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On joint analysing XMM-NuSTAR spectra of active galactic nuclei

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https://doi.org/10.52396/JUSTC-2023-0160
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  • A recently released XMM-Newton note revealed a significant calibration issue between nuclear spectroscopic telescope array (NuSTAR) and XMM-Newton European Photon Imaging Camera (EPIC) and provided an empirical correction to the EPIC effective area. To quantify the bias caused by the calibration issue in the joint analysis of XMM-NuSTAR spectra and verify the effectiveness of the correction, in this work, we perform joint-fitting of the NuSTAR and EPIC-pn spectra for a large sample of 104 observation pairs of 44 X-ray bright active galactic nuclei (AGN). The spectra were extracted after requiring perfect simultaneity between the XMM-Newton and NuSTAR exposures (merging Good Time Intervals, GTIs from two missions) to avoid bias due to the rapid spectral variability of the AGN. Before the correction, the EPIC-pn spectra are systematically harder than the corresponding NuSTAR spectra by $\Delta \varGamma \sim 0.1 $, subsequently yielding significantly underestimated cutoff energy Ecut and the strength of reflection component R when performing joint-fitting. We confirm that the correction is highly effective and can commendably erase the discrepancy in best-fit Γ, Ecut, and R. We thus urge the community to apply the correction when joint-fitting XMM-NuSTAR spectra, but note that the correction is limited to 3–12 keV and therefore not applicable when the soft X-ray band data are included. Besides, we show that as merging GTIs from two missions would cause severe loss of NuSTAR net exposure time, in many cases, joint-fitting yields no advantage compared with utilizing NuSTAR data alone. We finally present a technical note on filtering periods of high background flares for XMM-Newton EPIC-pn exposures in the Small Window mode.
    Γ, Ecut, and R derived through fitting the NuSTAR spectra alone versus joint-fitting XMM-NuSTAR spectra (before/after the ARF correction).
    A recently released XMM-Newton note revealed a significant calibration issue between nuclear spectroscopic telescope array (NuSTAR) and XMM-Newton European Photon Imaging Camera (EPIC) and provided an empirical correction to the EPIC effective area. To quantify the bias caused by the calibration issue in the joint analysis of XMM-NuSTAR spectra and verify the effectiveness of the correction, in this work, we perform joint-fitting of the NuSTAR and EPIC-pn spectra for a large sample of 104 observation pairs of 44 X-ray bright active galactic nuclei (AGN). The spectra were extracted after requiring perfect simultaneity between the XMM-Newton and NuSTAR exposures (merging Good Time Intervals, GTIs from two missions) to avoid bias due to the rapid spectral variability of the AGN. Before the correction, the EPIC-pn spectra are systematically harder than the corresponding NuSTAR spectra by $\Delta \varGamma \sim 0.1 $, subsequently yielding significantly underestimated cutoff energy Ecut and the strength of reflection component R when performing joint-fitting. We confirm that the correction is highly effective and can commendably erase the discrepancy in best-fit Γ, Ecut, and R. We thus urge the community to apply the correction when joint-fitting XMM-NuSTAR spectra, but note that the correction is limited to 3–12 keV and therefore not applicable when the soft X-ray band data are included. Besides, we show that as merging GTIs from two missions would cause severe loss of NuSTAR net exposure time, in many cases, joint-fitting yields no advantage compared with utilizing NuSTAR data alone. We finally present a technical note on filtering periods of high background flares for XMM-Newton EPIC-pn exposures in the Small Window mode.
    • In this work, we perform joint-fitting of NuSTAR and EPIC-pn spectra for a large sample of 104 observation pairs of 44 X-ray bright AGN. EPIC-pn spectra are systematically more difficult than those of NuSTAR (∆Γ ~ 0.1), leading to an underestimated of the cutoff energy Ecut and the reflection component R when performing joint-fitting before correcting the calibration issue.
    • The empirical correction of the effective area implemented in latest XMM-Newton calibration files (but would not be applied by default) is highly effective and could commendably erase the discrepancy in the derived best-fit Γ, Ecut, and R.
    • For this sample, requiring perfect simultaneity between the NuSTAR and EPIC-pn spectra leads to severse loss of the net exposure time of NuSTAR. Consequently, fitting NuSTAR spectra jointly with simultaneous EPIC-pn data does not always improve the constraints on the key spectral parameters.
    • For XMM-Newton EPIC-pn observations in small window (SW) mode, insufficient filtering of high background flares could bias the spectral fitting results due to the background vignetting effect, which is no longer negligible in case of background flares. A threshold of 0.05 counts/s to filter background flares appears appropriate for EPIC-pn SW mode.

