Relativistic Reflection Model Research Articles

X-ray spectropolarimetry of the bright atoll Serpens X-1

We present simultaneous X-ray polarimetric and spectral observations of the bright atoll source Ser X-1 carried out with the Imaging X-ray Polarimetry Explorer (IXPE), NICER, and NuSTAR. We obtain an upper limit of 2% (99% confidence level) on the polarization degree in the 2–8 keV energy band. We detect four type-I X-ray bursts, two of which during the IXPE observation. This is the first time that has IXPE observed type-I X-ray bursts, and it allows us to place an upper limit on their polarization degree; however, due to the limited total number of counts in each burst, we obtain a relatively high upper limit (80%). We confirm the presence of reflection features in the X-ray spectrum, notably a broad iron line. Fitting the data with a relativistic reflection model, we derive a disk inclination of 25°. The spectral and polarization properties are comparable with other atolls observed by IXPE, suggesting a similar accretion geometry, and the relatively low polarization is consistent with the low inclination.

Exploring the high-density reflection model for the soft excess in RBS 1124

Abstract ‘Bare’ active galactic nuclei (AGN) are a subclass of Type 1 AGN that show little or no intrinsic absorption. They offer an unobscured view of the central regions of the AGN and therefore serve as ideal targets to study the relativistic reflection features originating from the innermost regions of the accretion disc. We present a detailed broadband spectral analysis (0.3 – 70 keV) of one of the most luminous bare AGN in the local universe, RBS 1124 (z = 0.208) using a new, co-ordinated high signal-to-noise observation obtained by XMM-Newton and NuSTAR. The source exhibits a power-law continuum with Γ ∼ 1.8 along with a soft excess below 2 keV, a weak neutral iron line and curvature at high energies (∼30 keV). The broadband spectrum, including the soft excess and the high-energy continuum, is well fit by the relativistic reflection model when the accretion disc is allowed to have densities of log(ne/cm−3) ≳ 19.2. Our analysis therefore suggests that when high-density effects are considered, relativistic reflection remains a viable explanation for the soft excess.

Spectral and timing analysis of NGC 7314

The analysis of X rays observations from the active galactic nuclei is a good tool to study the properties of the supermassive black hole, the physics of the accretion disk and the corona, and to model their geometries. We present a detailed spectral and timing analysis of the Seyfert galaxy NGC 7314, we used all available public XMM-Newton and NuSTAR observations from 2001 to 2016. In order to improve the spectral modeling, we used both relativistic and non relativistic reflection models. We confirmed the presence of narrow and Broad Iron lines and we obtained constraints for other spectral proprieties. For the timing analysis, we first studied the behaviour of the power density spectrum, then we studied the time delays between the direct X rays emission and the reflected emission from the accretion disk, we measure reverberation in at least one observation, but we do not see it in others, suggesting it may be a transient phenomenon.

The exploration of the soft X-ray excess in FSRQ 4C+21.35 during the 2014 X-ray flare

ABSTRACT We report the soft X-ray excess feature and an opposite tendency between the optical/UV and X-ray bands during the flare that occurred in 2014 in flat-spectrum radio quasar (FSRQ) 4C+21.35 with the observations by Swift-XRT and Swift-UVOT. These unusual phenomena are rarely reported in FSRQs. The flare was covered from the optical to the X-ray while no prominent activity in the γ-ray was observed, which could be an indication of weaker jet activity during the flare. We carry out the spectral energy distribution ranging from the optical to X-ray based on four models which are the warm corona model (Model-1), the relativistic reflection model (Model-2), the corona with the jet model (Model-3), and the relativistic reflection with the jet model (Model-4). We find some extreme variation of the parameters in Model-2 and Model-4 and they are unable to explain the correlations between the optical/UV and X-ray bands. Model-1 and Model-3 can obtain reasonable parameters and provide an explanation for the observed opposite tendency between the optical/UV and X-ray bands. However, considering the weak jet activity during the X-ray flare, Model-1 is much preferred for describing the soft X-ray excess, hard X-ray emission, as well as the opposite tendency between the optical/UV and X-rays. Based on Model-1, it is suggested that the flare in 2014 is probably due to the variation of the geometry of the corona, which is expected to be more strictly tested with further observations in the future.

