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Dramatic change in the X-ray spectrum of symbiotic recurrent nova T CrB

ATel #10046; Luna G. J. M. (IAFE/CONICET), Mukai K. (NASA GSFC), Sokoloski J. L., Lucy A. (Columbia), Nelson T. (Pittsburgh), Nuñez N. (ICATE/CONICET)
on 3 Feb 2017; 17:32 UT
Credential Certification: Gerardo Juan Manuel Luna (gjmluna@iafe.uba.ar)

Subjects: X-ray, Nova

We report the results from two Swift XRT/UVOT observations of the recurrent novae T CrB, performed on 01/18/2017 and 01/29/2017. In April 2016, T CrB reached the peak of an optical brightening event (Delta V~1, Delta B~1.5) that started at the beginning of 2015 (Munari et al. 2016) and is slowly declining toward the quiescence level of V~10. A similar brightening event of unknown origin was also observed around 8 years before the nova eruptions of 1866 and 1946, when T CrB brightened up to V~3 (Schaefer 2014). Using the BAT transient monitor, we have made the surprising discovery that T CrB has undergone a shallow decline in its hard X-ray flux starting at roughly the time of the optical peak in April 2016. For the past 200 days, T CrB has been undetected by the Swift BAT Daily Monitor. The overall decline in high energy flux is confirmed by our Swift XRT observations, with source count rates of 0.009 c/s on 01/18/2017 and 0.014 c/s on 01/29/2017 in the 0.3-10 keV energy range. Comparing an XRT count rate of 0.082 c/s measured in the 0.3-10.0 keV range in 2005 with our two recent Swift XRT observations, the X-ray flux has declined by a factor of ~10 and ~6 in the two observations, respectively. Even more interesting is that the hardness ratio, F(2.0-10.0 keV)/F(0.3-2.0 keV), decreased a factor of ~100. This change is due to the appearance of a new, distinct, soft X-ray component with E <~ 1 keV. While previous X-ray observations (Luna et al. 2008, I?kiewicz et al. 2016) suggested that T CrB had a delta-type X-ray spectrum as described by Luna et al. (2013), the detection of the soft component indicates that it now has a mixed beta/delta-type spectrum. The hard component of the XRT spectra can be described by a highly absorbed (nh > 10^23 cm^-2), optically thin thermal plasma (kT > 5 keV) while the new soft component can be described by either an optically thin thermal plasma with a temperature of kT = 0.07+-0.02 keV or a blackbody with about the same temperature. The blackbody normalization implies an unabsorbed luminosity of a 6e31 ergs/s (d/1 kpc)^2. The Swift/UVOT UVM2 observations saturated, with magnitudes < 10. Previous observations (between 2005 and 2015) showed a UVM2 magnitude of about 14.90. In Kennea et al. (2009) and Luna et al. (2008) we reported Swift BAT and Suzaku observations and proposed that the high-energy X-rays originate from a highly absorbed, optically thin thermal plasma in the most internal part of the accretion disk, i.e. the boundary layer. The current decline in BAT flux could thus be due to a change in the optical depth of this region, likely after an increase in the accretion rate through the disk (as in dwarf novae outbursts). Such a change could also explain the optical brightening. The drop in X-ray hardness ratio as well as the increase in UV flux would also be expected in this picture. Swift detected both effects in our recent observations. We will continue monitoring this unique event with Swift, and we encourage further observations at other wavelengths. We thank the Swift team, in particular Neil Gehrels, for the fast response time in executing this target-of-opportunity observation.