Likely spectral transition of Aql X-1 seen by SVOM/ECLAIRs.

ATel #16843; Author list: Sebastien Le Stum, Floriane Cangemi, Alexis Coleiro, Antoine Foisseau (APC, France), Jerome Rodriguez, Nicolas Dagoneau, Stephane Schanne, Frederic Chateau, Herve Le Provost (CEA, F), Wenjin Xie (NAOC, China), Jean-Luc Atteia, Laurent Bouchet, Sebastien Guillot, Juliette Alaux (IRAP, F), Tais Maiolino (LUPM, F), Karine Mercier, Marie-Claire Charmeau, Stefano Crepaldi (CNES, F) Jian-Yan Wei (NAOC), Bertrand Cordier (CEA), Shuang-Nan Zhang (IHEP, CN), Stephane Basa (LAM), Olivier Godet (IRAP), Arnaud Claret (CEA), Zi-Gao Dai (USTC), Frederic Daigne (IAP), Jin-Song Deng (NAOC), Andrea Goldwurm (APC), Diego Gotz (CEA), Xu-Hui Han (NAOC), Cyril Lachaud (APC), En-Wei Liang (GXU), Yu-Lei Qiu (NAOC), Susanna Vergani (Obs. Paris), Jing Wang (NAOC), Chao Wu (NAOC), Li-Ping Xin (NAOC), Bing Zhang (UNLV)
on 4 Oct 2024; 16:13 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Floriane Cangemi (cangemi@apc.in2p3.fr)

Tweet

on board the SVOM mission. The source was detected by the "Quicklook Analysis" of X-band data in the 4-20 keV energy range during 3 observations respectively made on 2024-09-28T20:10:19 UTC, 2024-09-30T00:46:15 UTC, and 2024-10-02T03:24:16 UTC, and with exposures of 497, 899, and 408 seconds. The source is not detected above 20 keV. The 4-20 keV spectrum on 2024-09-28 can be well fitted (chi-square = 2.06/4 dof) by a model consisting of blackbody (bbodyrad) emission and a power-law. The derived unabsorbed flux in the 4-20 keV band is in the range [2.8e-9-6.7e-9] erg/s/cm2 (90% confidence interval). For the two other observations (2024-09-30 and 2024-10-02), the spectra are more accurately described by a black body alone without requiring the addition of a power-law (chi-square = 3.50/5 dof and chi-square = 12.11/5 dof respectively). The unabsorbed flux ranges (90% confidence interval) are then [8.1e-9-9.6e-9] and [8.1e-9-1.0e-8] erg/s/cm2, respectively. We find black body temperatures of 2.0 +/- 0.3 keV, 1.8 +/- 0.1 keV, and 1.6 +/- 0.1 keV respectively for the three observations. As the mission is still in commissioning phase, systematics are still under evaluation and we advise caution regarding these spectral parameters.

This thermally dominated spectrum is at odds with the spectrum previously reported by NuSTAR on 2024-09-23 (ATel #16829, thermal Comptonised/cut-off power law spectrum with a photon index of 1.86 +/- 0.02). INTEGRAL also performed a public ToO observation on 2024-09-26 (about 44 ks effective exposure for IBIS). A preliminary analysis of 3-200 keV (near real-time) JEM X-1 and IBIS spectra also shows that the spectrum was compatible with a pure cut-off power law at that time, with a photon index of 1.93 +/- 0.04 and a cut-off energy of 46 +/- 3 keV.

The spectral change between the NuSTAR/INTEGRAL and SVOM observations suggests that a transition to a soft state took place. This is further supported by a significant decrease of the 15-50 keV flux in the BAT transient monitor (https://swift.gsfc.nasa.gov/results/transients/AqlX-1/), while the soft X-ray flux monitored with MAXI remains at a similarly high level (http://maxi.riken.jp/star_data/J1911+005/J1911+005.html).

Renewed activity of this source was additionally reported in ATels #16821, #16822, #16823, #16826, #16840, and #16841. Further multi-wavelength follow-up observations are encouraged to investigate the evolution of the source.

The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. ECLAIRs was developed jointly by APC, CEA, CNES and IRAP.