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

Taichi Kato tkato at kusastro.kyoto-u.ac.jp
Sun Sep 14 12:53:39 JST 2008

   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 
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, 

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). 

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