ATEL #4911
Title: GRB 121225A might be an X-ray burst from Swift J1741.5-6548
Author: 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
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.).
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