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
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Credential Certification: Craig B. Markwardt (craigm@lheamail.gsfc.nasa.gov)

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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).