MAXI/GSC observation of the bright X-ray outburst from the Galactic center region
ATel #16983; M. Nakajima, H. Negoro, Y. Kudo (Nihon U.), T. Mihara (RIKEN), H. Shibui, K. Takagi, H. Takahashi, K. Tatano, H. Nishio (Nihon U.), T. Kawamuro, S. Yamada, S. Wang, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, Y. Kawakubo, H. Hiramatsu, H. Nishikawa, Y. Kondo, S. Sasao, A. Yoshida (AGU), Y. Tsuboi, H. Sugai, N. Nagashima (Chuo U.), M. Shidatsu, Y. Niida (Ehime U.), I. Takahashi, M. Niwano, N. Higuchi, Y. Yatsu (Tokyo Tech), S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, S. Ogawa, M. Kurihara (JAXA), Y. Ueda, Y. Okada, K. Fujiwara (Kyoto U.), M. Yamauchi, Y. Otsuki, T. Hasegawa, M. Nishio (Miyazaki U.), K. Yamaoka (Nagoya U.), M. Sugizaki (Kanazawa U.), W. Iwakiri (Chiba U.)
on 17 Jan 2025; 03:30 UT
Credential Certification: Motoki Nakajima (nakajima.motoki@nihon-u.ac.jp)
Subjects: X-ray, AGN, Binary, Black Hole, Transient
We report further observations of the bright X-ray outburst from the vicinity of the Galactic center (ATel #16975). In our previous report, we tentatively identified the source as either KS 1741-293 or a new hard X-ray transient, MAXI J1744-294. However, with additional data, another possibility of the source identification has emerged when we assume a nearby source SAX J1747.0-2853 is in quiescence and exclude it from the position fitting.
Assuming that the source flux was constant over 26 scan transits from 12:59 UT on January 13 to 11:27 on January 15, we obtain the source position at
(R.A., Dec) = (266.229 deg, -29.207 deg) = (17 44 54, -29 12 25) (J2000)
with a statistical 90% C.L. elliptical error region with long and short radii of 0.17 deg and 0.12 deg, respectively. The roll angle of the long axis from the north direction is 18.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). The X-ray flux averaged over the scan was 252 ± 14 mCrab (4.0-10.0keV, 1 sigma error). This error region includes the position of Sgr A*, raising the possibility that the outburst may be associated with it. We note that the region also include a hard transient in 2016 CXOGC J174540.0-290031 (or a new transient SWIFT J174540.2-290037), which locates at only 10" south from the Sgr A* (ATel #9109).
Furthermore, the independent 15â50 keV monitoring Sgr A* (including CXOGC J174540.0-290031) data of Swift/BAT also suggests that Sgr A* or a compact source to the direction of the galactic center is in an X-ray outburst (https://swift.gsfc.nasa.gov/results/transients/weak/SgrAstar/), and other sources such as KS 1741-293, 2E 1742.9-2929 and 1E 1740.7-2942 are quiet. The duration (20 days) and flux (0.2 Crab, 15-50 keV) of the outburst in the BAT light curve are similar to those of the 2016 outburst from CXOGC J174540.0-290031.
The X-ray spectrum of the outburst, observed between MJD 60682 [2025 Jan. 7] - 60691 [Jan. 16], was extracted from the Galactic center region using the MAXI on-demand process (http://maxi.riken.jp/mxondem/). Since MAXI/GSC does not have sufficient spatial resolution to distinguish individual sources around the Galactic center, data from the pre-outburst period (MJD 60610 [2024 Oct.27] - 60620 [2024 Nov.6] ) was used as a background model. In this way, the galactic ridge emission and nearby persistent sources should also be subtracted. The 2â20 keV spectrum is well fitted by an absorbed power-law model with an additional 6.6 ± 0.2 keV iron line. The best-fit spectral parameters are:
Photon index: Γ = 2.5 ± 0.4
Hydrogen column density: NH = (2.4 ± 0.9) × 1023 cm-2
The quoted errors are 90% confidence levels. The unabsorbed flux in the 2â20 keV band is 4.7 × 10-9 photons/cm2/s, corresponding to a luminosity of LX = 3.6 × 1037 erg/s (assuming a distance of 8 kpc). These spectral parameters suggest that the source's nature is consistent with the steep power-law state of a black hole X-ray binary. We also note that the very bright iron-line is similar to that seen in the microquasar V4641 Sgr.
The 4-10 keV flux remained constant level of 0.25 photons/cm2/s, while the 15-50 keV flux decreased from 0.06 counts/cm2/s on January 14, 2024 (MJD 60689), to 0.04 counts/cm2/s on January 16, 2024 (MJD 60691). This suggests that the outburst source is undergoing a state transition.
Further observations are strongly encouraged to reveal the source nature.