4U 1630-47 still in outburst: Potential hard X-ray flaring recently detected by INTEGRAL and Swift/BAT

ATel #15924; T. Bouchet, J. Rodriguez (CEA-Saclay/DAP-AIM), J. Wilms (Remeis-Observatory, FAU Erlangen-Nuernberg), K. Pottschmidt (CRESST/UMBC/GSFC), F. Cangemi (U. Paris Cite/APC), V. Grinberg (ESA/ESTEC), P. Laurent (CEA-Saclay/DAP-AIM), P. Thalhammer (Remeis-Observatory, FAU Erlangen-Nuernberg)
on 2 Mar 2023; 10:46 UT
Credential Certification: Jerome Rodriguez (jrodriguez@cea.fr)

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The Black Hole Candidate (BHC) 4U1630-47 started a new outburst on 2022 July 28. It was first detected with MAXI and then during a Swift/XRT follow-up observation on August 25 (ATel#15575).

This peculiar BHC typically shows periodic outbursts approximately every 600 days with a duration of about 4 months. In comparison the current outburst has been going on for 7 months. Long outbursts with a duration of about one year or more have previously only been recorded in 2002 and 2012. If the current outburst continues to develop into another of these long outbursts, it could indicate that they reoccur on a ~10 year timescale

The source was in the field of view (FoV) of INTEGRAL during 9 satellite revolutions since September 2022: The first INTEGRAL JEM-X detection happened on 2022 September 06 (revolution 2545) with a 3-10 keV flux of 270 mCrab and 30 mCrab in the 10-35 keV band, showing that the source was still active in the X-rays. It was not detected with IBIS with 3-sigma upper limits of 6 and 10 mCrab in the 30-50 keV and 50-100 keV bands, indicating that the source was very soft at that time. On 2022 October 04 (rev 2556) the source was detected with INTEGRAL/IBIS with a 30-50 keV flux of ~20 mCrab. It was also detected with JEM-X with fluxes of 290 and 48 mCrab in the 3-10 and 10-35 keV bands.

After these observations 4U1630-47 was not in the INTEGRAL FoV until the recent Galactic Center Survey (PI J. Wilms) where initially it was seen in a much brighter state by both JEM-X and IBIS: On 2023 January 20 (revolution 2597) 4U 1630-47 was detected by IBIS at fluxes of 58, 41, 79 mCrab in the 30-50, 50-100 and 100-200 keV bands (7.5 ks effective exposure) and by JEM-X at fluxes of 430 and 88 mCrab in the 3-10 and 10-35 keV bands. A good description of the spectrum is obtained for a power-law with a very soft photon index of Γ=2.4 +/- 0.1 (reduced chi2 = 1.2 for 10 degrees of freedom) with no apparent cutoff. The next revolutions (2599 and 2601) saw a similar photon index of Γ=2.2 +/- 0.3 (red chi2=0.97 for 10 degrees of freedom) on 2023 Jan 25 and 2023 Feb 01, with a combined exposure of 3 ks.

The BHC was not detected on 2023 February 06 (revolution 2603) with a 3-sigma upper limit of 4 and 7 mCrab in the 30-50 and 50-100 keV ranges of IBIS, despite a total exposure of 11 ks and an off- axis angle of only 5 degrees.

We note that Swift/BAT has seen a 15-50 keV flux consistently above 40 mCrab during the slight hardening between January 08 and February 02, and has been measuring a flux above 100 mCrab since 2023 February 18, showing a slight increase on a daily basis. All these results might indicate that 4U1630-47 is in a period of hard X-ray flaring activity, with fast transitions occurring. The recent Swift/BAT behavior may be the signature of another (bright) flare.

Hard X-ray flares have been associated in some sources with ejections of material that became visible in radio observations. Multi-wavelength follow-up observations are encouraged. 4U1630-47 will re-enter the FoV of INTEGRAL during revolution 2614 (2023 March 08).

The latest X-ray light curves can be found on the following pages:

Swift/BAT - https://swift.gsfc.nasa.gov/results/transients/weak/4U1630-472/

MAXI - http://maxi.riken.jp/star_data/J1634-473/J1634-473.html