spectral state of the source seems to have remained rather constant since then. During the first observations of our 2019 campaign (MJD 58574, April 1st) the Swift/XRT+INTEGRAL/IBIS spectra are well fitted with an absorbed power law (N_H=6.0 (+-0.1) e22 cm-2, Gamma=1.78 (+-0.03) E_cut=97 (-12 +16) keV leading to an unabsorbed 2-200 keV flux of 5.3e-9 erg/cm2/s, showing only slight softening of the spectral slope.

Consistently with the decrease of X-ray fluxes mentioned in ATELs #12742 and #12743, our latest observations performed on May 13th (MJD 58616) show the source continues its dimming. The most recent Swift/XRT-INTEGRAL/IBIS spectra are fitted with an absorbed cut-off power law with Gamma=1.8 (+-0.1) and Ecut=69 (-23 +55) keV corresponding to a 2-200 keV unabsorbed flux of 1.7e-9erg/cm2/s. (Remark that XRT was operated in photon counting mode and that the inner central pixels had to be excised to avoid pile up).

This points towards GRS 1915+105 being in a significantly harder state than previously when in the so-called chi-class (C-state according to the classification of Belloni et al. 2000) in the past. In particular during the specific chi observation observed with INTEGRAL and RXTE (and discussed in Rodriguez et al. 2008) the spectrum of the source was much softer, with a low Comptonisation temperature and a hard X-ray tail.

The 2-200 keV flux was also 20 times higher than our most recent observation. This and the fact the all spectra of the recent observations are also particularly similar to standard hard state black hole spectra such as those from the prototypical source GX 339-4 which may indicate that GRS 1915+105 is decreasing towards quiescence through a low hard state.

More observations, and especially monitorings at various wavelengths, are encouraged to study this very rare stage for this source. We warmly thank the INTEGRAL and Swift planners and mission operations centers for having made this program possible over the years. references: Belloni et al. 2000, MNRAS, 355, 271; Rodriguez et al. 2008, ApJ, 675, 1449