Abstract
We study X-ray and soft gamma-ray spectra from the hard state of the accreting black-hole binary MAXI J1820+070. We perform an analysis of two joint spectra from NuSTAR and INTEGRAL, ...covering the range of 3–650 keV, and of an average joint spectrum over the rise of the hard state, covering the 3–2200 keV range. The spectra are well modeled by Comptonization of soft seed photons. However, the distributions of the scattering electrons are not purely thermal; we find they have substantial high-energy tails, well modeled as power laws. The photon tail in the average spectrum is detected well beyond the threshold for electron–positron pair production, 511 keV. This allows us to calculate the rate of the electron–positron pair production and put a lower limit on the size of the source from pair equilibrium. At the fitted Thomson optical depth of the Comptonizing plasma, the limit is about 4 gravitational radii. If we adopt the sizes estimated by us from the reflection spectroscopy of >20 gravitational radii, the fractional pair abundance becomes much less than unity. The low pair abundance is confirmed by the lack of both an annihilation feature and of a pair absorption cutoff above 511 keV in the average spectrum.
ABSTRACT
We have obtained new solutions and methods for the process of thermal Comptonization. We modify the solution to the kinetic equation of Sunyaev and Titarchuk to allow its application up to ...mildly relativistic electron temperatures and optical depths $\gtrsim {1}$. The solution can be used for spectral fitting of X-ray spectra from astrophysical sources. We also have developed an accurate Monte Carlo method for calculating spectra and timing properties of thermal Comptonization sources. The accuracy of our kinetic equation solution is verified by comparison with the Monte Carlo results. We also compare our results with those of other publicly available methods. Furthermore, based on our Monte Carlo code, we present distributions of the photon emission times and the evolution of the average photon energy for both up and down scattering.
Improved spectral models for relativistic reflection Niedźwiecki, Andrzej; Szanecki, Michał; Zdziarski, Andrzej A
Monthly Notices of the Royal Astronomical Society,
05/2019, Letnik:
485, Številka:
2
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
We have developed improved spectral models of relativistic reflection in the lamppost and disc-corona geometries. The models calculate photon transfer in the Kerr metric and give the ...observed photon-energy spectra produced by either thermal Comptonization or an e-folded power law incident on a cold ionized disc. Radiative processes in the primary X-ray source and in the disc are described with the currently most precise available models. Our implementation of the lamppost geometry takes into account the presence of primary sources on both sides of the disc, which is important when the disc is truncated. We thoroughly discuss the differences between our models and the previous ones.
The accreting black hole binary XTE J1752-223 was observed in a stable hard state for 25 days by the Rossi X-ray Timing Explorer (RXTE), yielding a 3-140 keV spectrum of unprecedented statistical ...quality. Its published model required a single-Comptonization spectrum reflecting from a disk close to the innermost stable circular orbit. We studied that model as well as a number of other single-Comptonization models (yielding similarly low inner radii), but found they violate a number of basic physical constraints, e.g., their compactness is much above the maximum allowed by pair equilibrium. We also studied the contemporaneous 0.55-6 keV spectrum from the Swift/X-ray Telescope and found it well fitted by an absorbed power law and a disk blackbody with the innermost temperature of 0.1 keV. The normalization of the disk blackbody corresponds to an inner radius of 20 gravitational radii and its temperature, to irradiation of the truncated disk by a hot inner flow. We have also developed a Comptonization/reflection model including the disk irradiation and intrinsic dissipation, but found that it does not yield any satisfactory fits. On the other hand, we found that the ≤10 keV band from RXTE is much better fitted by a reflection from a disk with the inner radius 100 gravitational radii, which model then underpredicts the spectrum at >10 keV by <10%. We argue that the most plausible explanation of the above results is inhomogeneity of the source, with the local spectra hardening with the decreasing radius. Our results support the presence of a complex Comptonization region and a large disk truncation radius in this source.
Abstract
A fraction of the electromagnetic radiation emitted from the surface of a geometrically thin and optically thick accretion disk of a black hole returns to the disk because of the strong ...light bending in the vicinity of the compact object (returning radiation). While such radiation clearly affects the observed spectrum of the source, it is often neglected in theoretical models. In the present paper, we study the impact of the returning radiation on relativistic reflection spectra. Assuming neutral material in the disk, we estimate the systematic uncertainties on the measurement of the properties of the system when we fit the data with a theoretical model that neglects the returning radiation. Our NICER simulations show that the inclination angle of the disk and the black hole spin parameter tend to be overestimated for low viewing angles, while no clear bias is observed for high viewing angles. The iron abundance of the disk is never overestimated. In the most extreme cases (in particular, for maximally rotating black holes), the returning radiation flattens the radial emissivity beyond a few gravitational radii. In such cases, it also produces residuals that cannot be compensated for by adjusting the parameters of models that neglect the returning radiation. This may be an important issue for the interpretation of data from future X-ray missions (e.g., Athena). When we simulate some observations with NuSTAR and fit data above 10 keV, we find that some conclusions that are valid for the NICER simulations are no longer true (e.g., we can obtain a high iron abundance).