The complex spectral behavior of the newly discovered neutron star X-ray binary Swift J1858.6-0814

ABSTRACT We report on the NuSTAR observation of the newly discovered neutron star X-ray binary Swift J1858.6-0814 taken on 23rd March 2019. The light curve of the source exhibits several large flares during some time intervals of this observation. The source is softer in the high-intensity interval where the large flaring activity mainly occurs. We perform time-resolved spectroscopy on the source by extracting spectra for two different intensity intervals. The source was observed with a 3 − 79keV luminosity of ∼9.68 × 1036 ergs/s and ∼4.78 × 1036 ergs/s for high and low-intensity interval, respectively assuming a distance of 15 kpc. We find a large value of the absorbing column density ($\rm {N_{H}}\sim 1.1\times 10^{23}$ cm−2), and it appears to be uncorrelated with the observed flux of the source. Each spectrum shows evidence of Fe Kα emission in the 5 − 7keV energy band, an absorption edge around ∼7 − 8keV, and a broad Compton hump above 15keV, indicating the presence of a reflection spectrum. The observed features are well explained by the contribution of a relativistic reflection model and a partially covering absorption model. From the best-fit spectral model, we found an inner disc radius to be $4.87_{-0.96}^{+1.63}\,\,R_{ISCO}$ (for the high-intensity interval) and $5.68_{-2.78}^{+9.54}\,\,R_{ISCO}$ (for the low-intensity interval), indicating a significant disc truncation. The disk inclination is found to be relatively low, i < 330. We further place an upper limit on this source’s magnetic field strength considering the disc is truncated at the magnetospheric radius.

Accretion properties and estimation of spin of galactic black hole candidate Swift J1728.9–3613with NuSTAR during its 2019 outburst

ABSTRACT Black hole X-ray binaries (BHXRBs) play a crucial role in understanding the accretion of matter onto a black hole. Here, we focus on exploring the transient BHXRB Swift J1728.9–3613 discovered by Swift/BAT and MAXI/GSC during its January 2019 outburst. We present measurements on its accretion properties, long time-scale variability, and spin. To probe these properties, we make use of several NICER observations and an unexplored data set from NuSTAR, as well as long-term light curves from MAXI/GSC. In our timing analysis, we provide estimates of the cross-correlation functions between light curves in various energy bands. In our spectral analysis, we employ numerous phenomenological models to constrain the parameters of the system, including flavours of the relativistic reflection model Relxill to model the Fe Kα line and the >15 keV reflection hump. Our analysis reveals that: (i) Over the course of the outburst, the total energy released was ∼5.2 × 1044 ergs, corresponding to roughly 90 per cent of the mass of Mars being devoured. (ii) We find a continuum lag of 8.4 ± 1.9 d between light curves in the 2–4 and 10–20 keV bands, which could be related to the viscous inflow time-scale of matter in the standard disc. (iii) Spectral analysis reveals a spin parameter of ∼0.6–0.7 with an inclination angle of ∼45°–70° and an accretion rate during the NuSTAR observation of ${\sim}17\ \hbox{per cent}\ L_{\rm Edd}$.

Evolution of the truncation radius in GX 339−4 from XMM–Newton spectral timing

ABSTRACT We investigate the changing geometry of the accretion disc in the X-ray binary GX 339−4 using a combination of spectral and timing models to fit six XMM–Newton observations taken during the end of the 2015 X-ray outburst. We use progressively more complex models of the X-ray reflection spectra, from simple disc lines to full relativistic reflection models. For the timing analysis, the Power Spectral Densities (PSDs) are generated from the light curves in the 0.3–0.7 and 0.7–1.5 keV energy bands. We fit PSD models that assume the standard accretion disc truncates at a specific radius, inside of which are two hot-flow zones, one spectrally soft and one spectrally hard, separated by a transition radius. Finally, we combine and jointly fit the full reflection spectral models and the truncated disc PSD models. Our final model is consistent with the spectroscopic and timing data. It suggests that the truncation radius of the disc increases towards the end of the outburst, which is self-consistent with the obtained reflection fraction that is smaller. During this, the source spectrum becomes harder and the soft excess becomes more prominent. The disc truncation radius that increases as the source flux decreases is intermediate between the results of previous studies. However, our analysis of the model performance leads us to believe that higher quality data and better models are required to fully understand the GX 339-4 system and reduce the effect of systematic uncertainties.

Soft excess in AGN with relativistic X‐ray reflection

The soft X‐ray excess, emission below 2 keV over the X‐ray power‐law, is a marked spectral component in the X‐ray spectra of many Seyfert 1 galaxies. We investigate if the observed soft X‐ray excess in a sample of Seyfert 1s is in accordance with the prediction of the relativistic reflection model by analyzing the XMM‐Newton and NuSTAR spectra. The fractional difference in the soft excess (SE) obtained from the blurred reflection emission predicted (from NuSTAR) and the observed (from XMM‐Newton) luminosities show that the reflection model underestimates the SE emission in our sample. The results point to alternative models (for example, warm Comptonization) to explain the soft X‐ray excess in AGN.

