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Refined position of the X-ray transient MAXI J1630-276 and its observed properties during the outburst

ATel #11173; H. Negoro, T. Kawase (Nihon U.), S. Ueno, H. Tomida, M. Ishikawa, Y. Sugawara, N. Isobe, R. Shimomukai (JAXA), T. Mihara, M. Sugizaki, S. Nakahira, W. Iwakiri, M. Shidatsu, F. Yatabe, Y. Takao, M. Matsuoka (RIKEN), N. Kawai, S. Sugita, T. Yoshii, Y. Tachibana, S. Harita, K. Morita (Tokyo Tech), A. Yoshida, T. Sakamoto, M. Serino, Y. Kawakubo, Y. Kitaoka, T. Hashimoto (AGU), H. Tsunemi, T. Yoneyama (Osaka U.), M. Nakajima, A. Sakamaki (Nihon U.), Y. Ueda, T. Hori, A. Tanimoto, S. Oda (Kyoto U.), Y. Tsuboi, Y. Nakamura, R. Sasaki, H. Kawai (Chuo U.), M. Yamauchi, C. Hanyu, K, Hidaka (Miyazaki U.), T. Kawamuro (NAOJ), and K. Yamaoka (Nagoya U.)
on 16 Jan 2018; 05:03 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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

Referred to by ATel #: 11198

The X-ray transient MAXI J1630-276 was discovered on 2017 November 17 (MJD 57074) at relatively high galactic latitude (b ~ 14.1 deg) with MAXI/GSC (Negoro et al. ATel #10984). The source was close to the sun, but it will be soon observable with Swift/XRT.
Using MAXI/GSC 4-10 keV data of 32 scan transits from 11:42 UT on 2017 November 17 to 11:35 on November 19, we obtained the source position at
(R.A., Dec) = (247.563 deg, -27.547 deg) = (16 30 15, -27 32 49) (J2000)
with a statistical 90% C.L. elliptical error region with long and short radii of 0.16 deg and 0.14 deg, respectively. The roll angle of long axis from the north direction is 0.0 deg counterclockwise.
We also obtained the source position using 2-4 keV data at
(R.A., Dec) = (247.860 deg, -27.406 deg) = (16 31 26, -27 24 21) (J2000)
with a statistical 90% C.L. elliptical error region with long and short radii of 0.26 deg and 0.17 deg, respectively. The roll angle of long axis from the north direction is 11.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius) in both cases. These error regions are hardly overlapped, but consistent with the previously reported position (ATel #10984).

After an unobservable period from November 23 (MJD 58080) to December 6 (MJD 58093) to avoid solar X-rays, the 2-10 keV count rate obtained with the image fitting method (Morii et al. 2016, PASJ, 68S, a11) was 0.035(+0.009, -0.008) counts/s/cm2 (~16 mCrab) on December 7, and then decreased exponentially with a time constant of 16.9 +/- 3.3 days. From December 24 (MJD 58111), the source flux was below the detection limit of GSC.

A significant spectral change has not been recognized during the outburst. An averaged GSC spectrum obtained from November 16 to December 24 was described by an absorbed power-law with a photon index of 2.0 +/- 0.3 and a hydrogen column density nH of (2.8+/-1.5)E22 atoms/cm2 (reduced chi-square: 0.63), or with an absorbed blackbody with a temperature of kT = 1.5 +/- 0.2 keV and nH of < 8.2E21 atoms/cm2 at the 90% confidence level for this nH (reduced chi-square: 0.76). Note that the total Galactic nH to the direction is 1.5E21 atoms/cm2 (https://heasarc.gsfc.nasa.gov/cgi-bin/Tools/w3nh/w3nh.pl).

The nature of the source is still quite unknown, but a low mass X-ray binary hosting a neutron-star or a black-hole is most likely. Swift/XRT followup observations are planned.

MAXI data for MAXI J1630-276