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Joint Swift XRT and NuSTAR Observations of TXS 0506+056

ATel #10845; D. B. Fox (PSU), J. J. DeLaunay (PSU), A. Keivani (PSU), P. A. Evans (U. Leicester), C. F. Turley (PSU), J. A. Kennea (PSU), D. F. Cowen (PSU), J. P. Osborne (U. Leicester), M. Santander (UA) & F. E. Marshall (GSFC)
on 12 Oct 2017; 16:54 UT
Credential Certification: Derek B. Fox (dfox@astro.psu.edu)

Subjects: X-ray, Gamma Ray, >GeV, TeV, Neutrinos, AGN, Blazar, Quasar

Referred to by ATel #: 10861

We have carried out joint Swift and NuSTAR observations of the BL Lac-type blazar TXS 0506+056, identified in our previous Swift XRT observations (GCN #21930) as one of nine bright X-ray sources within the localization region of the likely-cosmic IceCube EHE neutrino IceCube-170922A (GCN #21916). Subsequent reports have revealed that TXS 0506+056 is in a GeV flaring state as observed by Fermi LAT (Atel #10791) and AGILE (ATel #10801), and is detected at E>100 GeV energies by MAGIC (ATel #10817), albeit undetected at E>160 GeV by VERITAS (ATel #10833).

Our NuSTAR observations over 02:23 to 17:48 UTC on 29 Sep 2017 yield 23.9 ks (24.5 ks) exposure in the A (B) units, respectively, after processing with NuSTAR standard software tools (SAAMODE=strict). The source is well detected with count rates of 21.3 ct ks-1 (20.8 ct ks-1) in the A (B) units. We obtain a satisfactory fit to the two spectra with a single power-law model having photon index Gamma=1.71(11) and flux 4.6(10)e-12 erg cm-2 s-1 (3-100 keV). Uncertainties are quoted at 90%-confidence.

For joint analysis purposes, we select Swift XRT data from the 27 Sep and 30 Sep epochs of our ongoing monitoring program on TXS 0506+056; processing these data with the online tools of the UK Swift Science Data Centre (Evans et al. 2009, MNRAS, 397, 1177) yields 6.9 ks exposure, with the source exhibiting a count rate of 88 ct ks-1. The XRT spectrum is satisfactorily fit with an absorbed power-law model with N_H=1.2(5)e21 cm-2 and photon index Gamma=2.7(3), giving flux 1.9(4)e-12 erg cm-2 s-1 (0.3-10 keV). This absorbing column is consistent with the Galactic column along the line of sight, N_H=1.11e21 cm-2 as per the LAB Survey (via HEASARC).

Jointly analyzed, the spectra are not consistent with any single power-law fit (with absorption) over 0.3 keV to 100 keV. Fixing the column density to the Galactic value, we obtain a satisfactory fit with a double power-law spectral model that mimics the parameters of the separate fits: absorbing column N_H=1.11e21 cm-2 (fixed), soft photon index Gamma_s=3.24(37) and flux of 1.1(3)e-12 erg cm-2 s-1 over 0.3-10 keV, and hard photon index Gamma_h=1.51(15) and flux 5.3(2)e-12 erg cm-2 s-1 over 3-100 keV.

We note that the hard spectral component (Gamma_h=1.51 +/- 0.15) dominating the NuSTAR bandpass is rising, in the nu*F_nu sense, toward higher energies, and is harder than the Fermi LAT spectrum (ATel #10791), which has Gamma_LAT=2.0(1) and is flat in nu*F_nu over 0.1-300 GeV. Extrapolating the NuSTAR hard spectral component to higher energies, it reaches the 5.8e-11 erg cm-2 s-1 flux of the Fermi nu*F_nu "plateau" at E~40 MeV, just below the lower end of the LAT bandpass.

We express our appreciation to the Swift and NuSTAR science operations teams responsible for carrying out these observations. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester.