arXiv subjects

Christos Panagiotou

Publications and source records attributed to Christos Panagiotou.

44 records · Page 3Linked to original sources

arXiv2022

A physical model for the UV/optical power spectra of AGN

The UV/optical variability of AGN has long been thought to be driven by the X-ray illumination of the accretion disk. However, recent multi-wavelength campaigns of nearby Seyfert galaxies seem to challenge this paradigm, with an apparent discrepancy between observations and the underlying theory. In order to further probe the connection between the UV/optical and X-ray variability in AGN we developed a physical model to reproduce the UV/optical power spectra (PSDs) of AGN assuming the thermal reprocessing of the X-rays in the disk. This model offers a novel way to probe the innermost regions of AGN. We use our model to study the variability of NGC 5548 and we infer that the X-ray and UV/optical PSDs as well as the interband UV/optical time lags are all well reproduced. We also derive constraints on the source physical parameters, such as the X-ray corona height and the accretion rate. Our results suggest that X-ray disk reprocessing accounts for the full variability properties of this AGN, within the considered time scales. Using earlier data of NGC 5548, we also show that our model can reproduce its PSD in different epochs, establishing the feasibility of using PSD modelling to investigate the time evolution of a source.

astro-ph.GA↗

arXiv2022

SRGA J181414.6-225604: A new Galactic symbiotic X-ray binary outburst triggered by an intense mass loss episode of a heavily obscured Mira variable

We present the discovery and multi-wavelength characterization of SRGA J181414.6-225604, a Galactic hard X-ray transient discovered during the ongoing SRG/ART-XC sky survey. Using data from the Palomar Gattini-IR survey, we identify a spatially and temporally coincident variable infrared (IR) source, IRAS 18111-2257, and classify it as a very late-type (M7-M8), long period ($1502 \pm 24$ days) and luminous ($M_K\approx -9.9 \pm 0.2$) O-rich Mira donor star located at a distance of $\approx 14.6^{+2.9}_{-2.3}$ kpc. Combining multi-color photometric data over the last $\approx 25$ years, we show that the IR counterpart underwent a recent (starting $\approx 800$ days before the X-ray flare) enhanced mass loss (reaching $\approx 2.1 \times 10^{-5}$ M$_\odot$ yr$^{-1}$) episode resulting in an expanding dust shell obscuring the underlying star. Multi-epoch follow-up from Swift, NICER and NuSTAR reveal a $\approx 200$ day long X-ray outburst reaching a peak luminosity of $L_X \approx 2.5 \times 10^{36}$ erg s$^{-1}$, characterized by a heavily absorbed ($N_{\rm H} \approx 6\times 10^{22}$ cm$^{-2}$) X-ray spectrum consistent with an optically thick Comptonized plasma. The X-ray spectral and timing behavior suggest the presence of clumpy wind accretion together with a dense ionized nebula overabundant in silicate material surrounding the compact object. Together, we show that SRGA J181414.6-225604 is a new symbiotic X-ray binary in outburst, triggered by an intense dust formation episode of a highly evolved donor. Our results offer the first direct confirmation for the speculated connection between enhanced late-stage donor mass loss and active lifetimes of the symbiotic X-ray binaries.

astro-ph.HE↗

arXiv2021

NuSTAR view of heavily absorbed AGN: The $R-N_\text{H}$ correlation

The nature of the putative torus and the outer geometry of active galactic nuclei (AGN) are still rather unknown and the subject of active research. Improving our understanding of them is crucial for developing a physical picture for the structure of AGN. The main goal of this work is to investigate the outer geometry of AGN by studying the observed hard X-ray spectrum of obscured sources. We primarily aim at researching the reflected emission in these sources. To that end, we analysed archived NuSTAR observations of a sample of nearby AGN, whose X-ray emission has been found to be heavily absorbed, with $10^{23} 1.5\cdot 10^{24}\text{ cm}^{-2}$) regime.

astro-ph.GA↗

arXiv2020

NuSTAR view of Swift/BAT AGN: The $R$-$Γ$ correlation

The reflection hump is a prominent feature in the hard X-ray spectrum of active galactic nuclei (AGN). Its exact shape and its correlation to other quantities provide valuable information about the inner and outer regions of an AGN. Our main goal is to study the reflection hump in a large sample of nearby AGN. We aim to investigate the evolution of reflection with absorption and its correlation with the spectral index. We analysed archived NuSTAR observations of the 70-month BAT catalogue AGN. By performing a detailed spectral analysis, we were able to constrain the spectral parameters and to investigate the reflection emission in a large sample of individual sources. The reflection strength was found to be strongly correlated with the power-law slope in unabsorbed sources, pointing towards disc reflection for these sources. Different possible explanations were tested and the most likely one is that the corona is moving either towards or away from the disc with a moderately relativistic velocity. An $R-Γ$ correlation was not detected for absorbed sources. In addition, these AGN feature harder spectra, suggesting intrinsic differences between the two classes or a slab geometry for the X-ray source.

