MAXI/GSC detection of a soft X-ray transient MAXI J0637-430

ATel #13256; H. Negoro (Nihon U.), K. Miike (Miyazaki U.), M. Nakajima, W. Maruyama, M. Aoki, K. Kobayashi (Nihon U.), T. Mihara, T. Tamagawa, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, H. Nishida, A. Yoshida (AGU), Y. Tsuboi, W. Iwakiri, R. Sasaki, H. Kawai, T. Sato (Chuo U.), M. Shidatsu (Ehime U.), N. Kawai, M. Oeda, K. Shiraishi (Tokyo Tech), S. Nakahira, Y. Sugawara, S. Ueno, H. Tomida, M. Ishikawa, 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, K. Hattori (Osaka U.), M. Yamauchi, S. Iwahori, Y. Kurihara, K. Kurogi (Miyazaki U.), T. Kawamuro (NAOJ), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), M. Sugizaki (NAOC) report on behalf of the MAXI team:
on 3 Nov 2019; 10:48 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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The MAXI/GSC nova alert system triggered on a bright uncatalogued soft X-ray transient source at 06:37 UT on 2018 November 02. Assuming that the source flux was constant over seven scan transits from 05:04 on November 2 to 07:21 on November 3, we obtain the following source positions using 2-4 keV and 4-10 keV data.

2-4 keV band:
(R.A., Dec) = (99.729 deg, -42.766 deg) = (06 38 54, -42 45 57) (J2000)
with a statistical 90% C.L. elliptical error region with long and short radii of 0.08 deg and 0.07 deg, respectively. The roll angle of the long axis from the north direction is 90.0 deg counterclockwise. The X-ray flux averaged over the scan was 59 +- 6 mCrab (2.0-4.0 keV, 1 sigma error).
4-10 keV band:
(R.A., Dec) = (99.433 deg, -43.054 deg) = (06 37 43, -43 03 15) (J2000)
with a 90% C.L. statistical error of 0.30 deg. The X-ray flux averaged over the scan was 32 +- 6 mCrab (4.0-10.0keV, 1 sigma error).

If we take account of additional systematic uncertainties of 0.1 deg (90% containment radius) in both results, the two positions 0.36 deg apart are marginally consistent. There is no known bright X-ray source in the error regions, we tentatively name the source MAXI J0637-430. Due to the softness of the source spectrum, the positional uncertainty in the 2-4 keV band is small. However, we use the 4-10 keV position for the name because 4-10 keV band data are better calibrated than 2-4 keV data for the positional determination. We also note that 1H 0635-431 is in the 4-10 keV error region. 1H 0635-431 is likely an AGN (Marshall et al., 1995, ApJ, 439, 90). Currently, however, the possibility that this new source is 1H 0635-431 can not be excluded because there is little information about 1H 0635-431.

The 2-4 keV flux has been increasing since the first detection, and reached about 200 mCrab on November 3. On the other hand, the 4-10 keV flux has been almost constant at about 50 mCrab.

The soft spectrum suggests that the source is a black hole or neutron star binary in a soft state. We highly encourage followup observations of the new X-ray nova.

Trigger Information about MAX J0637-430