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Catalog

    Figure  1.  An example (XMM-Newton obsid 0741330101) of the adopted regions when processing the XMM-Newton EPIC-pn data in SW mode. The image is extracted at 3–10 keV with evselect and plotted in logarithmic scale with ds9. The green circle is the source region, which is optimally determined by eregionanalyse, while the two white circles with a radius of 40 arcsec are the background regions. Note that part of the source region is outside the FOV (often the case for the SW mode), which will be automatically corrected by arfgen. Outside the red circle with a radius of 100 arcsec is the source-free region, used to filter intervals with a flaring background.

    Figure  2.  Usable fraction of the GTI after filtering the periods showing flaring background with different thresholds, for the 85 XMM-Newton exposures in SW mode.

    Figure  3.  Example NuSTAR FPMA 3–78 keV (ObsID: 60501049002) and XMM-Newton EPIC-pn 3–10 keV (ObsID: 0852210101) light curves (with a time bin of 500 s) of Mrk 1383, to illustrate the merge of NuSTAR and EPIC-pn GTIs. The grey shades represent the dropped time intervals for each instrument, mainly due to the Earth occultation for NuSTAR, and flaring background for EPIC-pn. The blue dots represent the remaining data after merging the GTIs, while the red open circles mark the data dropped due to the merging of GTI. For EPIC-pn (lower panel), we over-plot the background rate curve (grey stars) used to filter the intervals with flaring background, and the corresponding data points filtered out from the source light curve (grey circles). The duration of the exposures and net exposure time (before/after merging GTIs) are labelled, respectively.

    Figure  4.  The left panel: photon index $ \varGamma $ of the NuSTAR spectra versus those of the EPIC-pn spectra before (red square) or after (blue circle) correcting the effective area (corresponding to $ \varGamma^{\rm Nu} $, $ \varGamma^{\rm pn} $, and $ \varGamma^{\rm pn-Cor} $ in Table 1, respectively). The colored solid lines show the linear regression results (in comparison with the black 1∶1 line), with the shadows showing the 1$ \sigma $ uncertainty derived through bootstrapping the sample. The middle and right panels show the case for observations before/after 2017-01-01.

    Figure  5.  Absorption column density $ N_{\rm H} $ of the NuSTAR spectra versus those of the EPIC-pn spectra before (red square) or after (blue circle) correcting the effective area. The colored solid lines show the linear regression results derived by asurv (in comparison with the black 1∶1 line), with the shadows showing the 1$ \sigma $ uncertainty derived through bootstrapping the sample.

    Figure  6.  $ \Gamma $, $ E_{{\rm{cut}}} $ and $ R $ derived through fitting the NuSTAR spectra alone versus joint-fitting with EPIC-pn spectra. For $ E_{{\rm{cut}}} $ and $ R $, we perform linear regression in logarithmic space with asurv[42] to handle the censored data points (as hollow markers). The colored solid lines show the linear regression results (in comparison with the black 1∶1 line), with the shadow showing the 1$ \sigma $ uncertainty derived through bootstrapping the sample.

    Figure  7.  The photon index $ \varGamma $ (y-axis) derived using the effective area corrected XMM-Newton EPIC-pn spectra, after filtering the periods showing flaring background with a threshold of 0.05 counts/s (blue squares) or 0.4 counts/s (brown circles), versus those derived using NuSTAR spectra (x-axis), for the 33 observation pairs with significant flaring background. The colored solid lines show the linear regression results (in comparison with the black 1∶1 line), with the shadow showing the 1$ \sigma $ uncertainty derived through bootstrapping the sample.

    Figure  8.  Lost fraction of the NuSTAR net exposure after requiring a perfect simultaneity between NuSTAR and XMM-Newton data.

    Figure  9.  The distribution of the relative errors of $ \varGamma $, $ R $ and $ E_{\rm cut} $, with the detection fraction of $ R $ and $ E_{\rm cut} $ provided in the legend. The top panels show the distributions of $ \log_{10}\left( {\rm \dfrac{upper\,error}{value}} \right) $ of the three parameters, while the bottom panels show those of $ \log_{10}\left( {\rm \dfrac{lower\,error}{value}} \right) $. Blue boxes show the results of the joint-fitting of NuSTAR and EPIC-pn, while the orange boxes show the result of fitting the NuSTAR spectra only (from the whole NuSTAR exposure without matching EPIC-pn GTI).

