INTEGRAL detection of X-ray transient source AX J1620.1-5002

We report the detection of the X-ray transient source AX J1620.1-5002, with the IBIS/ISGRI and JEM-X detectors aboard the INTEGRAL observatory, during the Galactic Plane Scan program (PI: Dr A. Bazzano). The source was observed during revolutions 1913 and 1914; observations were performed between 2018-01-29 15:06:58 and 2018-01-30 10:24:00 UTC (exposure time 326 ks).

In a combined analysis of the two revolutions, we detect the source at RA = 245.03, Dec = -50.01, 3' error radius, and with an overall detection significance in the 22-60 keV band of 9 sigma. The flux was measured in the 22-60 keV as 1.2 +/- 0.2^-10 erg cm^-2 s^-1 (11.31 +/- 1.3 mCrab), with an effective exposure time of 23 ks.

In revolution 1913 we measure the flux in the 22-60 keV energy band as 1.3 +/- 0.3^-10 erg cm^-2 s^-1 (12.31 +/- 2.9 mCrab), with an effective exposure time of 4 ks. In revolution 1914 we measure the flux in the 22-60 keV energy band as 1.0 +/- 0.2^-10 erg cm^-2 s^-1 (9.23 +/- 1.3 mCrab), with an effective exposure time of 18 ks.

AX J1620.1-5002 was only in the JEM-X FOV during revolution 1914 and well detected at RA= 245.07, Dec=-50.02, 90% 2' error radius. Flux values are as following: 3-10 keV: 7 +/- 1 mCrab (1.1 +/- 0.2 ^-10 erg cm^-2 s^-1) at 7 sigma for 10 ks exposure and10-25 keV: 13 +/- 5 mCrab (1.3 +/- 0.5^-10 erg cm^-2 s^-1) at 3 sigma for 9.6 ks exposure.

We believe this to be the first confirmed detection of AX J1620.1-5002 at hard X-ray energies. We note that in October 2017, MAXI observed a new source, MAXI J1621-501 (ATel #10869). Its measured position is offset by ~12' from AX J1620.1-5002 and the MAXI error circle is ~13'; i.e., consistent with the position of both AX J1620.1-5002 and our detection.

The nature of AX J1620.1-5002 is still not known but is a suggested candidate magnetically accreting white dwarf (Degenaar et al. 2012 A&A, 540, 22), but an HMXB nature cannot be completely ruled out (Gemma et al. 2014, ApJSS, 212, 13).