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Title: The Variable Hard X-Ray Emission of NGC 4945 as Observed by NUSTAR

Abstract

Here, we present a broadband (~0.5-79 keV) spectral and temporal analysis of multiple NuSTAR observations combined with archival Suzaku and Chandra data of NGC 4945, the brightest extragalactic source at 100 keV. We observe hard X-ray (>10 keV) flux and spectral variability, with flux variations of a factor of two on timescales of 20 ks. A variable primary continuum dominates the high-energy spectrum (>10 keV) in all states, while the reflected/scattered flux that dominates at E <10 keV stays approximately constant. From modeling the complex reflection/transmission spectrum, we derive a Compton depth along the line of sight of τThomson ~ 2.9, and a global covering factor for the circumnuclear gas of ~0.15. This agrees with the constraints derived from the high-energy variability, which implies that most of the high-energy flux is transmitted rather than Compton-scattered. This demonstrates the effectiveness of spectral analysis at constraining the geometric properties of the circumnuclear gas, and validates similar methods used for analyzing the spectra of other bright, Compton-thick active galactic nuclei (AGNs). The lower limits on the e-folding energy are between 200 and 300 keV, consistent with previous BeppoSAX, Suzaku, and Swift Burst Alert Telescope observations. The accretion rate, estimated from the X-ray luminositymore » and assuming a bolometric correction typical of type 2 AGN, is in the range ~0.1-0.3 λEdd depending on the flux state. As a result, the substantial observed X-ray luminosity variability of NGC 4945 implies that large errors can arise from using single-epoch X-ray data to derive L/L Edd values for obscured AGNs.« less

Authors:
ORCiD logo [1];  [2];  [3];  [4];  [5];  [6];  [7]; ORCiD logo [8];  [8];  [9];  [10];  [11];  [12];  [9];  [13];  [14];  [9];  [7];  [15];  [16] more »;  [8];  [17] « less
  1. ASDC-ASI, Roma (Italy); INAF-Osservatorio Astronomico di Roma, Monte Porzio Catone (RM) (Italy)
  2. INAF-Osservatorio Astronomico di Bologna, Bologna (Italy)
  3. INAF-Osservatorio Astronomico di Roma, Monte Porzio Catone (RM) (Italy)
  4. Pontificia Univ. Catolica de Chile, Santiago (Chile)
  5. INAF-Osservatorio Astrofisico di Arcetri, Firenze (Italy); Harvard-Smithsonian Center for Astrophysics, Cambridge, MA (United States)
  6. Pontificia Univ. Catolica de Chile, Santiago (Chile); Space Science Institute, Boulder, CO (United States)
  7. The Pennsylvania State Univ., University Park, PA (United States)
  8. California Inst. of Technology (CalTech), Pasadena, CA (United States)
  9. Durham Univ., Durham (United Kingdom)
  10. Univ. of California, Berkeley, CA (United States)
  11. Technical Univ. of Denmark, Lyngby (Denmark)
  12. Univ. of California, Berkeley, CA (United States); Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
  13. Columbia Univ., New York, NY (United States)
  14. ETH Zurich, Zurich (Switzerland)
  15. SLAC National Accelerator Lab., Menlo Park, CA (United States)
  16. Univ. 'a Roma Tre, Roma (Italy)
  17. NASA Goddard Space Flight Center (GSFC), Greenbelt, MD (United States)
Publication Date:
Research Org.:
SLAC National Accelerator Lab., Menlo Park, CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC); National Aeronautics and Space Administration (NASA)
OSTI Identifier:
1165903
Report Number(s):
SLAC-PUB-16156
Journal ID: ISSN 1538-4357; arXiv:1407.3974
Grant/Contract Number:
AC02-76SF00515
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
The Astrophysical Journal (Online)
Additional Journal Information:
Journal Name: The Astrophysical Journal (Online); Journal Volume: 793; Journal Issue: 1; Journal ID: ISSN 1538-4357
Publisher:
Institute of Physics (IOP)
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS; Astrophysics; ASTRO; GRQC; galaxies: active; galaxies: individual (NGC 4945); X-rays: galaxies

Citation Formats

Puccetti, Simonetta, Comastri, Andrea, Fiore, Fabrizio, Arevalo, Patricia, Risaliti, Guido, Bauer, Franz E., Brandt, William N., Stern, Daniel, Harrison, Fiona A., Alexander, David M., Boggs, Steve E., Christensen, Finn E., Craig, William W., Gandhi, Poshak, Hailey, Charles J., Koss, Michael J., Lansbury, George B., Luo, Bin, Madejski, Greg M., Matt, Giorgio, Walton, Dominic J., and Zhang, Will. The Variable Hard X-Ray Emission of NGC 4945 as Observed by NUSTAR. United States: N. p., 2014. Web. doi:10.1088/0004-637X/793/1/26.