Broad-band X-ray properties of black holes GRS 1758−258 and 1E 1740.7−2942: AstroSat and NuSTAR results

ABSTRACT We present the results on broad-band X-ray properties of persistent black hole binaries GRS 1758−258 and 1E 1740.7−2942 using AstroSat, NuSTAR, and Swift–XRT observations carried out during 2016–2022. We perform spectral modelling of both sources after eliminating the contamination in their LAXPC spectra from nearby X-ray sources. Preliminary spectral modelling using Comptonization and line emission (∼6.4 keV) models suggest that GRS 1758−258 occupies both dim-soft state (kTbb = 0.37 ± 0.01 keV, Γ ∼ 5.9, $L_{bol}=1{{\ \rm per\ cent}}$ of Eddington luminosity LEdd) and hard state (Γ = 1.64–2.22, kTe = 4–45 keV, Lbol = 1–5 per cent LEdd) that requires a multicolour disc blackbody model (kTin = 0.54 ± 0.01 keV) occasionally. 1E 1740.7−2942 instead is found only in hard state (Γ = 1.67–2.32, kTe = 5–16 keV, Lbol = 1–2 per cent LEdd). Reflection properties of both sources are studied by applying relativistic reflection model RELXILL to the broad-band spectra. Our results from AstroSat and NuSTAR consistently unveiled the presence of a Comptonizing region along with an ionized reflection region (ionization parameter log ξ = 2.7–3.8 and 2.7–4.7 erg cm s−1 in GRS 1758−258 and 1E 1740.7−2942, respectively) in both sources. Reflection modelling revealed GRS 1758−258 to have a high metal abundance ($A_{fe}=3.9^{+0.4}_{-0.3}$ times solar metal abundance) and inclination angle (i) of 61 ± 2°. In case of 1E 1740.7−2942, i is constrained to be 55 ± 1°. Finally, we discuss the implication of our findings in the context of accretion dynamics by comparing our results with the previous studies.

Comparing reflection and absorption models for the soft X-ray variability in the NLS1 AGN UGC 11763

ABSTRACT We present a spectral analysis of two XMM–Newton observations of the narrow-line Seyfert 1 galaxy UGC 11763. UGC 11763 shows very different soft X-ray spectral shapes in the two observations separated by 12 yr. Three spectral models are considered to explain the multi-epoch X-ray variability of UGC 11763, one based on the relativistic disc reflection model, one based on multiple partially covering absorbers combined with the warm corona model, and a hybrid model. In the first model, the X-ray variability of UGC 11763 is caused by the emission from a compact coronal region with a variable size. The resulting disc reflection component changes accordingly. A warm absorption model with a modest column density is required in this model too. In the partially covering absorption scenario, the X-ray variability of UGC 11763 is caused by the variable covering factors of two absorbers located within a region of r < ≈100rg. Moreover, the temperature and strength of the warm corona have to change significantly too to explain the variable underlying soft X-ray emission. Lastly, we investigate the possibility of variable intrinsic power-law emission from the hot corona combined with variable absorption in UGC 11763 without changing the geometry of the corona in the third model. This hybrid model provides a slightly better fit than the partially covering absorption model with improvements in fitting the iron emission band. Current CCD-resolution data cannot distinguish these spectral models for UGC 11763. Future high-resolution X-ray missions, e.g. Athena and XRISM, will test them by resolving different spectral components.

Broadband X-Ray Spectroscopy and Estimation of Spin of the Galactic Black Hole Candidate GRS 1758–258

We present the results of a broadband (0.5–78 keV) X-ray spectral study of the persistent Galactic black hole X-ray binary GRS 1758–258 observed simultaneously by Swift and NuSTAR. Fitting with an absorbed power-law model revealed a broad Fe line and reflection hump in the spectrum. We used different flavors of the relativistic reflection model for the spectral analysis. All models indicate the spin of the black hole in GRS 1758–258 is >0.92. The source was in the low hard state during the observation, with the hot electron temperature of the corona estimated to be kT e ∼ 140 keV. The black hole is found to be accreting at ∼1.5% of the Eddington limit during the observation, assuming the black hole mass of 10 M ⊙ and distance of 8 kpc.

The spin of new black hole candidate: MAXI J1803−298 observed by NuSTAR and NICER

ABSTRACT MAXI J1803−298, a newly-discovered Galactic transient, and black hole candidate was first detected by MAXI/GSC on 2021 May 01. In this paper, we present a detailed spectral analysis of MAXI J1803−298. Utilizing the X-ray reflection fitting method, we perform a joint fit to the spectra of MAXI J1803−298, respectively, observed by NuSTAR and NICER/XTI on the same day over the energy range between 0.7–79.0 keV in SIMS state, and the observed inner radius of the accretion disc is demonstrated to have extended to the ISCO radius (Rin =1.08 RISCO). Implementing the relativistic reflection model relxillCp, we found its spin (and the inclination angle i) can be constrained to be close to an extreme value, 0.991 (i ∼ 70 °), at 68 per cent confidence interval. The results suggest that MAXI J1803−298 may be a fast-rotating black hole with a large inclination angle.

