INTEGRAL detection of hard X-ray activity from MAXI J1810-222

ATel #13540; L. Ducci (IAAT, Germany; ISDC, Switzerland), V. Grinberg (IAAT, Germany), E. Kuulkers (ESA, The Netherlands), A. Paizis, (INAF-IASF Milano, Italy), J. Wilms (ECAP/FAU, Germany), L. Sidoli (INAF-IASF Milano, Italy), C. Ferrigno, E. Bozzo, V. Savchenko (ISDC, Switzerland)
on 3 Mar 2020; 11:19 UT
Credential Certification: Lorenzo Ducci (lorenzo.ducci@unige.ch)

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During the INTEGRAL observations of the Galactic Center region carried out from 27 February 2020 at 15:08 to 28 February 2020 at 10:59 (INTEGRAL revolution 2199) and from 29 February 2020 at 14:16 to 1 March 2020 at 8:54 (INTEGRAL revolution 2200; total exposure time 97 ks), significant hard X-ray emission was detected with IBIS/ISGRI from the X-ray source MAXI J1810-222 (ATels #12254, #12264, #12283, #12398, #12487, #12496, #12521, #12910).

The source was detected in the IBIS/ISGRI mosaic images at a flux of 14 ± 3 mCrab (18 ± 3 mCrab) in the 28-40 keV (40-80 keV) energy band. The average IBIS/ISGRI spectrum (28-100 keV) can be reasonably well fit with a power law model with photon index 1.7 ± 0.4 (χ²_red= = 0.53, 9 d.o.f.). All uncertainties are given at 68% confidence level. MAXI J1810-222 was outside the field of view of the JEM-X instrument during the whole observation.

MAXI J1810-222 was discovered on 1 December 2018, during a soft state modeled with a blackbody with a temperature of about kT=0.4 keV and a power law with photon index of 3.8 (ATels: #12398, #12283). On 3 July 2019, the source showed a transition to a harder spectral state, with a disc blackbody temperature of about 0.2 keV and a power law with photon index of 2.8. The 2-10 keV absorbed flux during the hard state was about 5.8 mCrab (ATel #12910). If the flux and spectral state of MAXI J1810-222 had remained constant since then, an extrapolation to the IBIS/ISGRI energy bands would lead to a ~10 times lower flux compared to current one measured by IBIS/ISGRI.

Additional INTEGRAL observations of the region around the source are already planned for the coming week.

Further observations of the source in soft X-rays, as well as in other energy domains, are encouraged.