INTEGRAL detection of hard X-ray activity from Terzan 1 (likely XB 1732-304) and XTE J1810-189

ATel #13983; E. Bozzo (ISDC, Switzerland), J. Wilms (ECAP, Germany), E. Kuulkers (ESA/ESTEC, Netherlands - on behalf of the Galactic bulge monitoring program team), L. Sidoli (INAF-IASF, Italy), C. Sanchez-Fernandez (ESA/ESAC, Spain), A. Paizis (INAF-IASF, Italy), J. Chenevez (DTU, Denmark), C. Ferrigno, V. Savchenko (ISDC, Switzerland), L. Ducci (ISDC, Switzerland; IAAT, Germany)
on 31 Aug 2020; 13:30 UT
Credential Certification: E. Bozzo (enrico.bozzo@unige.ch)

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During the INTEGRAL monitoring observations of the region around the Galactic Center performed from 2020 August 10 at 12:04 to August 30 at 2:52 (UTC), renewed activity has been detected from positions consistent with those of the two transient X-ray binaries XB 1732-304 and XTE J1810-189. These data covered the satellite revolutions 2261, 2265, 2266, 2267, and 2268.

In the IBIS/ISGRI mosaic obtained by combining all data together, XB 1732-304 (XTE J1810-189) is detected at a significance of 15 sigma (9 sigma) in the 28-40 keV energy range and the correspondingly estimated X-ray flux is of ~12 mCrab (~12 mCrab). XB 1732-304 is also detected in the mosaic obtained from the JEM-X data covering the same revolutions. The estimated 3-10 keV X-ray flux is of roughly 8 mCrab. XTE J1810-189 was inside the JEM-X field of view only for 3 pointings during revolution 2265 and it was not detected (total effective exposure time 2 ks). We estimate a rough upper limit of 15 mCrab in the 3-10 keV energy band.

Given the uncertainties affecting the near real time version of the JEM-X data, we only extracted the IBIS/ISGRI spectrum of the two sources. The spectrum obtained for XB 1732-304 could be well described by using a power-law of photon index 2.3+/-0.3, with an estimated 20-100 keV flux of 2.4E-10 erg/cm^2/s (total effective exposure time 115 ks). In the case of XTE J1810-189, the IBIS/ISGRI spectrum could be fit by using a power-law with a photon index of 2.1+/-0.5. The estimated flux in the 20-100 keV energy range is of 2.0E-10 erg/cm^2/s (total effective exposure time 80 ks).

We searched for type-I bursts in the JEM-X lightcurve of XB 1732-304 but no convincing evidence for bursts could be found.

By looking at the IBIS/ISGRI mosaics of the individual revolutions, we noticed that the flux of XB 1732-304 remained virtually constant within the period covered by the revolutions 2261-2268. This seems in agreement with what has been observed by Swift/BAT (https://swift.gsfc.nasa.gov/results/transients/weak/XB1732-304/). The Swift/BAT data also suggest that XB 1732-304 displayed a rather constant hard X-ray activity since April 2020 (see also ATel #13630). The flux recorded from XTE J1810-189 during the period covered by the INTEGRAL revolutions 2261-2268 shows evidence of a significant on-going increase (about 50% and raising).

We remark that XB 1732-304 is located within the globular cluster Terzan 1. The limited position accuracy of the INTEGRAL/JEM-X instrument (roughly 3 arcminutes) cannot distinguish between different possible transient sources within the globular cluster. To refine the X-ray localization of this transient down to the arcsecond accuracy level, a follow-up observation with Swift X-ray has been approved. This will be reported in a forthcoming astronomer Telegram.

Further INTEGRAL observations of the region including the two sources are already planned for the coming weeks.