Dirk Grupe (Swift MOC, PSU) and Phil Evans (U Leicester) report
on behalf of the Swift team:
We report on further Swift XRT observations of the field of the
Fermi GBM and LAT detected GRB 140104B (Xiong, GCN Circ. 15687,
G. Vianello GCN Circ. 15684, respectively).
We detected an uncatalogued X-ray source in the first Swift
observation of the field of GRB 140104B starting at 15.5 hours
after the Fermi trigger (Grupe, GCN circ 15691) at
Ra-2000 = 14 35 01.0
Dec-2000 = -09 01 40.7
with an uncertainty of 4.0". The XRT count rate of this source
was (8.1+/-1.6)e-3 counts s^-1. Note that we misidentified this
source in the previous circular (15691) with the star TYC
5564-19-1, which however, is 2.2' away from this X-ray position.
We re-observed this source on 2014-01-14 for 4.6 ks again with
Swift (9.5 days after the trigger). We do not detect this source
in the XRT any more, suggesting that this may be the X-ray
afterglow of GRB 140104B. The 3sigma upper limit at the position
of this source is 1.3e-3 counts s^-1. Note, however, that this is
not conclusive. This source can still be a highly variable
background AGN (see e.g. Grupe et al. 2010, ApJS, 187, 64).
If this source is the X-ray afterglow of GRB 140104B, the X-ray
spectrum appears to be rather soft. A spectral analysis of the
X-ray data shows that the X-ray photon index is Gamma =
2.7+/-0.68 which is rather steep (see Grupe et al. 2013, ApJS,
209, 20; Evans et al., 2009, MNRAS, 397, 1177).
The absorption column density is consistent with the
Galactic value (5.63e20 cm^-2). This steep X-ray spectral index
and the steep energy spectral index and the high fluence in the
10-1000 keV band of the burst reported by Xiong (GCN circular
15687) suggests a low redshift nature of the burst (see Grupe et
al. 2013, ApJ, 209, 20).
However, the steep X-ray photon index of Gamma = 2.7
does not exclude a possible AGN nature of the source. In a
matter of fact, if this is a background AGN then the steep X-ray
spectrum suggest that this is most-likely a Narrow Line
Seyfert 1 galaxy. This is also the type of AGN with the
strongest X-ray variability (e.g. Boller et al. 1996,
Leighly 1999, Grupe et al. 2010).
This report is an official product of the Swift team.