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Swift/XRT detection of PS17bgn/2017bcc

ATel #10177; K. Sokolovsky (IAASARS NOA/ASC Lebedev/SAI MSU), L. Wyrzykowski, A. Hamanowicz, M. Gromadzki (Warsaw Observatory)
on 17 Mar 2017; 14:55 UT
Credential Certification: Kirill Sokolovsky (kirx@scan.sai.msu.ru)

Subjects: Optical, Ultra-Violet, X-ray, AGN, Black Hole, Supernovae, Transient, Tidal Disruption Event

PS17bgn was discovered on 2017-02-18 0.3 arcsec. from the center of the galaxy SDSS J113152.97+295944.8. Spectroscopic observations with the NTT (PESSTO, ATel #10105) on 2017-02-20 and NOT 2017-02-28 suggest this may be a superluminous Type II supernova (SLSN-II) or an AGN flare at z=0.148, however, the re-examination of the spectrum shows it is at redshift z=0.133, lower than the one reported in ATel #10105. The spectrum shows very broad Halpha and Hbeta with possibly two components, there is a hint for broad HeII and the blue continuum which makes this object a candidate tidal disruption event (TDE).

Swift observed PS17bgn for 2.1ks on 2017-03-10. Swift/XRT detected a previously uncataloged X-ray source with the net count rate of 0.0286 +/-0.0037 cts/s at the position of the transient. Fixing the absorbing column density to the Galactic value in the direction of the source, n_H1 = 1.78x10^20 (Kalberla et al. 2005 A&A, 440, 775), the spectrum can be fit with the absorbed power law with the photon index of 1.7 +/-0.3 and the unabsorbed 0.3-10 keV flux of 1.3x10^-12 ergs/cm^2/s. The relatively hard photon index suggests non-thermal origin of the observed X-ray emission.

Swift/UVOT detected the source in six ultraviolet and optical bands:

Band  mag   err  
 W2  18.59  0.11 
 M2  18.64  0.11 
 W1  18.61  0.15 
 U   18.46  0.17 
 B   18.90  0.17  
 V   18.01  0.20 
Taking into account the Galactic reddening of E(B-V)=0.02 (Schlafly et al. 2011 ApJ, 737, 103) these magnitudes suggest a black-body emission with the temperature between 10000 K (all bands) and 17000 K (only W2, M2, W1 bands included in the fit). The host galaxy light (which is expected to be stronger at longer wavelength) was not subtracted before performing the black-body spectrum fit.

The detected non-thermal X-ray emission, optical-ultraviolet color temperature >~10000 K and optical emission lines are, in principle, consistent with all three possible explanations for the PS17bgn/2017bcc event: SLSN-II, AGN flare or TDE. Further observations are needed to clarify the nature of this transient.

We thank Brad Cenko and the Swift team for making this ToO observation possible.