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Swift broad band observations of the Black Hole transient GRS 1716-249

ATel #10036; Melania Del Santo, Antonino D'Ai' (INAF/IASF-Palermo), Tiziana Bassi (INAF/IASF-Palermo, Universita' di Palermo), Alberto Segreto (INAF/IASF-Palermo), Tomaso Belloni (INAF/OAB), Giancarlo Cusumano, Valentina La Parola (INAF/IASF-Palermo)
on 1 Feb 2017; 16:12 UT
Credential Certification: Melania Del Santo (melania@ifc.inaf.it)

Subjects: X-ray, Binary, Black Hole, Transient

Referred to by ATel #: 10069, 10196, 10236, 10296, 10371

We report on Swift observations of the ongoing outburst of the Black Hole Transient (BHT) GRS 1716-249 (ATel #9876, #9895). We analyzed both XRT and BAT data of three Swift ToO pointings performed on 2017 January 28, 29 and 30 (Target ID 34924, segments 1, 2, 3). The XRT count rate is about 90 count/s and therefore the observations have been performed in window-timing mode.
However, in the segment 3 a short XRT snapshot (75 s) has been performed in photon-counting mode. Despite the strong pile-up, this allowed us to obtain the source position (with xrtcentroid tool):
RA(hh mm ss.s) = 17 19 36.80
Dec(dd mm ss.s) = -25 00 59.95
with an uncertainty of 3.8".
The position is consistent with the optical counterpart of GRS 1716-249, Nova Ophiuchi 1993, (Della Valle et al., 1994, A&A, 290, 803) and thus we confirm the association between the MAXI outbursting source and the known black-hole candidate GRS 1716-249 (ATel #9876).

Since the spectral shape did not change in the three observations, we fit together the three XRT spectra and the simultaneous BAT averaged spectrum (extending up to 120 keV). A preliminary fit (Chi2_red=1.17, 2071 d.o.f.) with an absorbed cutoff power-law provides Gamma=1.59 +/- 0.01 with a lower limit on the high-energy cutoff (above 120 keV). A fit with the physical model COMPTT (thermal Comptonisation) provides the following parameters: kTe=38+11/-5 keV and tau=1.5+0.2/-0.3 (Chi2_red=1.18, 2071 d.o.f.), typical of the BHT in hard state. The absorption N_H is (0.48 +/- 0.01)E22 cm^-2, consistent with the value found by Tanaka 1993 (IAU Circ. 5877).
The 2-10 keV and 15-150 keV fluxes of GRS 1716-249 are 4.8E-9 erg/cm2/s and 1.5E-8 erg/cm2/s, respectively. The unabsorbed bolometric flux is 2.7E-8 erg/cm2/s which results in a luminosity of 2E+37 erg/s for a distance of 2.4 kpc (Della Valle et al., 1994, A&A, 290, 803).

The average power-density spectrum for the three XRT observations is well fitted by a zero-centered Lorentzian component with break at a characteristic frequency of 1.09 +/- 0.06 Hz and an integrated fractional rms of 11.8 +/- 0.5 %. The break frequency is rather high for a typical hard state and the fractional rms correspondingly low. These values are more consistent with a hard-intermediate state, at variance with the energy spectrum.