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First Detection of X-rays from 8 Symbiotic Binaries

ATEL # 3053; G. J. M. Luna (SAO), J. Sokoloski (Columbia), K. Mukai (NASA/GSFC) and T. Nelson (NASA/GSFC)
on 23 Nov 2010; 22:07 UT
Password Certification: Gerardo Juan Manuel Luna (gluna@cfa.harvard.edu)

Subjects: X-ray, Binaries, Cataclysmic Variables

As part of our Swift fill-in program (PI. J. Sokoloski), we report the Swift/XRT detection of 8 symbiotic stars not previously detected in X-rays. These 8 new X-ray sources all show spectra extending to energies greater than 2.4 keV. The X-ray spectra are consistent with emission from a boundary layer around an accreting white dwarf, as in the symbiotic stars RT Cru, T CrB, CD -57 3057, and CH Cyg (Luna & Sokoloski 2007, ApJ, 671, 741; Smith et al. 2008, PASJ, 60, S43; Kennea et al. 2009, ApJ, 701, 1992). The newly detected sources, the Swift/XRT count rates, the exposure times, and the S/N ratios of the detections (assuming that uncertainties are dominated by Poisson statistics) are: NQ Gem: 0.025 c/s; 10.1 ks; S/N=14.8 Hen 3-461: 0.013 c/s; 9.9 ks; S/N=11.1 BI Cru: 0.0038 c/s; 10.7 ks; S/N=6.6 CD -28 3719: 0.0029 c/s; 10.2 ks; S/N=5.6 ER Del: 0.0034 c/s; 10.6 ks; S/N=5.9 ZZ CMi: 0.0037 c/s; 11.6 ks; S/N=6.5 V347 Nor: 0.0017 c/s; 15.6 ks; S/N=4.8 Y Cra: 0.0043 c/s; 9.8 ks; S/N=6.6 We detected all sources at the positions listed in the catalogue by Belczynski et al. (2000), except Y Cra. In the case of Y Cra, the X-ray source is detected at alpha=18h 14m 20.425s, delta=-42deg 50' 22.143", while the coordinates from the catalogue are alpha=18h 14m 22.951s, delta=-42deg 50' 32.40". This discrepancy in position is 10 times the radius of the Swift/XRT error circle (3 arcsec). Nonetheless, as SIMBAD does not list any other source around this position, we tentatively identify this X-ray source as the symbiotic star Y Cra. Preliminary fits show that all of the newly discovered X-ray emitting symbiotic stars are strongly absorbed (nH~10^22-23 cm^-2) and variable, which might explain their lack of detection in previous surveys such as the RASS. We thank the Swift team for scheduling these observations.


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