NuSTAR observations of the blazar PKS 0735+178

ATel #15113; Qi Feng (Barnard College/Columbia University), Weidong Jin (U. Alabama), Kaya Mori (Columbia University), Reshmi Mukherjee (Barnard College/Columbia University), Marcos Santander (U. Alabama), Jooyun Woo (Columbia University)
on 14 Dec 2021; 22:08 UT
Credential Certification: Qi Feng (qifeng@nevis.columbia.edu)

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We report on the recent target-of-opportunity NuSTAR observations of the blazar PKS 0735+178, located 2.2 deg away from the best-fit position of the IceCube neutrino event IC211208A (GCN #31191). Elevated fluxes from PKS 0735+178 were recently reported in the radio band (ATel #15105), in the optical band by MASTER (ATel #15098), in the X-ray band by Swift-XRT (ATel #15102, #15109), and in the GeV gamma-ray band by Fermi-LAT (ATel #15099) as part of the follow-up observations of the region around the IceCube event.

Two NuSTAR observations of PKS 0735+178 were performed:

1. from 2021 Dec 11 14:00 UT to Dec 12 01:30 UT, and
2. from 2012 Dec 13 12:35 UT to Dec 13 23:30 UT.

The data were processed with the NuSTAR Data Analysis Software (v2.0.0). The exposures for the first observation were 22.2 ks and 21.9 ks for the focal plane modules A and B, respectively. Those for the second observation were 21.6 ks and 21.3 ks.

The spectra from both modules A and B were simultaneously and adequately fitted between 3 keV and 40 keV with a single power-law model, without any evidence for curvature or absorption. The best-fit models for the two NuSTAR observations yield photon indices of 1.79 +/- 0.07 and 1.64 +/- 0.08, fluxes (3 -- 40 keV) of (3.2 +/- 0.2) x 10^-12 erg cm^-2 s^-1 and (3.0 +/- 0.2) x 10^-12 erg cm^-2 s^-1, and chi-squared (degree of freedom) values of 107 (108) and 76 (92), respectively.

The hard-X-ray spectrum of the source is significantly harder than the recently reported soft-X-ray spectrum, which yielded photon indices of ~2.5 -- 2.7 and uncertainties around 0.2, observed from 0.3 to 10 keV by Swift-XRT on 2021 Dec 10, 12, and 13 UT (ATel #15102, #15109). Earlier observations of the source from 0.3 keV to 10 keV between 2020 Apr and 2021 Oct by eROSITA suggested X-ray variability both in the flux and spectrum (the photon index ranging between 1.2 and 2.0; ATel #15108). The hard-X-ray flux is consistent with being constant over the two-day interval between the two NuSTAR observations. Further multi-wavelength follow-up observations are encouraged to study the variability of the source on longer timescales.

We thank the NuSTAR Science Operations Center, in particular K. Forster, for making these observations possible.