NuSTAR observation of the blazar PKS 0446+11 as a follow-up on IceCube-240105A

ATel #16417; Jooyun Woo (Columbia University), Atreya Acharyya (University of Southern Denmark, Odense, Denmark), Qi Feng (University of Utah), Kaya Mori (Columbia University), Reshmi Mukherjee (Barnard College/Columbia University), Martin Pohl (University of Potsdam, Potsdam-Golm, Germany and DESY, Zeuthen, Germany), Raj Prince (Polish Academy of Sciences, Warsaw, Poland), Marcos Santander (University of Alabama), Ruo-Yu Shang (Barnard College/Columbia University)
on 25 Jan 2024; 04:00 UT
Credential Certification: Jooyun Woo (jw3855@columbia.edu)

Tweet

We report on the NuSTAR target-of-opportunity (ToO) observation of PKS 0446+11, a flat-spectrum radio quasar located in the 90% uncertainty region (0.4 deg away from the best-fit position) of IceCube-240105A (GCN #35485, #35498). A flaring activity of PKS 0446+11 was detected in the X-ray band by Swift-XRT (ATel #16397), GeV gamma-ray band by Fermi-LAT (ATel #16398), radio band as part of TELAMON (ATel #16399), RATAN-600 and MOJAVE VLBA program (ATel #16409), and optical band by 1.3-meter Devasthal Fast Optical Telescope (DFOT, ATel #16402) and the Large Array Survey Telescope (LAST, ATel #16407). ATel #16414 reported the neutrino flux upper limit from the recent 63-day window with good data (2023-10-20 12:00:00.0 UTC to 2023-12-22 12:00:00.0 UTC). The NuSTAR observation of PKS 0446+11 was performed on 2024-01-11 at UT 12:36:09 for 44.8 ks (net exposure 35.7 ks after data cleaning). The data were processed using NuSTAR Data Analysis Software v2.1.2 and CALDB 20230718. Our preliminary analysis shows that the source is bright and significantly detected up to 50 keV. Its spectrum in the 3 - 50 keV range is well explained by an absorbed power law with a best-fit photon index of 1.75 +/- 0.03. The hydrogen column density was fixed to the Galactic value (2.48e-21 /cm2). Compared with the 0.3 - 10 keV spectrum observed with Swift (photon index 1.4 +/- 0.5 on 2024-01-06 and 1.21 +/- 0.22 on 2024-01-08, ATel #16397), the 3 - 50 keV spectrum is consistent with the first Swift observation within the error bar, and softer than the second Swift observation. Note that the Swift analysis did not account for absorption, and hence underestimated the photon index. The 3-50 keV unabsorbed flux is (8.07 +/- 0.25)e-12 erg/s/cm2. The spectral indices from the NuSTAR observation and the first Swift exposure are consistent within uncertainties with the NuSTAR+Swift index reported by Marcotulli et al. (ApJ 839 96, 2017) in a 2015 observation of the source. NuSTAR is a focusing hard X-ray telescope (3 - 79 keV) with a sub-arcminute angular resolution. NuSTAR’s broad high-energy coverage is minimally affected by absorption and uniquely suited to study blazars and other extreme phenomena. We thank the NuSTAR Science Operations Center for making this observation possible.