Puccetti, Simonetta, Comastri, Andrea, Fiore, Fabrizio, Arevalo, Patricia, Risaliti, Guido, Bauer, Franz E., Brandt, William N., Stern, Daniel, Harrison, Fiona A., Alexander, David M., Boggs, Steve E., Christensen, Finn E., Craig, William W., Gandhi, Poshak, Hailey, Charles J., Koss, Michael J., Lansbury, George B., Luo, Bin, Madejski, Greg M., Matt, Giorgio, Walton, Dominic J., & Zhang, Will. The Variable Hard X-Ray Emission of NGC 4945 as Observed by NUSTAR. United States. doi:10.1088/0004-637X/793/1/26.
Puccetti, Simonetta, Comastri, Andrea, Fiore, Fabrizio, Arevalo, Patricia, Risaliti, Guido, Bauer, Franz E., Brandt, William N., Stern, Daniel, Harrison, Fiona A., Alexander, David M., Boggs, Steve E., Christensen, Finn E., Craig, William W., Gandhi, Poshak, Hailey, Charles J., Koss, Michael J., Lansbury, George B., Luo, Bin, Madejski, Greg M., Matt, Giorgio, Walton, Dominic J., and Zhang, Will. Tue . "The Variable Hard X-Ray Emission of NGC 4945 as Observed by NUSTAR". United States. doi:10.1088/0004-637X/793/1/26. https://www.osti.gov/servlets/purl/1165903.
@article{osti_1165903,
title = {The Variable Hard X-Ray Emission of NGC 4945 as Observed by NUSTAR},
author = {Puccetti, Simonetta and Comastri, Andrea and Fiore, Fabrizio and Arevalo, Patricia and Risaliti, Guido and Bauer, Franz E. and Brandt, William N. and Stern, Daniel and Harrison, Fiona A. and Alexander, David M. and Boggs, Steve E. and Christensen, Finn E. and Craig, William W. and Gandhi, Poshak and Hailey, Charles J. and Koss, Michael J. and Lansbury, George B. and Luo, Bin and Madejski, Greg M. and Matt, Giorgio and Walton, Dominic J. and Zhang, Will},
abstractNote = {Here, we present a broadband (~0.5-79 keV) spectral and temporal analysis of multiple NuSTAR observations combined with archival Suzaku and Chandra data of NGC 4945, the brightest extragalactic source at 100 keV. We observe hard X-ray (>10 keV) flux and spectral variability, with flux variations of a factor of two on timescales of 20 ks. A variable primary continuum dominates the high-energy spectrum (>10 keV) in all states, while the reflected/scattered flux that dominates at E <10 keV stays approximately constant. From modeling the complex reflection/transmission spectrum, we derive a Compton depth along the line of sight of τThomson ~ 2.9, and a global covering factor for the circumnuclear gas of ~0.15. This agrees with the constraints derived from the high-energy variability, which implies that most of the high-energy flux is transmitted rather than Compton-scattered. This demonstrates the effectiveness of spectral analysis at constraining the geometric properties of the circumnuclear gas, and validates similar methods used for analyzing the spectra of other bright, Compton-thick active galactic nuclei (AGNs). The lower limits on the e-folding energy are between 200 and 300 keV, consistent with previous BeppoSAX, Suzaku, and Swift Burst Alert Telescope observations. The accretion rate, estimated from the X-ray luminosity and assuming a bolometric correction typical of type 2 AGN, is in the range ~0.1-0.3 λEdd depending on the flux state. As a result, the substantial observed X-ray luminosity variability of NGC 4945 implies that large errors can arise from using single-epoch X-ray data to derive L/L Edd values for obscured AGNs.},
doi = {10.1088/0004-637X/793/1/26},
journal = {The Astrophysical Journal (Online)},
number = 1,
volume = 793,
place = {United States},
year = {Tue Sep 02 00:00:00 EDT 2014},
month = {Tue Sep 02 00:00:00 EDT 2014}
}