    [1]
    Harrison F A, Craig W W, Christensen F E, et al. The nuclear spectroscopic telescope array ( NuSTAR) high-energy X-ray mission. The Astrophysical Journal, 2013, 770: 103. doi: 10.1088/0004-637x/770/2/103
    [2]
    Haardt F, Maraschi L. A two-phase model for the X-ray emission from Seyfert galaxies. The Astrophysical Journal, 1991, 380: L51. doi: 10.1086/186171
    [3]
    Haardt F, Maraschi L. X-ray spectra from two-phase accretion disks. The Astrophysical Journal, 1993, 413: 507. doi: 10.1086/173020
    [4]
    Brenneman L W, Madejski G, Fuerst F, et al. The broad-band X-ray spectrum of ic 4329a from a joint NuSTAR/Suzaku observation. The Astrophysical Journal, 2014, 788: 61. doi: 10.1088/0004-637x/788/1/61
    [5]
    Matt G, Baloković M, Marinucci A, et al. The hard X-ray spectrum of NGC 5506 as seen by NuSTAR. Monthly Notices of the Royal Astronomical Society, 2015, 447: 3029–3033. doi: 10.1093/mnras/stu2653
    [6]
    Fabian A C, Lohfink A, Kara E, et al. Properties of AGN coronae in the NuSTAR era. Monthly Notices of the Royal Astronomical Society, 2015, 451: 4375–4383. doi: 10.1093/mnras/stv1218
    [7]
    Kamraj N, Harrison F, Baloković M, et al. Coronal properties of Swift/BAT-selected Seyfert 1 AGNs observed with NuSTAR. 2018, 866: 124.
    [8]
    Baloković M, Harrison F A, Madejski G, et al. NuSTAR survey of obscured swift/BAT-selected active galactic nuclei. II. Median high-energy cutoff in Seyfert II hard X-ray spectra. The Astrophysical Journal, 2020, 905: 41. doi: 10.3847/1538-4357/abc342
    [9]
    Kang J L, Wang J X. The X-ray coronae in NuSTAR bright active galactic nuclei. The Astrophysical Journal, 2022, 929: 141. doi: 10.3847/1538-4357/ac5d49
    [10]
    Parker M L, Wilkins D R, Fabian A C, et al. The NuSTAR spectrum of Mrk 335: extreme relativistic effects within two gravitational radii of the event horizon. Monthly Notices of the Royal Astronomical Society, 2014, 443: 1723–1732. doi: 10.1093/mnras/stu1246
    [11]
    Kara E, Zoghbi A, Marinucci A, et al. Iron K and Compton hump reverberation in SWIFT J2127. 4+5654 and NGC 1365 revealed by NuSTAR and XMM–Newton. Monthly Notices of the Royal Astronomical Society, 2015, 446: 737–749. doi: 10.1093/mnras/stu2136
    [12]
    Wilkins D R, Gallo L C. Driving extreme variability: The evolving corona and evidence for jet launching in Markarian 335. Monthly Notices of the Royal Astronomical Society, 2015, 449: 129–146. doi: 10.1093/mnras/stv162
    [13]
    Panagiotou C, Walter R. Reflection geometries in absorbed and unabsorbed AGN. Astronomy & Astrophysics, 2019, 626: A40. doi: 10.1051/0004-6361/201935052
    [14]
    Jansen F, Lumb D, Altieri B, et al. XMM-Newton observatory. Astronomy & Astrophysics, 2001, 365: L1–L6. doi: 10.1051/0004-6361:20000036
    [15]
    Cappi M, De Marco B, Ponti G, et al. Anatomy of the AGN in NGC 5548 VIII. XMM-Newton’s EPIC detailed view of an unexpected variable multilayer absorber. Astronomy & Astrophysics, 2016, 592: A27. doi: 10.1051/0004-6361/201628464
    [16]
    Ponti G, Bianchi S, Muños-Darias T, et al. NuSTAR + XMM-Newton monitoring of the neutron star transient AX J1745.6-2901. Monthly Notices of the Royal Astronomical Society, 2018, 473: 2304–2323. doi: 10.1093/mnras/stx2425
    [17]
    Middei R, Bianchi S, Petrucci P O, et al. High-energy monitoring of NGC 4593 II. Broad-band spectral analysis:testing the two-corona model. Monthly Notices of the Royal Astronomical Society, 2019, 483: 4695–4705. doi: 10.1093/mnras/sty3379
    [18]
    Rani P, Stalin C S, Goswami K D. Study of X-ray variability and coronae of Seyfert galaxies using NuSTAR. Monthly Notices of the Royal Astronomical Society, 2019, 484: 5113–5128. doi: 10.1093/mnras/stz275
    [19]
    Kang J, Wang J, Kang W. NuSTAR hard X-ray spectra of radio galaxies. The Astrophysical Journal, 2020, 901: 111. doi: 10.3847/1538-4357/abadf5
    [20]
    Panagiotou C, Walter R. NuSTAR view of Swift/BAT AGN: The R–Γ correlation. Astronomy & Astrophysics, 2020, 640: A31. doi: 10.1051/0004-6361/201937390
    [21]
    Akylas A, Georgantopoulos I. Distribution of the coronal temperature in Seyfert 1 galaxies. Astronomy & Astrophysics, 2021, 655: A60. doi: 10.1051/0004-6361/202141186
    [22]
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