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ATEL # 1577; J. Greiner, G. Sala, T. Kruehler (MPE Garching)
on 13 Jun 2008; 21:12 UT
Password Certification: J. Greiner (email@example.com)
Subjects: Optical, X-ray, Transients
We have performed a Chandra ToO observation of XTE J1719-291 which was found to flare up in RXTE/PCA data between March 5-26 (Markwardt & Swank 2008, ATEL #1442). The goal was to improve upon the localization obtained by Swift/XRT on March 30 (Degenaar et al. 2008, ATEL #1451) to allow optical/NIR identification in this crowded region. We have obtained a 2.16 ksec HRC-I observation starting on 2008-04-27 at 18:23:25. The one X-ray source detected in the field of view is located within the Swift/XRT error circle, at RA(2000.0) = 17h 19m 17.18s, Decl.(2000) = -29d 04' 10.0" with an uncertainty of 0.2" (90% confidence, statistical). We detect a total of 255 counts. Using the spectral parameters from the Swift/XRT measurement of April 3 (9), namely a powerlaw of slope -2.8(-2.5) and an absorbing column of 6.7(6.0)*1021 cm-2, we derive an unabsorbed 2-10 keV flux of 1.4 (2.0) *10-12 erg cm-2 s-1. Thus, at the time of our measurement XTE J1719-291 was at similar luminosity as during the April 3 Swift/XRT observation, and thus had returned from the bright state on April 9 (Degenaar et al. 2008, ATEL #1467), consistent with the finding of Degenaar & Wijnands (2008), ATEL #1541 during a Swift observation on April 30.
We also observed XTE J1719-291 with the 7-channel imager GROND (Greiner et al. 2008, PASP 120, 405) on two epochs: April 11 between 9:08-9:42 UT (effective exposure of 1120 sec in g'r'i'z' and 900 sec in JHK), and May 4 at 8:21-8:53 UT (effective exposure of 1475 sec in g'r'i'z' and 1200 sec in JHK).
We detect a source at RA(2000)=17h 19m 17.18s and DEC(2000)= -29d 04' 09.98" with uncertainties of 0.3", coincident with the Chandra position. The object is clearly visible in the first epoch, but not detected in the second epoch (see the r'-band images of both epochs at URL http://www.mpe.mpg.de/~jcg/xrt/xrtj1719m291.html). We suggest this object to be the optical counterpart of the Chandra, Swift and RXTE source.
Photometry was done against SA100-241 in g'r'i'z', and against 2MASS for JHK, but is strongly compromised by a nearby bright object which in JHK is even saturated. Using PSF fitting, we measure g' = 22.98, r' = 22.6, i'=22.3 and z'=22.6 (all in AB system), with statistical errors of ±0.15 mag. This is surprisingly blue considering the large absorption seen in the Swift X-ray spectrum. Applying the corresponding AV=3.3 correction, our g'r'i'z' SED is best fit with a spectrum Fν ~ ν3, even bluer than the Rayleigh-Jeans tail of a putative accretion disk.
The upper limits during the second epoch are g' > 23.7 ± 0.1, r' > 23.5 ± 0.1, i' > 22.9 ± 0.1, z' > 22.9 ± 0.1, J > 20.2 ± 0.1, H > 19.4 ± 0.1, K > 18.6 ± 0.1 (JHK in Vega system), i.e. the source had faded by 0.7 mag, in concordance with the fading seen at X-rays during that time (Degenaar & Wijnands 2008, ATEL #1541). From the upper limits (and assuming a distance of 8 kpc) we derive an absolute V magnitude of >5.8, suggesting a companion K0V or later (if this transient is a binary system).