Hard X-ray activity from the direction to Sgr A* revealed by INTEGRAL

ATel #9000; S. A. Grebenev, R. A. Krivonos, S. V. Molkov, R. A. Sunyaev (Space Research Institute, Moscow)
on 28 Apr 2016; 00:56 UT
Credential Certification: Sergei Grebenev (sergei@hea.iki.rssi.ru)

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Observing the Galactic center field with INTEGRAL in mid-March - April, 2016, we revealed a steady increase in the hard X-ray flux from a few arcmin vicinity of Sgr A^* (the nucleus of our Galaxy). According to the IBIS/ISGRI telescope the 20-60 keV flux from this region was raised during the last month from 3.7+/-1.0 mCrab (that is close to its average level) to 17.7+/-1.5 mCrab. Currently it became the dominating source of hard X-rays in the central (2.5 deg in radius) region of the Galaxy (see below the link to the light curve of the source and the mosaic images).

The INTEGRAL/JEM-X telescope working in the standard 4-20 keV X-ray band also detected some unusual brightening (by a factor of 2-3) in this direction. There are several X-ray transients known located within a few arcmin of Sgr A^*: the X-ray burster AX J1745.6-2901, X-ray novae A 1742-289 and CXOGC J174540.0-290031, magnetar SGR J1745-2900, VFXB CXOGC J174535.5-290124 and others, historical sources 1E 1742.5-2859 and 1E 1742.2-2857 (see Degenaar et al. 2015, for review). Neither IBIS nor JEM-X are able to resolve them. But our fast inspection of the soft X-ray image obtained by SWIFT/XRT on April 24 under the program of Galactic center monitoring allows most of them, including the well-known AX J1745.6-2901, to be excluded from the list of candidates for the source of this activity.

Sgr A^* is assumed to be a quasi persistent source on a time scale of months and days but exhibiting bright X-ray flares on a time scale of hours and minutes. Note in this connection that Ponti et al. (2015) reported recently a steady increase in the bright flaring rate of Sgr A^*.

We encourage follow-up observations of this interesting source.

Hard X-ray activity from the direction to Sgr A*