astro-ph.GA↗

arXiv2020

Multiwavelength power-spectrum analysis of NGC 5548

NGC 5548 was recently monitored intensively from NIR to X-rays as part of the STORM campaign. Its disc emission was found to lag behind the observed X-rays, while the measured time lag was increasing with wavelength. These results are consistent with the assumption that short term variability in AGN emission is driven by the X-ray illumination of the accretion disc. In this work, we studied the power spectrum of UV/optical and X-ray emission of NGC 5548, using the data of the STORM campaign as well as previous Swift data, in order to investigate the relation between the UV/optical and X-ray variability and to examine its consistency with the above picture. We demonstrate that even the power spectrum results are compatible with a standard disc being illuminated by X-rays, with low accretion rates, but the details are not entirely consistent with the results from the modelling of the "τ vs λ" relation. The differences indicate that the inner disc might be covered by a "warm corona" which does not allow the detection of UV/optical emission from the inner disc. Finally, we found strong evidence that the UV emission of NGC 5548 is not stationary.

astro-ph.GA↗

arXiv2019

Reflection geometries in absorbed and unabsorbed AGN

The hard X-ray emission of active galactic nuclei (AGN), and in particular, the reflection component, is shaped by the innermost and outer regions of the galactic nucleus. Our main goal is to investigate the variation of the Compton hump amongst a population of sources and correlate it with other spectral properties to constrain the source geometry. We studied the NuSTAR hard X-ray spectra of a sample of 83 AGN and performed a detailed spectral analysis of each of them. Based on their spectral shape, we divided the sample into five categories and also studied their stacked spectra. We found a stronger reflection in mildly obscured sources, which verifies the results reported in previous works. In addition, the reflection behaviour, and probably origin, varies with absorption. The accretion disc seems to be the main reflector in unabsorbed sources. A clumpy torus seems to produce most of the reflection in obscured sources. The filling factor of the clouds surrounding the active nucleus is a key parameter that drives the appearance of AGN. Finally, we found that the Fe line and the Compton hump are roughly correlated, as expected.

astro-ph.GA↗

arXiv2017

The environment of the wind-wind collision region of $η$ Carinae

$η$ Carinae is a colliding wind binary hosting two of the most massive stars and featuring the strongest wind collision mechanical luminosity. The wind collision region of this system is detected in X-rays and $γ$-rays and offers a unique laboratory for the study of particle acceleration and wind magneto-hydrodynamics. Our main goal is to use X-ray observations of $η$ Carinae around periastron to constrain the wind collision zone geometry and understand the reasons for its variability. We analysed 10 Nuclear Spectroscopic Telescope Array (NuSTAR) observations, which were obtained around the 2014 periastron. The NuSTAR array monitored the source from 3 to 30 keV, which allowed us to grasp the continuum and absorption parameters with very good accuracy. We were able to identify several physical components and probe their variability. The X-ray flux varied in a similar way as observed during previous periastrons and largely as expected if generated in the wind collision region. The flux detected within ~10 days of periastron is lower than expected, suggesting a partial disruption of the central region of the wind collision zone. The Fe K$α$ line is likely broadened by the electrons heated along the complex shock fronts. The variability of its equivalent width indicates that the fluorescence region has a complex geometry and that the source obscuration varies quickly with the line of sight.

astro-ph.HE↗

arXiv2017

Fermi acceleration under control: $η$ Carinae

We used data from the Fermi Large Area Telescope obtained during the last 7 years and spanning two passages of $η$ Carinae at periastron and compared them with the predictions of particle acceleration in hydrodynamic simulations. Two emission components can be distinguished. The low-energy component cuts off below 10 GeV and its flux, modulated by the orbital motion, varies by a factor less than 2. Short-term variability occurs at periastron. The flux of the high energy component varies by a factor 3-4 but differently during the two periastrons. The variabilities observed at low-energy, including some details of them, and these observed at high-energy during the first half of the observations, do match the prediction of the simulation, assuming a surface magnetic field in the range 0.4-1 kG. The high-energy component and the thermal X-ray emission were weaker than expected around the second periastron suggesting a modification of the wind density in the inner wind collision zone. Diffuse shock acceleration in the complex geometry of the wind collision zone of $η$ Carinae provides a convincing match to the observations and new diagnostic tools to probe the geometry and energetics of the system.

astro-ph.HE↗