S. Nakahira (JAXA), N. Kawai (Tokyo Tech), T. Sakamoto (AGU), K. Yamaoka (Nagoya U),$B!!(B
H. Negoro (Nihon U.), M. Serino (RIKEN), P. A. Evans (U. Leicester), K. L. Page (U Leicester)
on behalf of the MAXI team and the Swift-MAXI collaboration
The Swift ToO observation of GRB141029A (Nakahira et al., GCN16981)
was performed starting from 2014-10-29T09:00:37
with 4-point tiling to cover the MAXI error circle.
We found three X-ray sources in the MAXI error circle
as shown in the Swift observation summary page
One of the sources ("source 1") showed suggestive fading between the
two tilings centered at 15500 s and 20700 s after the MAXI detection
with count rates of 2.7 +- 0.3 c/s and 1.7 +- 0.2 c/s respectively.
Assuming a power-law spectrum with photon index of 2.4 the X-ray flux
at the first Swift pointing is estimated as 6.8 x 10^-11 erg/cm2/s,
two order of magnitude fainter than 5.6 x 10^-9 erg/cm2/s at the MAXI detection.
The fading behavior was confirmed by additional Swift TOO observation
conducted from 2014-10-30T10:14:59 with a detected count rate of 0.27+-0.02 c/s.
This fading source matches a catalogued X-ray source 1RXS J043657.1-161258,
whose position in the ROSAT All-Sky Survey Bright Source Catalog is
9.1" away from the Swift position with a 3-sigma positional error of 40.1".
The source flux in the catalog corresponds to 9.3 x 10^-12 erg/s/cm2,
indicating that the fluxes in the present Swift observations were
in much elevated states.
The source was also reported to be variable (Fuhrmeister and Schmitt 2003).
Thus, we conclude that 1RXS J043657.1-161258 is the most likely source of the MAXI transient.
This optical counterpart of 1RXS J043657.1-161258 has been studied,
and found to be variable with with possible rotational period of P=0.3787d or P =2.74d
(Kiraga et al. 2012).
These characteristics of the source suggest that the X-ray transient
observed by MAXI was a strong flare from an active star system
such as RS CVn star, dMe star, or a cataclysmic variable.
Non-detection of an optical counterpart of source 3 reported by Xu et al. ( GCN16994)
further supports that this transient was not a gamma-ray burst.
We thank the Swift team for performing the observation.