NuSTAR observation of the hard state of MAXI J1848-015

ATel #14327; T. Mihara (RIKEN), H. Negoro (Nihon U.), M. Shidatsu (Ehime U.), M. Sugizaki (NAOC), M. Serino (AGU), S. N. Pike, H. Miyasaka (Caltech/SRL), M. Nakajima, M. Aoki, R. Takagi, K. Kobayashi, K, Asakura, K, Seino (Nihon U.), T. Tamagawa, M. Matsuoka (RIKEN), T. Sakamoto, S. Sugita, H. Nishida, K. Komachi, A. Yoshida (AGU), Y. Tsuboi, W. Iwakiri, R. Sasaki, H. Kawai, Y. Okamoto, S. Kitakoga (Chuo 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) report on behalf of the MAXI team
on 18 Jan 2021; 12:36 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Tatehiro Mihara (mihara@crab.riken.jp)

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We observed the MAXI transient MAXI J1848-015 (ATel #14282) with NuSTAR again in 2020 Dec. 31 8:46 - 2021 Jan. 1 4:16 UTC. Although the source had hardly seen in the MAXI image (ATel #14292) after the first NuSTAR observation on Dec. 23 (ATel #14290), we detected the source with the 2-10 keV flux of (9.1 ± 0.1) x 10^-11 erg/cm²/s (= 4 mCrab). The 3-20 keV spectrum was roughly characterized by an absorbed power-law with a photon index of ~2.0. A cutoff is required above 20 keV, which cannot be represented by a reflection model alone. Using the cutoffpl model on XSPEC we obtained a cutoff energy of 41 keV Still the residuals remain at an iron line emission and a hump around 20 keV, both of which can be represented by a reflection component. The pexmon model, a cutoff power-law continuum and its reflection model, better reproduce the entire NuSTAR spectrum over 3-60keV. To explain the broadening of the iron line, we considered the relativistic blurring by adding the kdblur model. Thus we finally obtained N_H = (8.0 ± 0.7) x 10²² cm^-2, powerlaw index = 1.82 ± 0.05, E_fold = 32 ± 3 keV, solid angle of reflector normalized by 2 π = 0.87 ± 0.10, and inclination = 26.8 ± 2.4 degrees (90% confidence errors). Although the cutoff energy is low compared with a hard-state spectrum of the black hole binary (BHB) and the iron line is relatively strong in BHB, the spectrum could be explained by a BHB or a neutron star LMXB in the hard state. The "soft-to-hard state transition" hypothesis to explain the sudden flux decrease on Dec. 23 (ATel #14292) was confirmed.
The source location is offset from AX J1848.8-0129 in the same way as in the first NuSTAR observation (ATel #14290). However, due to a larger positional systematic error than the usual we cannot identify or separate the two sources yet. Since the source is close to the sun, the star tracker in the telescope direction cannot be used, A further localization is needed in X-ray, radio or others.