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.