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Observed X-ray properties of MAXI J1810-222 and the possible nature of the source

ATel #12264; W. Maruyama, H. Negoro (Nihon U.), T. Mihara, S. Nakahira (RIKEN), N. Kawai (Tokyo Tech), M. Serino (AGU), M. Nakajima, A. Sakamaki, M. Aoki, K. Kobayashi (Nihon U.), F. Yatabe, Y. Takao, M. Matsuoka (RIKEN), T. Sakamoto, S. Sugita, Y. Kawakubo, T. Hashimoto, A. Yoshida (AGU), M. Sugizaki, Y. Tachibana, K. Morita, T. Oeda, K. Shiraishi (Tokyo Tech), S. Ueno, H. Tomida, M. Ishikawa, Y. Sugawara, N. Isobe, R. Shimomukai, T. Midooka (JAXA), Y. Ueda, A. Tanimoto, T. Morita, S. Yamada, S. Ogawa (Kyoto U.), Y. Tsuboi, W. Iwakiri, R. Sasaki, H. Kawai, T. Sato (Chuo U.), H. Tsunemi, T. Yoneyama, K. Asakura, S. Ide (Osaka U.), M. Yamauchi, K. Hidaka, S. Iwahori (Miyazaki U.), T. Kawamuro (NAOJ), K. Yamaoka (Nagoya U.), M. Shidatsu (Ehime U.) report on behalf of the MAXI team:
on 4 Dec 2018; 13:44 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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

Referred to by ATel #: 12283, 12398, 12496, 12521, 12910, 13540

We further report analysis results using MAXI/GSC data of the newly discovered soft X-ray transient MAXI J1810-222 (Negoro et al. ATel #12254).

Longterm GSC light curves show that the 2-4 keV X-ray flux has gradually increased since the beginning of November (~ MJD 58425) except for the period from November 18 to 27 when MAXI could not observe the region, and that the 4-10 keV X-ray flux shows slight excess of 0.0034 +/- 0.0023 c/cm^2/s (~ 3 mCrab) only from MJD 58420 to 58430. On December 2 (MJD 58454), the 2-4 keV and 4-10 keV fluxes changed from 0.027 +/- 0.004 c/cm^2/s (on December 1) to 0.014 +/- 0.004 c/cm^2/s, and from 0.002 +/- 0.004 c/cm^2/s to 0.007 +/- 0.004 c/cm^2/s, respectively, which may indicate the source is undergoing some transition.

The time averaged energy spectrum obtained on November 29-31 is well represented by a blackbody model with a temperature of 0.36 (+0.10, -0.08) keV, or by an absorbed blackbody model with a temperate of 0.34 (+0.09, -0.08) keV and a fixed column density to the direction of 6.36e-21 atoms/cm^2 obtained at HEASARC (https://heasarc.gsfc.nasa.gov/cgi-bin/Tools/w3nh/w3nh.pl). The flux and unabsorbed flux of the above models are 2.2e-10 erg/cm^2/s in the 2-10 keV band in both case, and 1.29e-9 and 0.92e-9 erg/cm^2/s, respectively, in the 0.8-60 keV band. The size of the emission region, the blackbody radius, is hardly constrained, but the best-fit parameters show the radii of 80 (d/10 kpc) km and 100 (d/10kpc) km for the models, respectively. Optically thin thermal plasma models with kT ~ 0.7-0.8 keV also represent the observed spectrum. The 90% upper-limit of the 4-20 keV flux is 1.5e-11 erg/cm^2/s if we assume the power-law model with a photon index of 1.

We also obtained the refined position of the source using 15 scan data from 22:15 on November 30 to 20:34 on December 1:
(R.A., Dec) = (272.634 deg, -22.309 deg) = (18 10 32, -22 18 32) (J2000)
with a statistical 90% C.L. elliptical error region with long and short radii of 0.27 deg and 0.21 deg, respectively. The roll angle of the long axis from the north direction is 101.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius).

Similar soft X-ray spectra were obtained in Aql X-1 with a luminosity of less than 5e35 erg/s at 0.8-100 keV (Sakurai et al. 2014, PASJ, 66, 10, also see MAXI J1807+132, Shidatsu et al. 2017, ApJ, 850, 115). If this is the case, the source distance becomes less than 1.9 kpc. A (massive) black hole binary in the soft state at a distance of, say, ~10 kpc might also be possible. However, in both cases, a hard power-law or tail component must be present, which is not confirmed, yet. Some magnetars also exhibit such a soft thermal component with kT ~ 0.5 keV (e.g., Enoto et al. 2017, ApJS, 231, 8, and references therein). In such case, the source distance must be ~1 kpc from the blackbody radius, and a hard-tail is also expected. Finally, we note that these is a possibility of a new type of X-ray star because MAXI has not detected such a long-lived, soft X-ray transient without seeing a hard state.

Followup observations especially in the hard X-ray band to detect a hard tail are encouraged to reveal the nature of the source.