On Measuring the Variation of High-energy Cutoff in Active Galactic Nuclei

Zhang, Jixian; Wang, Junxian; Zhu, Fei-Fan

doi:10.3847/1538-4357/aacf92

Cited by 16 publications

(36 citation statements)

References 51 publications

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“…[64]) could reproduce a flatter PSD in harder band, thus weaker "softer-when-brighter" trends at shorter timescales, it is unable to explain other essential observations in literature, including the dynamical/geometrical variations of the corona in AGNs (e.g., refs. [25,26]), and the "hotter-when-brighter" pattern recently observed [21].…”

Section: Summariesmentioning

confidence: 70%

“…[58]. One key difference is that, in our scheme the long term "softer-when-brighter" trend is not driven by corona cooling but corona heating and inflation, as recent NuSTAR observations show hints of hotter corona at higher fluxes in individual AGNs [21]. We also show in our scheme that the long term spectral variation in individual AGNs should be intrinsically different from that between AGNs with different accretion rates (see Figure 5).…”

Section: Short Term Brighteningmentioning

confidence: 79%

“…The inflation of the hotter corona leads to smaller opacity (likely also due to higher escape velocity of outflowing electrons), thus softer X-ray spectrum could be yielded. The hotter temperature and/or the higher outflowing velocity are responsible for the higher cutoff energy observed during the brighter and softer phases [21]. Note the…”

Section: The Underlying Processesmentioning

confidence: 96%

“…Furthermore, the effect of corona temperature has not been considered in the inward-propagation model, which simply assumes softer X-ray emission is produced at larger radii. Assuming in the scheme the outer corona where softer X-ray emission is produced has lower temperature, the inward-propagation model would yield lower effective corona temperature during the brighter and softer phases, contrary to the "hotterwhen-brighter" pattern seen in AGNs [21]. Also note that the studies, which compared the inward-propagation model with AGN observations (e.g., ref.…”

Section: The Underlying Processesmentioning

confidence: 98%

“…Alternatively, the rapid X-ray spectral variabilities in individual AGNs could be attributed to variations of the coronal properties. For instance, Zhang et al [21] found with a small sample of sources that AGNs tend to have larger hard X-ray cutoff energies (thus hotter corona) when they brighten in X-ray. This is in direct contrast to the corona cooling hypothesis aforementioned, and requires a smaller corona opacity at brighter phases to produce the softer spectra (e.g., ref.…”

Section: Introductionmentioning

confidence: 99%

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More than softer-when-brighter: The X-ray powerlaw spectral variability in NGC 4051

Wang

Cai

et al. 2020

Sci. China Phys. Mech. Astron.

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The powerlaw X-ray spectra of active galactic nuclei at moderate to high accretion rates normally appear softer when they brighten, for which the underlying mechanisms are yet unclear. Utilizing XMM-Newton observations and excluding photons < 2 keV to avoid contamination from the soft excess, in this work we scrutinize the powerlaw spectral variability of NCG 4051 from two new aspects. We first find that a best-fit "softer-when-brighter" relation is statistically insufficient to explain the observed spectral variabilities, and intervals deviated from the empirical relation are clearly visible in the light curve of 2-4 keV/4-10 keV count rate ratio. The deviations are seen not only between but also within individual XMM-Newton exposures, consistent with random variations of the corona geometry or inner structure (with timescales as short as ∼1 ks), in addition to those behind the smooth "softer-when-brighter" trend. We further find the "softer-when-brighter" trend gradually weakens with the decreasing timescale (from ∼ 100 ks down to 0.5 ks). These findings indicate that the powerlaw spectral slope is not solely determined by its brightness. We propose a two-tier geometry, including flares/nano-flares on top of the inner disc and an embedding extended corona (heated by the flares, in analogy to solar corona) to explain the observations together with other observational clues in literature. Rapid spectral variabilities could be due to individual flares/nano-flares, while slow ones are driven by the variations in the global activity of inner disc region (akin to the variation of solar activity, but not the accretion rate) accompanied with heating/cooling and inflation/contraction of the extended corona.

