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Rebrightening and X-ray spectral hardening of MAXI J1810-222

ATel #12910; H. Negoro (Nihon U.), J. Kennea (Penn State), W. Iwakiri (Chuo U.), M. Nakajima, W. Maruyama, M. Aoki, K. Kobayashi (Nihon U.), S. Nakahira, T. Mihara, T. Tamagawa, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, H. Nishida, A. Yoshida (AGU), Y. Tsuboi, R. Sasaki, H. Kawai, T. Sato (Chuo U.), M. Shidatsu (Ehime U.), N. Kawai, M. Sugizaki, M. Oeda, K. Shiraishi (Tokyo Tech), S. Ueno, H. Tomida, M. Ishikawa, Y. Sugawara, N. Isobe, R. Shimomukai, M. Tominaga (JAXA), Y. Ueda, A. Tanimoto, S. Yamada, S. Ogawa, K. Setoguchi, T. Yoshitake (Kyoto U.), H. Tsunemi, T. Yoneyama, K. Asakura, S. Ide (Osaka U.), M. Yamauchi, S. Iwahori, Y. Kurihara, K. Kurogi, K. Miike (Miyazaki U.), T. Kawamuro (NAOJ), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU) report on behalf of the MAXI team:
on 8 Jul 2019; 09:13 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

Subjects: X-ray, Black Hole, Neutron Star, Transient

Referred to by ATel #: 13540

The soft X-ray transient MAXI J1810-222 was discovered on December 1st, 2018 (ATel #12254, #12264). NuSTAR observations revealed a thermal-dominated soft X-ray spectrum represented by a model consisting of a blackbody or disk-blackbody with kT 0.4-0.5 keV and a steep power-law with a photon index of 3.6-3.8 (ATel #12283, #12398). A precise location and a similar soft X-ray spectrum were obtained with Swift XRT on February 9th (ATel #12487, also see #12496). No radio emission was observed on February 16th (#12521).

The MAXI/GSC nova-alert system triggered on the source on July 3 (MJD 58667), and we found that since the end of April (MJD 58590-58600) the 2-4 keV X-ray flux had increased again and peaked at 0.044 (+0.008,-0.007) c/cm^2/s (~41 mCrab) on July 1. Interestingly, the 4-10 keV flux also increased since the end of April and remained at about 5 mCrab, suggesting the source underwent a state transition.

To obtain a recent X-ray spectrum we requested a Swift XRT ToO observation. The XRT observation was carried out from 18:08 on July 5th, and we obtained a 1ks X-ray spectrum through the UK Swift Science Data Center at the University of Leicester (Evans et al. 2009, MNRAS, 397, 1177). The spectrum is no longer represented by a thermal-dominated model, but instead roughly represented by an absorbed power-law model with N_H = (7.3 +/- 0.2)x10^21 and a photon index of 3.47 +/- 0.05. The spectrum is well described by an absorbed blackbody plus power-law model with N_H = (6.6 +/- 0.4)x10^21, kT = 0.21 +/- 0.02 keV and a photon index of 2.8 +/- 0.1, or by an absorbed disk blackbody plus power-law model with N_H = (7.1 +/- 0.4)x10^21, kT_in = 0.26 +/- 0.02 keV and a photon index of 2.8 +/- 0.1. The absorbed fluxes in the later model are 2.8x10^-10 ergs/cm^2/s at 0.5-10 keV, and 1.4x10^-10 ergs/cm^2/s at 2-10 keV.

The nature of the source is still unclear. The normalization values of the blackbody and the disk blackbody models require large emission regions with a radius of about 100 km, which suggests that the source is a black hole. The steep power-law also implies that the source is in a steep power-law state. The (disk) blackbody temperature much lower than those of typical black hole binaries and the large flux changes in the soft X-ray flux, however, prevent us from reaching a definite conclusion.

Multi-wavelength followup observations are strongly encouraged to reveal the nature of this interesting source.
We would like to thank the Neil Gehrels Swift Observatory team for scheduling the ToO observation.

MAXI data for MAXI J1810-222