On Measuring the Variation of High Energy Cut-off in Active Galactic Nuclei

Abstract

The variation in the high energy cut-off E_c in active galactic nuclei uniquely probes the corona physics. In this work we show that the ratio of two NuSTAR spectra (in analogy to difference imaging technique widely used in astronomy) is uniquely useful in studying E_c variations. The spectra ratio could directly illustrate potential E_c variation between two spectra. By comparing with the ratio of two spectral fitting models, it also examines the reliability of the spectral-fitting measured E_c variation. Assisted with this technique, we revisit the 5 AGNs in literature (MCG -5-23-16, 3C 382, NGC 4593, NGC 5548 and Mrk 335) for which E_c (kT_e) variations have been claimed with NuSTAR observations. We show the claimed E_c variations appear inconsistent with the spectra ratios in three of them, thus need to be revised, demonstrating the striking usefulness of spectra ratio. We present thereby improved spectral fitting results and E_c variations. We also report a new source with E_c variations based on NuSTAR observations (radio galaxy 4C +74.26). We find the corona tends to be hotter when it brightens (hotter-when-brighter) in 3C 382, NGC 5548, Mrk 335 and 4C +74.27, but MCG -5-23-16 and NGC 4593 show no evidence of significant E_c variations. Meanwhile all 6 sources in this small sample appear softer-when-brighter. Changes in corona geometry are required to explain the observed hotter-when-brighter trends.

arXiv comment14 pages, 13 figures, ApJ in press

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• NuSTAR Survey of Obscured Swift/BAT-selected Active Galactic Nuclei: II. Median High-energy Cutoff in Seyfert II Hard X-ray Spectra
• Evolution of a Relativistic Outflow and X-ray Corona in the Extreme Changing-Look AGN 1ES 1927+654
• The first simultaneous X-ray broad-band view of Mrk 110 with XMM-Newton and NuSTAR
• NuSTAR hard X-ray spectra of radio galaxies
• Study of X-ray variability and coronae of Seyfert galaxies using NuSTAR
• Distinct High Energy Cutoff Variation Patterns in Two Seyfert Galaxies
• More than softer-when-brighter: the X-Ray powerlaw spectral variability in NGC 4051
• Search for coronal temperature variation in Seyfert galaxies
• X-ray spectral and timing analysis of the Compton Thick Seyfert 2 galaxy NGC 1068
• NuSTAR spectral analysis of three Seyfert galaxies: NGC 3227, NGC 5548 and MR 2251$-$178
• Hidden biases in flux-resolved X-ray spectroscopy
• On joint analysing XMM-NuSTAR spectra of active galactic nuclei
• A possible two-fold scenario for the disc-corona of the luminous AGN 1H 0419--577: a high-density disc or a warm corona
• The X-ray coronae in NuSTAR bright active galactic nuclei