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  • We present a broadband (∼0.5-79 keV) spectral and temporal analysis of multiple NuSTAR observations combined with archival Suzaku and Chandra data of NGC 4945, the brightest extragalactic source at 100 keV. We observe hard X-ray (>10 keV) flux and spectral variability, with flux variations of a factor of two on timescales of 20 ks. A variable primary continuum dominates the high-energy spectrum (>10 keV) in all states, while the reflected/scattered flux that dominates at E <10 keV stays approximately constant. From modeling the complex reflection/transmission spectrum, we derive a Compton depth along the line of sight of τ{sub Thomson} ∼more » 2.9, and a global covering factor for the circumnuclear gas of ∼0.15. This agrees with the constraints derived from the high-energy variability, which implies that most of the high-energy flux is transmitted rather than Compton-scattered. This demonstrates the effectiveness of spectral analysis at constraining the geometric properties of the circumnuclear gas, and validates similar methods used for analyzing the spectra of other bright, Compton-thick active galactic nuclei (AGNs). The lower limits on the e-folding energy are between 200 and 300 keV, consistent with previous BeppoSAX, Suzaku, and Swift Burst Alert Telescope observations. The accretion rate, estimated from the X-ray luminosity and assuming a bolometric correction typical of type 2 AGN, is in the range ∼0.1-0.3 λ{sub Edd} depending on the flux state. The substantial observed X-ray luminosity variability of NGC 4945 implies that large errors can arise from using single-epoch X-ray data to derive L/L {sub Edd} values for obscured AGNs.« less
  • We present Nuclear Spectroscopic Telescope Array (NuSTAR) hard X-ray observations of two X-ray weak broad absorption line (BAL) quasars, PG 1004+130 (radio loud) and PG 1700+518 (radio quiet). Many BAL quasars appear X-ray weak, probably due to absorption by the shielding gas between the nucleus and the accretion-disk wind. The two targets are among the optically brightest BAL quasars, yet they are known to be significantly X-ray weak at rest-frame 2-10 keV (16-120 times fainter than typical quasars). We would expect to obtain Almost-Equal-To 400-600 hard X-ray ({approx}> 10 keV) photons with NuSTAR, provided that these photons are not significantlymore » absorbed (N{sub H} {approx}< 10{sup 24} cm{sup -2}). However, both BAL quasars are only detected in the softer NuSTAR bands (e.g., 4-20 keV) but not in its harder bands (e.g., 20-30 keV), suggesting that either the shielding gas is highly Compton-thick or the two targets are intrinsically X-ray weak. We constrain the column densities for both to be N{sub H} Almost-Equal-To 7 Multiplication-Sign 10{sup 24} cm{sup -2} if the weak hard X-ray emission is caused by obscuration from the shielding gas. We discuss a few possibilities for how PG 1004+130 could have Compton-thick shielding gas without strong Fe K{alpha} line emission; dilution from jet-linked X-ray emission is one likely explanation. We also discuss the intrinsic X-ray weakness scenario based on a coronal-quenching model relevant to the shielding gas and disk wind of BAL quasars. Motivated by our NuSTAR results, we perform a Chandra stacking analysis with the Large Bright Quasar Survey BAL quasar sample and place statistical constraints upon the fraction of intrinsically X-ray weak BAL quasars; this fraction is likely 17%-40%.« less
