X-rays follow-up of the Gamma-ray flaring FSRQ PKS 0346-27
ATel #11455; Roberto Nesci INAF/IAPS Roma
on 21 Mar 2018; 10:17 UT
Credential Certification: Roberto Nesci (Roberto.Nesci@iaps.inaf.it)
Subjects: Infra-Red, Optical, X-ray, AGN, Quasar
Referred to by ATel #: 12479
The Flat Spectrum Radio Quasar PKS 0346-27 underwent a strong Gamma-ray flare on 2018-02-02 (Angioni et al., ATel #11251). Optical/NIR follow up with the REM telescope (Nesci, ATel #11269) on 2018-02-07 showed the source in a bright state also in this frequency range. Independent indication of an optical peak on 2018-02-21 was given by the ASAS-SN survey (Vallely et al., ATel #11337). Archival observations of the Catalina Sky Survey DR2 (Drake, A.J. et al. 2009, ApJ, 696, 870) from 2005 to 2013 indicate an optical variability amplitude of more than 2 magnitudes, but never brighter than V=17.8.
We obtained a ToO exposure of 2300s with Swift-XRT and UVOT on 2018-03-19: PKS0346-27 was found at U=15.65 +-0.01 mag with a count-rate of 0.0704 +-0.0062 c/s in the 0.2-10 keV range. Spectral analysis with the interactive tool of the ASI SSDC gave a reasonable fit with a power-law photon index -2.08+-0.23, and a reduced chi2=0.44. The total flux in the 0.4-10 keV range is 2.0E-12 erg/cm2/s.
In a log(nu)-log(nu*F_nu) plot (Fig.1) the X-ray SED is practically flat, suggesting that in this energy range the contribution of Synchrotron emission and Inverse Compton emission are comparable.
The archival Swift observation in 2009-03-29 (3362s) gave U=17.53+-0.04 (U-V=-0.64) and an X-ray count rate of only 0.0174 +-0.0025 c/s, not enough to derive a meaningful spectral index. The ratio of the X-ray count rates between the two epochs is about 4.0 +-0.9, slightly smaller than the optical (U) flux ratio 5.6+-0.3.
The Fermi-LAT public data show that PKS 0346-27 is still much brighter than the quiescent level. Our REM monitoring is still ongoing. Radio observations at high frequency would be desirable to better constraint the emission model of this flare.
Fig. 1 shows the SED of PKS 0346-27 on 2018-03-19 from simultaneous Swift and REM data (crosses). The large dots are the UVOT data on 2009-03-29 and the XRT X-ray flux estimated from the count-rate, assuming the same photon index of the recent observation.
Acknowledgement: Part of this work is based on archival data, software and online services provided by the Space Science Data Center - ASI.
Figure 1