Reduction of the rate of flux increase during recent INTEGRAL hard state observations of the new transient black hole candidate MAXI J1348-630

ATel #12457; F. Cangemi, J. Rodriguez (CEA Saclay/lab AIM, Fr), T. Belloni (INAF OA Brera, It), M. Clavel (IPAG Grenoble, Fr), V. Grinberg (IAA Tuebingen, Ger) on behalf of a larger collaboration
on 30 Jan 2019; 21:49 UT
Credential Certification: Jerome Rodriguez (jrodriguez@cea.fr)

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The new Galactic transient MAXI J1348-630 (ATel #12425), which was subsequently detected with Swift, INTEGRAL, and in the optical band (GCN #23795, #23796, #23797, #23801, #23799, ATel #12434, #12441, #12430, #12439), has been classified as a black hole candidate after the identification, with NICER, of X-ray fast temporal and spectral behaviour typical of this class of objects (ATel #12447).

We triggered an INTEGRAL target of opportunity which started on 2019-01-29 10:23:09 (UTC) and will last over the entire satellite revolution (number 2051) until 2019-01-31 15:16:22 (UTC). The IBIS fluxes in the 30-50 keV and 50-100 keV bands have been constantly rising and reached 2.2 Crab and 3.9 Crab respectively until about MJD 58513.70 (or INTEGRAL science window #34). The rate of increase was, however, much flatter than during the previous observations of the field in revolutions 2049 and 2050 (ATel #12441), especially in the harder band. The hardness ratio has been steadily and monotonously decreasing, with no point of inflection, over the same time interval.

We extracted an average IBIS spectrum in the 30-500 keV band using scw 2 to 24. The spectrum is well modelled by a cutoff power law model with a reduced chi2 = 1.2 (57 degrees of freedom) with a photon index of 1.27 +/- 0.05 and an exponential cutoff at 66 +/-3 keV, with a 30-500 keV flux 7.16e-8 ergs/cm2/s. Overall the source behaviour is typical of a black hole hard state in the rising phase of its outburst, although we note that the power law photon index is extremely hard.

Note that the data analysed are near real time and not definitive; the results should then be taken with certain caution. All the analysis have been done with the latest version of the INTEGRAL data reduction software and calibrations (OSA 11.0). INTEGRAL ToO observations are currently planned until the end of the next revolution on 2019-02-03 03:25:43 (UTC).

We warmly thank the INTEGRAL planners at ESAC and the ISDC for their prompt reaction and help in the analysis of these data. Multiwavelength observations are strongly encouraged, while news and some of the current plan for observations are exchanged through the smartNet community web site .