A sudden decrease in soft X-ray flux of MAXI J1848-015

ATel #14292; H. Negoro (Nihon U.), T. Mihara (RIKEN), K, Seino, M. Nakajima, M. Aoki, K. Kobayashi, R. Takagi, K, Asakura (Nihon U.), T. Tamagawa, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, H. Nishida, K. Komachi, A. Yoshida (AGU), Y. Tsuboi, W. Iwakiri, R. Sasaki, H. Kawai, Y. Okamoto, S. Kitakoga (Chuo U.), M. Shidatsu (Ehime U.), N. Kawai, R. Adachi, 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, K. Kurogi, K. Miike (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 Dec 2020; 02:43 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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The X-ray transient MAXI J1848-015 was discovered with MAXI/GSC on December 20 (Takagi et al. ATel #14282), and the NuSTAR followup observation performed at 02:50-07:20 (UTC) on December 23 obtained a refined source position and an energy spectrum consistent with those of typical soft-state black hole binaries. (Pike et al. ATel #14290).

From December 21.0 to 23.25, MAXI/GSC 6-hr bin data show that the 2-4 keV and 4-10 keV fluxes are stable and the average fluxes are 0.075+/-0.006 photons/cm2/s (about 36 mCrab) and 0.053+/-0.005 photons/cm2/s (about 42 mCrab), respectively. From December 23.25 the fluxes linearly decreased in one day to below the 3-sigma detection limit of 15 mCrab. On the other hand, the 10-20 keV flux has not been significantly detected, but it shows a slight increase to 0.039+/-0.016 photons/cm2/s late on December 23 (23.75-24.0), which may be due to the hard state transition.

If the soft-to-hard state transition occurred just before the disappearance (~20 mCrab) and the bolometric flux at the transition is assumed to be about 2% of the Eddington luminosity (Maccarone 2003, A&A, 409, 697), the distance to the source can be estimated to be about 10 kpc for a 10 solar mass black hole. However, compared with other typical black hole X-ray novae, the period staying in a bright state and the decay constant of the flux (~1d) are too short, which imply that the transient is not a standard black-hole X-ray nova.

Finally we also note that if MAXI J1848-015 is identical to AX J1848.8-0129 (Sugizaki et al. 2001, ApJS, 134, 77) within the MAXI and NuSTAR error regions (ATel #14282, #14290), the source seems to be an unique black-hole transient showing a faint X-ray flux (a corresponding luminosity of an order of 1e34 erg/s if the above estimation for the distance is correct.) even in the quiescent state (see e.g., Menou et al. 1999, ApJ, 520, 276; Garcia et al. 2001, ApJ, 553, L47).

Followup observations especially in hard X-rays and radios are strongly encouraged to reveal the nature of this transient further.

MAXI data for MAXI J1848-015