Swift follow-up observation of XTE J1859+083
ATel #7067; K. L. Li and A. K.H. Kong (National Tsing Hua University, Taiwan)
on 14 Feb 2015; 12:26 UT
Credential Certification: Albert Kong (akong@phys.nthu.edu.tw)
Subjects: Optical, X-ray, Neutron Star, Transient, Pulsar
The recent outburst of XTE J1859+083 detected by MAXI (ATel #7034) was followed-up by a 500s Swift XRT observation on 13 February 2015. A bright X-ray source is clearly detected within the XTE J1859+083 error circle derived by BeppoSAX WFC (Corbet et al. 2009). The mean corrected count rate is 5.0 +/- 0.3 cts/s, at which pile-up affects the observational results.
Using the Swift/XRT product online tool (http://www.swift.ac.uk/user_objects/), the best-fit XRT coordinates are
UVOT Enhanced Position:
RA(J2000) = 18h 59m 01.57s (284.75654 deg)
Dec(J2000) = +08d 14' 44.2" (8.24561 deg)
Error radius = 1.9" (90% confidence).
The XRT spectrum can be well described by an absorbed power-law with a nH of 2.7(+1.2,-1.0)E22 cm^-2 and a photon index of 0.7 +/- 0.4. The best-fit model leads an unabsorbed flux of 7.1(+0.9,-0.8)E-10 erg cm^-2 s^-1, which is consistent with the MAXI (ATel #7034) and BAT (ATel #7037) measurements. Although a 9.80 s X-ray pulsation has been independently detected by RXTE/PCA (Marshall et al. 1999) and Fermi/GBM (ATel #7045), no pulsation signal has been found in this XRT observation. We caution that all the XRT results presented here could be biased by pile-up.
Although XTE J1859+083 was suggested to be a Be/NS binary (Corbet et al. 2009), no UVOT counterpart has been detected in the B-band, U-band, and white light images (upper limits: B > 20.21 mag; U > 19.89 mag; WL > 20.31). However, we found possible near-IR/optical counterparts in the 2MASS and USNO-B1.0 catalogs, which are
USNO-B1.0 0982-0467424 (offset: 1.1"):
R2 = 17.37 mag
I = 16.68 mag
2MASS 18590163+0814444 (offset: 1.0"):
I = 14.188 +/- 0.052 mag
J = 13.097 +/- 0.050 mag
K = 12.471 +/- 0.038 mag
We strongly encourage optical/IR follow-up observations to confirm the counterpart.
We thank the Swift PI, Neil Gehrels, and the Swift team for approving and scheduling this ToO observation.
