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INTEGRAL observations of MAXI J1813-095

ATel #11357; Felix Fuerst, G. Belanger, M. Parker, P. Kretschmar, G. Matzeu, C. Sanchez-Fernandez, (ESA/ESAC, Spain), L. Ducci (IAAT, Germany; ISDC, Switzerland), E. Bozzo, C. Ferrigno, V. Savchenko (ISDC, Switzerland)
on 26 Feb 2018; 15:22 UT
Credential Certification: Felix Fuerst (ff@caltech.edu)

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

We observed the new transient MAXI J1813-095 (see ATels #11323, #11326, #11332, #11356) from 2018 February 23th 02:57 to 18:13 UTC with INTEGRAL in order to measure the intrinsic properties of the broad-band X-ray spectrum and to obtain more information about the nature of the source. We obtained an effective exposure of about 30ks in both JEM-X and ISGRI. We used the quick look analysis of the NRT data provided by the ISDC for our analysis and combined them with contemporaneous Swift/XRT observations taken later on the same day (ObsID 00010563003). We find that the spectrum between 0.5--200keV is well described by an absorbed power-law with an exponential cutoff at high energies. The best fit parameters are: absorption column N_H = 1.18^{+0.15}_{-0.14}e22 cm^-2, photon index Gamma = 1.57\pm0.10, and cutoff energy E_cut = 140^{+120}_{-50} keV (90% uncertainties). The spectral parameters agree well with the ones reported for the soft spectrum by Kennea et al. (ATel #11326). The measured column density is about a factor of two above the expected Galactic column of ~5e21cm^-2 (Dickey & Lockman, 1990). The unabsorbed flux between 2--100keV is 7.0\pm0.4e-10 erg/s/cm^2, which corresponds to a luminosity of ~5e36 erg/s at 8kpc (see Russell et al., ATel #11356). The improvement using a cutoff-powerlaw over a straight power law seems significant with a Delta Chi^2 of 14 for one more variable parameter, resulting in a chance improvement possibility of 0.2% according to the Akaike Information Criterion (AIC). However, we caution that the exact values and significance may change with the consolidated data. The spectral shape is consistent with the source being a new black-hole transient in a low/hard state, as typically seen during the beginning of an outburst. Further multi-wavelength observations are encouraged to follow-up and monitor MAXI J1813 during its current outburst. We thank the INTEGRAL project scientist, the ISOC, and the ISDC for the prompt processing and scheduling of our DDT observation and their support with the analysis.