  • We present the first sub-arcminute images of the Galactic Center above 10 keV, obtained with NuSTAR. NuSTAR resolves the hard X-ray source IGR J17456–2901 into non-thermal X-ray filaments, molecular clouds, point sources, and a previously unknown central component of hard X-ray emission (CHXE). NuSTAR detects four non-thermal X-ray filaments, extending the detection of their power-law spectra with Γ ∼ 1.3–2.3 up to ∼50 keV. A morphological and spectral study of the filaments suggests that their origin may be heterogeneous, where previous studies suggested a common origin in young pulsar wind nebulae (PWNe). NuSTAR detects non-thermal X-ray continuum emission spatially correlated with the 6.4more » keV Fe Kα fluorescence line emission associated with two Sgr A molecular clouds: MC1 and the Bridge. Broadband X-ray spectral analysis with a Monte-Carlo based X-ray reflection model self-consistently determined their intrinsic column density (∼10{sup 23} cm{sup −2}), primary X-ray spectra (power-laws with Γ ∼ 2) and set a lower limit of the X-ray luminosity of Sgr A* flare illuminating the Sgr A clouds to L{sub X} ≳ 10{sup 38} erg s{sup −1}. Above ∼20 keV, hard X-ray emission in the central 10 pc region around Sgr A* consists of the candidate PWN G359.95–0.04 and the CHXE, possibly resulting from an unresolved population of massive CVs with white dwarf masses M{sub WD} ∼ 0.9 M{sub ⊙}. Spectral energy distribution analysis suggests that G359.95–0.04 is likely the hard X-ray counterpart of the ultra-high gamma-ray source HESS J1745–290, strongly favoring a leptonic origin of the GC TeV emission.« less
  • NuSTAR observed the bright Compton-thin, narrow-line Seyfert 1 galaxy, NGC 5506, for about 56 ks. In agreement with past observations, the spectrum is well fitted by a power law with Γ ~ 1.9, a distant reflection component and narrow ionized iron lines. A relativistically blurred reflection component is not required by the data. When an exponential high-energy cutoff is added to the power law, a value of 720more » $$+130\atop{-190}$$ keV (90 percent confidence level) is found. Even allowing for systematic uncertainties, we find a 3σ lower limit to the high-energy cutoff of 350 keV, the highest lower limit to the cutoff energy found so far in an AGN by NuSTAR.« less
  • Prior to the launch of NuSTAR, it was not feasible to spatially resolve the hard (E > 10 keV) emission from galaxies beyond the Local Group. The combined NuSTAR data set, comprised of three ∼165 ks observations, allows spatial characterization of the hard X-ray emission in the galaxy NGC 253 for the first time. As a follow up to our initial study of its nuclear region, we present the first results concerning the full galaxy from simultaneous NuSTAR, Chandra, and Very Long Baseline Array monitoring of the local starburst galaxy NGC 253. Above ∼10 keV, nearly all the emission is concentrated withinmore » 100'' of the galactic center, produced almost exclusively by three nuclear sources, an off-nuclear ultraluminous X-ray source (ULX), and a pulsar candidate that we identify for the first time in these observations. We detect 21 distinct sources in energy bands up to 25 keV, mostly consisting of intermediate state black hole X-ray binaries. The global X-ray emission of the galaxy—dominated by the off-nuclear ULX and nuclear sources, which are also likely ULXs—falls steeply (photon index ≳ 3) above 10 keV, consistent with other NuSTAR-observed ULXs, and no significant excess above the background is detected at E > 40 keV. We report upper limits on diffuse inverse Compton emission for a range of spatial models. For the most extended morphologies considered, these hard X-ray constraints disfavor a dominant inverse Compton component to explain the γ-ray emission detected with Fermi and H.E.S.S. If NGC 253 is typical of starburst galaxies at higher redshift, their contribution to the E > 10 keV cosmic X-ray background is <1%.« less