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GRS 1716-249 is going back to the hard state

ATel #10371; Tiziana Bassi (INAF/IASF Palermo, Universita' di Palermo), Melania Del Santo (INAF/IASF Palermo), Sara E. Motta (University of Oxford)
on 12 May 2017; 18:00 UT
Credential Certification: Melania Del Santo (melania@ifc.inaf.it)

Subjects: X-ray, Black Hole, Transient

We report on further Swift/XRT observations of the Black Hole Transient GRS 1716-249 (Nova Oph 93), in outburst since 2016 December 18 (ATel #9876, #9895). Thereafter, a number of multi-wavelength observations have been performed on this source (see e.g. ATel #10036, #10069, #10196, #10296).
After a few months in hard/hard-intermediate state, GRS 1716-249 underwent a hard-to-soft transition, showing significant spectral softening (Gamma about 1.85, Atel #10236) with respect to previous observations. We have analysed the ToO Swift/XRT observations performed after the transition and we found that the source reached its maximum spectral slope on April 11. We fitted the pile-up corrected spectrum obtained from this pointing and we found that a power-law model provides a reasonably good fit to the data (chi2_red=1.22(534 d.o.f.)), returning a spectral photon index of 2.01 +/- 0.01. The addition of a disk black body component improves the fit (chi2_red=1.08(532 d.o.f.)), returning a slightly lower photon index of 1.73 +/- 0.07 and a disk temperature of kT_in=0.48 +/- 0.02 keV (F-test probability of 2.62E-15). The absorption model Tbabs gives Galactic column density value of N_H=(0.62 +/- 0.01)E+22 cm^-2. The unabsorbed flux in the 0.5-10 keV is (8.1 +/- 0.1)E-09 ergs/cm^2/s.
Fits to the spectra extracted from the last two Swift/XRT pointings (performed on May 5 and May 11) show that the photon index decreased to 1.73 +/- 0.02 and 1.71 +/- 0.02, respectively. The 0.5-10 keV unabsorbed flux is (5.9 +/- 0.1)E-9 ergs/cm^2/s and (5.4 +/- 0.1)E-9 ergs/cm^2/s on May 5 and on May 11, respectively. These results indicate that the source is undergoing significant spectral hardening and might be transitioning back to the hard state.

We would like to thank the Swift Team for making these observations possible, the duty scientists as well as the science planners.