Swift observations of the galactic transient source Swift J1808.4-1754
ATel #6155; H. A. Krimm (CRESST/GSFC/USRA), J. A. Kennea (PSU) and S. T. Holland (STSci )
on 21 May 2014; 11:48 UT
Credential Certification: Hans A. Krimm (Hans.Krimm@nasa.gov)
Subjects: Ultra-Violet, X-ray, Binary, Black Hole, Neutron Star, Transient
Referred to by ATel #: 6170
The Swift satellite carried out an observation of the newly discovered galactic transient source Swift J1808.4-1754 (Krimm et al, ATel #6138). A total exposure of 1.3 ks in Photon Counting mode was obtained starting at 2014 May 19, 03:37:10.774 UT. A bright X-ray source is clearly detected in the Swift/XRT at an enhanced position (Goad et al., 2007, A&A, 476, 1401) of
RA = 272.10475, Dec = -17.89701
RA =18h 08m 25.14s, Dec = -17d 53'49.2''
glon = 12.167403, glat = +1.026395
Error radius 1.4''; (90% confidence)
The XRT spectrum is well fitted (Evans et al., 2009, MNRAS, 397, 1177) by a simple absorbed power law model with parameters:
nH = 4.31 (+1.06,-0.93) X 10^22 cm-2
Index = 0.93 +/- 0.29
Flux (observed; 0.3-10 keV) = (2.36 +/- 0.23) X 10^-10 erg cm-2 s-1
Flux (unabsorbed; 0.3-10 keV) = (3.22 +/- 0.32) X 10^-10 erg cm-2 s-1
W-stat=344 for 340 dof
The XRT light curve shows an average count rate of ~2.5 ct s-1 (0.3-10 keV) with significant variation on the time scale of ~100 seconds, though the time baseline is too short to determine if there is any periodicity.
The UVOT observed in the b band and no source was found in the integrated image. The position is consistent with a catalog source 2MASS 18082507-1753482, at RA=272.10446, dec=-17.896723. This star has quiescent magnitudes J=14.838, H=13.550, K=12.674. No star is found in the DSS image.
The BAT light curve shows a nearly constant rate for Swift J1808.4-1754 over the past 6 days of ~0.010 ct cm-2 s-1 (15-50 keV) or 45 mCrab.
The current observations do not allow us to determine the nature of the source. The ratio of BAT to XRT count rates is lower than what has normally been seen for black hole candidates in outburst like Swift J1539.2?6227 (Krimm et al, 2011, ApJ 735, 104), Swift J1910.2-0546 (Krimm et al, 2013, ApJSS 209, 14) or Swift J1745.1-2624 (Krimm 2013) and closer to the ratios seen for galactic neutron stars like Swift J0513.4-6547 (Krimm 2013) or Swift J1729.9-3437 (Krimm 2013).
Further Swift observations have been approved. Other observations are encouraged.
Swift/BAT transient monitor light curve for Swift J1808.4-1754