Abstract
We study X-ray spectra from the outburst rise of the accreting black hole binary MAXI J1820+070. We find that models having the disk inclinations within those of either the binary or the jet ...imply significant changes of the accretion disk inner radius during the luminous part of the hard spectral state, with that radius changing from >100 to ∼10 gravitational radii. The main trend is a decrease with the decreasing spectral hardness. Our analysis requires the accretion flow to be structured, with at least two components with different spectral slopes. The harder component dominates the bolometric luminosity and produces strong, narrow, X-ray reflection features. The softer component is responsible for the underlying broader reflection features. The data are compatible with the harder component having a large scale height, located downstream the disk truncation radius, and reflecting mostly from remote parts of the disk. The softer component forms a corona above the disk up to some transition radius. Our findings can explain the changes of the characteristic variability timescales, found in other works, as being driven by the changes of the disk characteristic radii.
Relativistic Reflection in NGC 4151 Szanecki, Michał; Niedźwiecki, Andrzej; Zdziarski, Andrzej A.
Astrophysical journal/The Astrophysical journal,
03/2021, Letnik:
909, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Abstract
We investigate the X-ray spectrum of the Seyfert galaxy NGC 4151 using the simultaneous Suzaku/NuSTAR observation and flux-resolved INTEGRAL spectra supplemented by Suzaku and XMM ...observations. Our best spectral solution indicates that the narrow Fe K
α
line is produced in Compton-thin matter at a distance of several hundred gravitational radii. In such a model, we find a weak but significant relativistic reflection from a disk truncated at about 10 gravitational radii when the source is in bright X-ray states. We do not find evidence either for or against the presence of relativistic reflection in the dim X-ray state. We also rule out models with X-ray emission dominated by a source located very close to the black hole horizon, which was proposed in previous works implementing the lamppost geometry for this source. We point out that accurate computation of the thermal Comptonization spectrum and its distortion by strong gravity is crucial in applications of the lamppost geometry to the NuSTAR data.
Abstract
We study X-ray and soft gamma-ray spectra from the hard state of the accreting black hole binary MAXI J1820+070. We perform an analysis of joint spectra from HXMT, NuSTAR, and INTEGRAL. We ...find an overall agreement between the spectra from all three satellites. Satisfactory fits to the data require substantial spectral complexity, with our models including two Comptonization regions and their associated disk reflection, a disk blackbody, and a narrow Fe K
α
line. Our fits confirm the presence of the truncation of the reflecting optically thick disk at least at >10 gravitational radii. However, we find that the HXMT data alone cannot significantly constrain the disk inner radii.
Abstract
The black hole spin parameter,
a
*
, was measured to be close to its maximum value of 1 in many accreting X-ray binaries. In particular,
a
*
≳ 0.9 was found in a number of studies of LMC ...X-1. These measurements were claimed to take into account both statistical and systematic uncertainties. We perform new measurements using a recent simultaneous observation by NICER and NuSTAR, providing a data set of high quality. We use the disk continuum method together with improved models for coronal Comptonization. With the standard relativistic disk model and optically thin Comptonization, we obtain values of
a
*
similar to those obtained before. We then consider modifications to the standard model. Using a color correction of 2, we find
a
*
≈ 0.64–0.84. We then consider disks with dissipation in surface layers. To account for that, we assume the standard disk is covered by a warm and optically thick Comptonizing layer. Our model with the lowest
χ
2
then yields
a
*
≈
0.40
−
0.32
+
0.41
. In order to test the presence of such effects in other sources, we also study an X-ray observation of Cyg X-1 by Suzaku in the soft state. We confirm the previous findings of
a
*
> 0.99 using the standard model, but then we find a weakly constrained
a
*
≈
0.82
−
0.74
+
0.16
when including an optically thick Comptonizing layer. We conclude that determinations of the spin using the continuum method can be highly sensitive to the assumptions about the disk structure.
ON THE LAMPPOST MODEL OF ACCRETING BLACK HOLES Nied wiecki, Andrzej; Zdziarski, Andrzej A.; Szanecki, Micha
Astrophysical journal. Letters,
04/2016, Letnik:
821, Številka:
1
Journal Article
Recenzirano
Odprti dostop
ABSTRACT We study the lamppost model, in which the X-ray source in accreting black hole (BH) systems is located on the rotation axis close to the horizon. We point out a number of inconsistencies in ...the widely used lamppost model relxilllp, e.g., neglecting the redshift of the photons emitted by the lamppost that are directly observed. They appear to invalidate those model fitting results for which the source distances from the horizon are within several gravitational radii. Furthermore, if those results were correct, most of the photons produced in the lamppost would be trapped by the BH, and the luminosity generated in the source as measured at infinity would be much larger than that observed. This appears to be in conflict with the observed smooth state transitions between the hard and soft states of X-ray binaries. The required increase of the accretion rate and the associated efficiency reduction also present a problem for active galactic nuclei. Then, those models imply the luminosity measured in the local frame is much higher than that produced in the source and measured at infinity, due to the additional effects of time dilation and redshift, and the electron temperature is significantly higher than that observed. We show that these conditions imply that the fitted sources would be out of the e pair equilibrium. On the other hand, the above issues pose relatively minor problems for sources at large distances from the BH, where relxilllp can still be used.
PDF
