MAXI/GSC detection of a new X-ray outburst from 4U 1705-32

ATel #14663; H. Negoro (Nihon U.), T. Mihara (RIKEN), M. Nakajima, K. Kobayashi, K. Asakura, K. Seino (Nihon U.), T. Tamagawa, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, K. Komachi, A. Yoshida (AGU), Y. Tsuboi, W. Iwakiri, H. Kawai, Y. Okamoto, S. Kitakoga (Chuo U.), M. Shidatsu (Ehime U.), N. Kawai, M. Niwano, R. Hosokawa (Tokyo Tech), S. Nakahira, Y. Sugawara, S. Ueno, H. Tomida, M. Ishikawa, M. Tominaga, T. Nagatsuka (JAXA), Y. Ueda, S. Yamada, S. Ogawa, K. Setoguchi, T. Yoshitake, Y. Goto, R. Uematsu (Kyoto U.), H. Tsunemi (Osaka U.), M. Yamauchi, Y. Nonaka, T. Sato, R. Hatsuda, R. Fukuoka (Miyazaki U.), T. Kawamuro (UDP/NAOJ), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), M. Sugizaki (NAOC) report on behalf of the MAXI team:
on 26 May 2021; 23:25 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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The MAXI/GSC nova alert system triggered on faint X-ray enhancement at 14:29 on March 25, 2021. Assuming that the source flux was constant over 8 transits from 09:02 on March 24 to 00:32 on March 25, we obtain the source position at
(R.A., Dec) = (257.241 deg, -32.495 deg) = (17 08 57, -32 29 41) (J2000)
with a statistical 90% C.L. elliptical error region with long and short radii of 0.47 deg and 0.37 deg, respectively. The roll angle of the long axis from the north direction is 109.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). The X-ray flux averaged over the scan was 25 +- 6 mCrab (4.0-10.0keV, 1 sigma error).

The low-mass X-ray binary 4U 1705-32 (aka 1RXS J170854.4–321857) lies within the error region. In case of 4U 1705-32, this is the first detection of the source with the GSC in the 11 years. We also find that the long-term (> 10 days) 2-10 keV X-ray flux from the region has increased by ~2 mCrab since around MJD 59200 (December 2020). The source flux was, however, still below a one-day detection limit of 5 mCrab. (Currently, we could not obtain a precise absolute source flux because of count leaks from nearby sources.) On March 24, 2021, the X-ray flux rapidly increased to about 20 mCrab, and on March 26 the 4-10 keV X-ray flux decreased, suggesting that the source enters a soft state.

Reported X-ray fluxes of 4U 1705-32 are below 5 mCrab for a long time after 1971-1973 when UHURU and OSO-7 observed the source at 5-25 mCrab and 6 +/- 2 mCrab, respectively, (in ’t Zand et al. 2005, A&A, 440, 287). Assuming the source distance is 13 kpc, the observed X-ray flux of 20 mCrab is corresponding to the luminosity of about 1e37 erg/s.

Tentatively, we also name the source MAXI J1708-32 until the identification as 4U 1705-32. Followup observations are encouraged.

MAXI data for 4U 1705-32