INTEGRAL sees the ongoing spectral transition of the black hole candidate MAXI J1348-630

ATel #12471; F. Cangemi (CEA Saclay/lab AIM, Fr), T. Belloni (INAF OA Brera, It), J. Rodriguez (CEA Saclay/lab AIM, Fr) on behalf of a larger collaboration
on 4 Feb 2019; 16:28 UT
Credential Certification: Jerome Rodriguez (jrodriguez@cea.fr)

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Ongoing INTEGRAL Target of Opportunity are following the evolution of the black hole candidate MAXI J1348-630 (Atel #12457).

The source fluxes were constantly increasing since MJD 58512.43 (revolution 2051, Atel #12457). The maximum occurred during the satellite perigee passage between MJD 58514.63 and MJD 58515.09, INTEGRAL sees a maximum of 2.5 Crab and 4.3 Crab in the 30-50 keV and 50-100 keV bands respectively. After that, the fluxes have started their decrease. Since the beginning of revolution 2053 (Feb 2nd 18:39:09), the decrease has accelerated (about 25% and 28% in half a day in the 30-50 keV and 50-100 keV band respectively) in conjunction with a stronger decrease of the hardness ratio.

The average spectra of revolution 2052 (MJD 58515.09 to 58517.14) and for the beginning of the revolution 2053 (MJD 58517.77 to 58518.34) show a progressive softening of the source: we found a photon index of 1.53 +/- 0.03 with an exponential cutoff at 71 +/- 3 keV for the revolution 2052 and a photon index of 1.67 +/- 0.04 and an exponential cutoff at 71 +/- 3 keV for the beginning of revolution 2053. The IBIS fluxes are 7.34e-08 ergs/cm^2/s and 5.41e-08 ergs/cm^2/s in the 30-500 keV band for the two revolutions respectively.

The analysis of the very last data (from MJD 58518.34 to 58518.51) shows a spectrum with a photon index of 1.81 +/- 0.06 and an exponential cutoff of 79 +/- 6 keV. The significant softening seen in the INTEGRAL data confirms that MAXI J1348-630 is heading towards a state transition, which might already have started (see also MAXI/GSC at lower energies, Atel #12469).

ToO will continue during the current INTEGRAL revolution (2053) until Feb 5th 23:07:47.