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Section: Summariesmentioning

confidence: 70%

Section: Short Term Brighteningmentioning

confidence: 79%

Section: The Underlying Processesmentioning

confidence: 96%

Section: The Underlying Processesmentioning

confidence: 98%

Section: Introductionmentioning

confidence: 99%

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More than softer-when-brighter: The X-ray powerlaw spectral variability in NGC 4051

Wang

Cai

et al. 2020

Sci. China Phys. Mech. Astron.

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The first simultaneous X-ray broadband view of Mrk 110 with XMM-Newton and NuSTAR

Porquet

Reeves

Grosso

et al. 2021

A&A

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Context. Soft and hard X-ray excesses, compared to the continuum power-law shape between ∼2−10 keV, are common features observed in the spectra of active galactic nuclei (AGN) and are associated with the accretion disc-corona system around the supermassive black hole. However, the dominant process at work is still highly debated and has been proposed to be either relativistic reflection or Comptonisation. Such an investigation can be problematic for AGN that have significant intrinsic absorption, either cold or warm, which can severely distort the observed continuum. Therefore, AGN with no (or very weak) intrinsic absorption along the line-of-sight, called bare AGN, are the best targets for directly probing disc-corona systems. Aims. We aim to characterise the main X-ray spectral physical components from the bright bare broad-line Seyfert 1 AGN Mrk 110, as well as the physical process(es) at work in its disc-corona system viewed almost face-on. Methods. We perform the X-ray broadband spectral analysis thanks to two simultaneous XMM-Newton and NuSTAR observations performed on November 16−17, 2019, and April 5−6, 2020. We also use a deep NuSTAR observation obtained in January 2017 for the spectral analysis above 3 keV. Results. The broadband X-ray spectra of Mrk 110 are characterised by the presence of a prominent and absorption-free smooth soft X-ray excess, moderately broad O VII and Fe Kα emission lines, and a lack of a strong Compton hump. The continuum above ∼3 keV is very similar at both epochs, while some variability (stronger when brighter) is present for the soft X-ray excess. A combination of soft and hard Comptonisation by a warm and hot corona, respectively, plus mildly relativistic disc reflection reproduce the broadband X-ray continuum very well. The inferred warm corona temperature, kTwarm ∼ 0.3 keV, is similar to the values found in other sub-Eddington AGN, whereas the hot corona temperature, kThot ∼ 21−31 keV (depending mainly on the assumed hot corona geometry), is found to be in the lower range of the values measured in AGN.

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NuSTARspectral analysis of three Seyfert galaxies: NGC 3227, NGC 5548, and MR 2251−178

Pal

Stalin

Mallick

et al. 2022

A&A

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Context. The observed nuclear X-ray emission in the radio-quiet category of active galactic nuclei (AGN) is believed to be from a compact region, the corona, which is situated in the vicinity of central supermassive black holes. The shape of the X-ray continuum depends on, among other factors, the temperature of the corona (kTe). The launch of the Nuclear Spectroscopic Telescope Array (NuSTAR) has led to the determination of the high energy cutoff (Ecut), and thereby kTe, in many AGN. In a handful of sources, multiple observations with NuSTAR have also revealed changes in Ecut. Aims. In this work we aimed to investigate the variation in kTe in three AGN, namely NGC 3227, NGC 5548, and MR 2251−178, using more than one epoch of data on a source from NuSTAR. Methods. We carried out a spectral analysis of multiple epochs of data acquired using NuSTAR on the three sources, including a few new observations that had not yet been published. By fitting a Comptonization model to the data, we determined the temperature of the corona and investigated changes in kTe, if there were any, in these sources. Results. In NGC 3227, we found evidence for variation in kTe. We found no correlation of kTe, photon index (Γ), reflection fraction (R), or optical depth (τ) with flux, while τ is found to anti-correlate with kTe. This could be due to more than one physical process at work in the source causing the change in kTe. Conclusive evidence for the variation in kTe is not found in MR 2251−178 or NGC 5548.

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On Measuring the Variation of High-energy Cutoff in Active Galactic Nuclei

Cited by 16 publications

References 51 publications

More than softer-when-brighter: The X-ray powerlaw spectral variability in NGC 4051

More than softer-when-brighter: The X-ray powerlaw spectral variability in NGC 4051

The first simultaneous X-ray broadband view of Mrk 110 with XMM-Newton and NuSTAR

NuSTARspectral analysis of three Seyfert galaxies: NGC 3227, NGC 5548, and MR 2251−178

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