INTEGRAL detection of renewed activity from BHC IGR J17091-3624: Developing towards a state transition?

ATel #17034; Rodriguez (CEA, F), C. Ferrigno (ISDC, CH), T. Bouchet (CEA/AIM, F), F. Cangemi (APC, F), V. Grinberg (ESA, NL), P. Laurent (CEA/AIM, F), K. Pottschmidt (CRESST/NASA/UMBC, USA), P. Thalhammer (Remeis Observatory, D), J. Wilms (Remeis Observatory, D)
on 12 Feb 2025; 09:58 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jerome Rodriguez (jrodriguez@cea.fr)

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The transient IGR J17091-3624 was detected back in 2003 with INTEGRAL (ATel #149). Its behavior was subsequently followed at various wavelengths, showing the presence of radio jets (e.g., ATels #152, #3167, Rodriguez et al. 2011), X-ray QPOs (e.g., ATel #3168), and state transitions characteristic for microquasars (e.g., ATels #3144, #3179). It attracted much interest when it showed peculiar X-ray flaring, reminiscent of the so-called heartbeat state seen in GRS 1915+105 (e.g., ATels #3230, #3299, Altamirano et al. 2011).

The last reported outburst of the source started in 2022 March (ATels #15282, #15283) and was followed by all type of black-hole transitions (ATels #15287, #15295, #15297, #15305). The last INTEGRAL/IBIS detection of this outburst occurred during satellite revolution 2558 (MJD 59861.0-59862.0 or 2022-10-09/10) with a 30-50 keV of about 2.1 cts/s (~23 mCrab). It was not detected in any INTEGRAL observations of the Galactic Centre region since then with a 30-50 keV 3-sigma upper limit of about 0.9 cts/s (~10 mCrab) during an observation covering MJD 59882.2 to 59882.4 (2022-10-30/31) and even down to a 30-50 keV 3-sigma upper limit of 0.3 cts/s (~3.3 mCrab) on MJD 59981.2-59982.8 (2023-02-06/07). The source remained undetected until the last observations of the field made from MJD 60603.2 to 60604.9 (2024-10-20/21), with a 30-50 keV 3-sigma upper limit of about 5.7 mCrab, suggesting the source was in quiescence since about MJD 59882.

INTEGRAL observations of the Galactic Centre restarted recently during satellite revolution 2878 (2025 February 7, 15h04 UTC, to 2025 February 8, 11h21 UTC, or MJD 60713.6 to 60714.5). IGR J17091-3624 was clearly detected (in Near-Real-Time data) with IBIS (it was outside of the JEM-X field of view) in mosaic images made in the 30-50, 50-100, and 100-150 keV energy ranges with respective fluxes of 5.8 cts/s (~63 mCrab; 30 sigma), 6.0 cts/s (~96 mCrab; 27 sigma), and 0.85cts/s (~97 mCrab; 7 sigma). Detection of the source was confirmed in revolution 2879 (2025 February 10, 6h49 UTC, to 2025 February 11, 03h17 UTC, or MJD 60716.3 to 60717.1), with fluxes of 4.5 cts/s (~49 mCrab; 30 sigma), 4.25 cts/s (~68 mCrab; 25 sigma), and 0.5 cts/s (~57 mCrab; 5 sigma) in the same energy bands.

The 35-500 keV IBIS spectrum of the revolution 2878 observation is well described with a powerlaw with Gamma=1.9+-0.2 (red. chi square = 1.1 with 13 dof), with a 30-200 flux of 1.6e-9erg/s/cm2. In revolution 2879 the spectrum is also powerlaw-like with Gamma=2.2+-0.2 (red. chi square = 1.1 with 13 dof) and a 30-200 keV flux 1.2e-9 erg/cm2/s.

This detection of IGR J17091-3624 after a roughly two year period of inactivity indicates that the source has entered a new outburst. The spectral analysis suggests it is already in a quite bright phase. It might be near the end of the bright hard state phase, moving toward a transition to a softer state, and this is usually associated with occurrences of bright radio flares / relativistic discrete ejections.

The next INTEGRAL observations of the field will occur on 2025 Feb. 14, 15, 17 and 20 (see https://www.cosmos.esa.int/web/integral/schedule-information for the precise observation times).

Multi-wavelength observations of this ongoing outburst are encouraged.

Refs: Altamirano et al. 2011, ApJ, 742, L17 ; Rodriguez et al. 2011, A&A, 533, L4