GRB 121225A might be an X-ray burst from Swift J1741.5-6548

ATel #4911; H. Negoro (Nihon U.), M. Serino (RIKEN), Y. Ogawa (Miyazaki U.), S. Ueno, H. Tomida, S. Nakahira, M. Kimura, M. Ishikawa (JAXA), M. Sugizaki, K. Morihana, T. Yamamoto, J. Sugimoto, T. Takagi, T. Mihara, M. Matsuoka (RIKEN), N. Kawai, M. Morii, R. Usui, K. Ishikawa (Tokyo Tech), A. Yoshida, T. Sakamoto, Y. Nakano (AGU), H. Tsunemi, M. Sasaki (OsakaU.), M. Nakajima, M. Asada (Nihon U.), Y. Ueda, K. Hiroi, M. Shidatsu, R. Sato, T. Kawamuro (Kyoto U.), Y. Tsuboi, M. Higa (Chuo U.), M. Yamauchi, Y. Nishimura, T. Hanayama, K. Yoshidome (Miyazaki U.), K. Yamaoka (ISAS) report on behalf of the MAXI team
on 24 Mar 2013; 16:39 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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Swift J1741.5-6548 is a hard X-ray transient recently discovered by Swift (Krimm et al. ATel #4902, also see #4906). The source position is marginally consistent with the position of GRB 121225A discovered by MAXI (Ogawa et al. GCN #14100), if the source variability and an additional systematic uncertainty of 0.2 deg are taken into account. The four corners of the error box (90 % C.L.) without the systematic uncertainty are as follows:

(R.A., Dec) = (+264.94 deg, -66.62 deg) = (17 39 46, -66 37 22)(J2000)
(R.A., Dec) = (+264.21 deg, -66.59 deg) = (17 36 51, -66 35 40)(J2000)
(R.A., Dec) = (+264.57 deg, -65.42 deg) = (17 38 18, -65 25 4)(J2000)
(R.A., Dec) = (+265.27 deg, -65.45 deg) = (17 41 4, -65 26 59)(J2000)

Assuming that the position of GRB 121225A is that of Swift J1741.5-6548, a collimator-response corrected light curve shows an exponential decay profile with a constant of 15-30 sec. Furthermore, the GSC spectrum is roughly described by an absorbed blackbody model with kT = 1.73 +/- 0.25 keV and the absorption column density N_H fixed to the value of 1.51E21 cm-2 obtained with Swift/XRT (Krimm et al. ATel #4902) (reduced Chi-squared = 1.50 for 8 d.o.f.). Assuming that the flux and the temperature during the scan transit are constant for simplicity, the 4-10 keV flux can be calculated to be 6.6E-9 erg/s/cm2, which is corresponding to the radius of the spherical emission region of 8.6^+1.4_-1.6 km for a 10 kpc distant source.

These results are consistent with a picture that GRB 121225A was an X-ray burst from Swift J1741.5-6548, and that Swift J1741.5-6548 is indeed a low-mass X-ray binary hosting a neutron star (Krimm et al. #4902).

It is also possible that MAXI detected the source in the middle of the burst during the scan observation, and the burst near the peak was brighter and hotter than observed. In fact, the collimator-response corrected light curve suggests that the flux at the beginning of the observation is 2-3 times larger than the above, averaged one. In this case, the bolometric luminosity becomes 1.6-2.4 times the Eddington luminosity for a 10 kpc distant source with 1.4 solar mass, which implies that the distance to the source is closer than 10 kpc, e.g., 6-8 kpc. This is likely to be consistent with with the relatively high galactic latitude position (-17.8 degrees) and the low absorption column density.

A GSC long-term light curve of the region shows a tendency of a gradual increase in X-ray intensity starting around the burst occurrence time, and currently the 2-20 keV flux is approximately 10 mCrab. This result does not make the connection between the burst and Swift J1741.5-6548 tightly. It should be noted, however, that X-ray bursts are often observed when a source is in the hard state.

On the other hand, the above results are against the tidal disruption hypothesis (also see Krimm et al.), and an absorbed power-law model gives worse fits to the observed spectrum, and the obtained power-law index is 3.0 +/- 0.6 and N_H is (1.8 +/- 0.8)E23 (reduced Chi-sq. = 1.86 for 7 d.o.f.), or the power-law index is 1.46 +/- 0.21 and N_H is 1.51E21 cm-2 fixed (reduced Chi-sq. = 3.16 for 8 d.o.f.).