[vsnet-alert 10528] SWIFT J1842.5-1124 and its optical counterpart

[Taichi Kato]

   This X-ray transient, showing QPOs with a frequency of 0.8 Hz,
is most likely a black-hole transient.  Although the source
relatively suffers from extinction, the optical counterpart is
well within reach of moderate telescope (particularly in longer
wavelengths).  Optical time-resolved photometry is encouraged to
study variations, such as superhumps and QPOs.  The reported strong
variability in the optical looks somewhat similar to that observed
in V404 Cyg.

===

RXTE and Swift Observations of SWIFT J1842.5-1124 

ATel #1716; C. B. Markwardt (CRESST/U.Md./NASA/GSFC), J. P. Halpern 
(Columbia U.), S. T. Holland (CRESST/USRA/NASA/GSFC), J. A. Kennea 
(PSU), H. A. Krimm (CRESST/USRA/NASA/GSFC), J. H. Swank (NASA/GSFC) 
on 12 Sep 2008; 19:24 UT 
Distributed as an Instant Email Notice (Transients) 

Password Certification: Craig B. Markwardt 
(craigm at lheamail.gsfc.nasa.gov) 

Subjects: X-ray, Request for Observations, Binaries, Black Holes, 
Neutron Stars, Transients 

We report on RXTE and Swift observations of SWIFT J1842.5-1124 (Krimm 
et al., ATel #1706), including timing, spectroscopy and a UV 
counterpart detection. The combined X-ray spectral and timing 
behaviors of the source are characteristic of a black hole in the 
hard spectral state. 

RXTE PCA observed on 2008-09-09 at 07:20 UT for 3 ksec. The spectrum 
is consistent with a combined black body and power law model (black 
body kT = 0.9 keV; photon index = 1.5), with a weak line near 6.4 keV 
(100 eV equivalent width). This line may be due to contamination from 
diffuse emission in the PCA field of view. The fluxes in the 2-10, 
10-20 and 20-40 keV bands are 7.7, 4.8 and 6.7, in units of 10-10 
erg/s/cm2, respectively. The black body component contributes about 
6% of the total 2-40 keV flux. 

We examined the timing properties of the source by extracting a power 
spectrum. There is a strong QPO present near a frequency of 0.8 Hz. 

In observations made with the Swift Ultraviolet/Optical Telescope 
(UVOT) on 2008-09-11 at 18:56 UT for 1.7 ksec, a bright source was 
found coincident with the position of SWIFT J1842.5-1124 (Krimm et 
al, ATel #1610). The refined UVOT position is 

         RA (J2000.0) =  18:42:17.45        Dec (J2000.0) = -11:25:03.9   

with an estimated uncertainty of 0.6 arcsec (radius, 90% confidence, 
statistical + systematic). There is strong evidence for a brightening 
in the UVOT u band between 2008-07-13 and 2008-09-11. The magnitudes 
are u = 19.16 +/- 0.07 on 2008-07-13 and 17.84 +/- 0.05 on 
2008-09-11. There is weak evidence for variability during 
observations on 2008-09-09 following the Swift Burst Alert Telescope 
(BAT) trigger (Krimm et al, ATel #1706). The V magnitude varied from 
17.37 +/- 0.27 to 16.79 +/- 0.25 at 1767.6 and 7342.7 seconds, 
respectively, after the Swift/BAT trigger. We note that the UVOT 
position differs by only 1.6" from a faint star on the DSS that is 
listed in the USNO B-1.0 catalog at (J2000.0) 18:42:17.40, 
-11:25:05.3. 

The source is highly variable as seen in the Swift/X-Ray Telescope 
(XRT). On 2008-09-09, the count rate was in the range of 2-7 cts/s 
(0.3-10 keV). In the first orbit on 2008-09-11 the source was 
approximately steady at 26.5 cts/s. In the second orbit the 
brightness varied rapidly between 1-16 cts/s. The 6.4 keV line is not 
seen in the XRT data. The XRT data is best fit by a blackbody plus 
absorbed power law with fit parameters: nH = (3.9 +/- 0.3) x 1021 
cm-2, kT = 0.20 +/- 0.02 keV, photon index = 1.25 +/- 0.4, chisq/dof 
= 1.5 (dof=267). 

The Swift/BAT count rate has remained constant between 2008-09-09 
(Krimm et al, ATel #1706) and 2008-09-11, and the spectrum shows 
signs of softening. The time averaged data for observations over 
10-11 Sep gives a photon index of 1.4 +/- 0.3 and a flux of (1.9 +/- 
0.2) x 10-9 erg/cm2/sec (chisq/dof = 1.5; dof=22).