NuSTAR Observation of MAXI J1810-222
ATel #12398; . Oeda, N. Kawai, M. Sugizaki(Tokyo Tech), W. Maruyama, H. Negoro (Nihon U.), T. Mihara, S. Nakahira (RIKEN), 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), Y. Tachibana, K. Morita, 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.)
on 16 Jan 2019; 10:53 UT
Credential Certification: Nobuyuki Kawai (nkawai@phys.titech.ac.jp)
Subjects: X-ray, Binary, Black Hole, Neutron Star
We analyzed NuSTAR data of MAXI J1810-222 taken on December 10-11 for 6.45 hours.
We found no significant variation in its 3-70 keV light curve. No periodicity was found in the range from 0.2 to 65535.6 sec.
Its spectrum, constructed from the entire data set, is well fit with a blackbody plus power law model with a reduced chi-squared of 1.10 (74 d.o.f). The spectral parameters are: blackbody temperature 0.42 +/-0.01 keV, the emission radius (33 +3 -2)(d/10kpc)km, and the power-law photon index 3.8 +/- 0.2. No absorption was required. The 3--20 keV flux is 3.4e-11 erg/cm^2/s (3-20 keV).
Use of a multi-color disk blackbody model instead of a single temperature blackbody provides a equally good fit and a very similar
3-20 keV flux with a reduced chi-squared of 1.009 (74 d.o.f). we obtain the inner disk temperature (Tin) 0.49 +/- 0.01 keV, the inner
disk radius R_in = 26.23 (+2.82, -2.18) km at 10 kpc, and the photon index of 3.6 +/- 0.2.
We find that the spectrum is better represented by a Comptonized blackbody (COMPTT in XSPEC) plus power-law model.
The parameters are: blackbody temperature 0.33(+0.01, -0.02) keV, electron temperature 20 keV (fixed), optical depth 0.01, and a photon index 2.8 (+0.2, -0.4). The reduced chi-squared is 0.90 (72 d.o.f).
The nature of the source is still in mystery. The relatively low blackbody temperature and the presence of a steep power-law component,
together with the lack of pulsations or X-ray bursts, may indicate that the source is an accreting black hole binary. Adopting the
innermost stable orbit of a non-spinning 10 M_sun black hole as the inner disk radius, we estimate its distance to be more than 30 kpc. On the other hand, the the blackbody emission radius is also consistent with that the source is a nearby (3-5 kpc) neutron star binary (see ATel #12264). Further follow-up observations are encouraged.
Finally, we note that in the previous report (ATel #12264), the unabsorbed 0.8-60 keV fluxes should be 0.92e-9 for the blackbody fit
and 1.29e-9 erg/cm2/s for the absorbed blackbody fit, respectively, and the fixed column density used in the latter fit (in obtaining the
temperature 0.34 keV) is 6.36e21 atoms/cm^2. We apologize for having caused any confusion.