NuSTAR Hard X-ray View of Low-luminosity Active Galactic Nuclei: High-energy Cutoff and Truncated Thin Disk. (arXiv:1811.10657v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Younes_G/0/1/0/all/0/1">George Younes</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+Ptak_A/0/1/0/all/0/1">Andrew Ptak</a> (2), <a href="http://arxiv.org/find/astro-ph/1/au:+Ho_L/0/1/0/all/0/1">Luis C. Ho</a> (3), <a href="http://arxiv.org/find/astro-ph/1/au:+Xie_F/0/1/0/all/0/1">Fu-Guo Xie</a> (4), <a href="http://arxiv.org/find/astro-ph/1/au:+Terasima_Y/0/1/0/all/0/1">Yuichi Terasima</a> (5), <a href="http://arxiv.org/find/astro-ph/1/au:+Yuan_F/0/1/0/all/0/1">Feng Yuan</a> (4), <a href="http://arxiv.org/find/astro-ph/1/au:+Huppenkothen_D/0/1/0/all/0/1">Daniela Huppenkothen</a> (6), <a href="http://arxiv.org/find/astro-ph/1/au:+Yukita_M/0/1/0/all/0/1">Mihoko Yukita</a> (2,7) ((1) The George Washington University, Washington DC, USA, (2) NASA Goddard Space Flight Center, MD, USA, (3) Peking University, Beijing, China, (4) Shanghai Astronomical Observatory, Shanghai, China, (5) Ehime University, Ehime, Japan, (6) University of Washington, Seattle WA, USA, (7) Johns Hopkins Universit, MD, USA)
We report the analysis of simultaneous XMM-Newton+NuSTAR observations of two
low-luminosity Active Galactic Nuclei (LLAGN), NGC 3998 and NGC 4579. We do not
detect any significant variability in either source over the ~3 day length of
the NuSTAR observations. The broad-band 0.5-60 keV spectrum of NGC 3998 is best
fit with a cutoff power-law, while the one for NGC 4579 is best fit with a
combination of a hot thermal plasma model, a power-law, and a blend of
Gaussians to fit an Fe complex observed between 6 and 7 keV. Our main spectral
results are the following: (1) neither source shows any reflection hump with a
$3sigma$ reflection fraction upper-limits $R<0.3$ and $R<0.18$ for NGC 3998
and NGC 4579, respectively; (2) the 6-7 keV line complex in NGC 4579 could
either be fit with a narrow Fe K line at 6.4 keV and a moderately broad Fe XXV
line, or 3 relatively narrow lines, which includes contribution from Fe XXVI;
(3) NGC 4579 flux is 60% brighter than previously detected with XMM-Newton,
accompanied by a hardening in the spectrum; (4) we measure a cutoff energy
$E_{rm cut}=107_{-18}^{+27}$ keV in NGC 3998, which represents the lowest and
best constrained high-energy cutoff ever measured for an LLAGN; (5) NGC 3998
spectrum is consistent with a Comptonization model with either a sphere
($tauapprox3pm1$) or slab ($tauapprox1.2pm0.6$) geometry, corresponding
to plasma temperatures between 40 and 150~keV. We discuss these results in the
context of hard X-ray emission from bright AGN, other LLAGN, and hot accretion
flow models.
We report the analysis of simultaneous XMM-Newton+NuSTAR observations of two
low-luminosity Active Galactic Nuclei (LLAGN), NGC 3998 and NGC 4579. We do not
detect any significant variability in either source over the ~3 day length of
the NuSTAR observations. The broad-band 0.5-60 keV spectrum of NGC 3998 is best
fit with a cutoff power-law, while the one for NGC 4579 is best fit with a
combination of a hot thermal plasma model, a power-law, and a blend of
Gaussians to fit an Fe complex observed between 6 and 7 keV. Our main spectral
results are the following: (1) neither source shows any reflection hump with a
$3sigma$ reflection fraction upper-limits $R<0.3$ and $R<0.18$ for NGC 3998
and NGC 4579, respectively; (2) the 6-7 keV line complex in NGC 4579 could
either be fit with a narrow Fe K line at 6.4 keV and a moderately broad Fe XXV
line, or 3 relatively narrow lines, which includes contribution from Fe XXVI;
(3) NGC 4579 flux is 60% brighter than previously detected with XMM-Newton,
accompanied by a hardening in the spectrum; (4) we measure a cutoff energy
$E_{rm cut}=107_{-18}^{+27}$ keV in NGC 3998, which represents the lowest and
best constrained high-energy cutoff ever measured for an LLAGN; (5) NGC 3998
spectrum is consistent with a Comptonization model with either a sphere
($tauapprox3pm1$) or slab ($tauapprox1.2pm0.6$) geometry, corresponding
to plasma temperatures between 40 and 150~keV. We discuss these results in the
context of hard X-ray emission from bright AGN, other LLAGN, and hot accretion
flow models.
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