Hard X-Ray broadband spectroscopy of Mrk 876: characterizing its spectrum

ABSTRACT Ever since the launch of the NuSTAR mission, the hard X-ray range is being covered to an unprecedented sensitivity. This range encodes the reflection features arising from active galactic nuclei (AGNs). Especially, the reflection of the primary radiation off the accretion disc carries the features of the manifestation of General Relativity described by the Kerr metric due to rotating supermassive black holes (SMBHs). We show the results of the broadband analyses of Mrk 876. The spectra exhibit the signature of a Compton hump at energies above 10 keV and a broadened and skewed excess at energies ∼6 keV. We establish this spectral excess to be statistically significant at 99.71 per cent (∼3σ) that is the post-trail probability through Monte Carlo simulations. Based on the spectral fit results and the significance of spectral features, the relativistic reflection model is favoured over the distant reflection scenario. The excess at ∼6 keV has a complex shape that we try to recover along with the Compton hump through a self-consistent X-ray reflection model. This allows inferring an upper limit to the black hole spin of a ≤ 0.85, while the inclination angle of the accretion disc results in i = 32.84$^{\circ }{}^{+12.22}_{-8.99}$, which is in agreement within the errors with a previous independent measurement (i = 15.4$^{\circ }{}^{+12.1}_{-6.8}$). While most spin measurements are biased towards high spin values, the black hole mass of Mrk 876 (2.4$\times 10^{8}\, \mbox{M}_\odot \le$ MSMBH $\le ~1.3 \times 10^{9}\, \mbox{M}_\odot$) lies in a range where moderately spinning SMBHs are expected. Moreover, the analyses of 12 Chandra observations reveal for the first time X-ray variability of Mrk 876 with an amplitude of 40 per cent.

X-ray timing and spectral analysis of reverberating active galactic nuclei

ABSTRACT We use the publicly available XMM–Newton archive to select a sample of 20 active galactic nuclei (AGNs) known to exhibit reverberation signatures caused by the reflection of X-rays from the corona off the accretion disc that feeds the central black hole engine. Inverse Compton scattering by energetic electrons, coupled with accretion disc fluctuations give rise to the highly variable observed X-ray spectrum, the behaviour of which is still not fully understood. We use 121 observations in 3–4 distinct spectral states for each source and calculate the time lags as a function of frequency. We fit the relativistic reflection model RELXILL and explore parameter correlations. The known scaling relationship between the black hole mass and time lag is well recovered and the continuum flux is coupled strongly to the disc reflection flux. We also find that 1H 0707−495 and IRAS 13224−3809 are well described using reflection and absorption modelling in a variety of flux states. The reflection fraction is strongly coupled to the power-law photon index and may be linked to dynamics of the emitting region. The data reveal hints of the power law evolutionary turnover when the 2–10 keV Eddington fraction is ∼0.02, the origin of which is not fully understood. Finally, we report the covering fraction is inversely correlated with the flux and power-law photon index in IRAS 13224−3809. These findings support recent studies of 1H 0707−495 where the covering fraction may contribute to the observed variability via flux modulations from non-uniform orbiting clouds.

The effect of returning radiation on relativistic reflection

ABSTRACT We study the effect of returning radiation on the shape of the X-ray reflection spectrum in the case of thin accretion discs. We show that the returning radiation mainly influences the observed reflection spectrum for a large black hole spin (a > 0.9) and a compact primary source of radiation close to the black hole at height h < 5rg, and that it dominates the reflected flux for extreme values of spin and compactness. The main effect of the returning radiation is to increase the irradiating flux on to the outer parts of the accretion disc, leading to stronger reflection and a flatter overall emissivity profile. By analysing simulated observations we show that neglecting returning radiation in existing studies of reflection-dominated sources has likely resulted in overestimating the height of the corona above the black hole. An updated version of the publicly available relxill suite of relativistic reflection models which includes returning radiation is also presented.

The evolution of the corona in MAXI J1535−571 through type-C quasi-periodic oscillations with Insight-HXMT

ABSTRACT Type-C quasi-periodic oscillations (QPOs) in black hole X-ray transients can appear when the source is in the low-hard and hard-intermediate states. The spectral-timing evolution of the type-C QPO in MAXI J1535−571 has been recently studied with Insight-HXMT. Here, we fit simultaneously the time-averaged energy spectrum, using a relativistic reflection model, and the fractional rms and phase-lag spectra of the type-C QPOs, using a recently developed time-dependent Comptonization model when the source was in the intermediate state. For the first time, we show that the time-dependent Comptonization model can successfully explain the X-ray data up to 100 keV. We find that in the hard-intermediate state the frequency of the type-C QPO decreases from 2.6 to 2.1 Hz, then increases to 3.3 Hz, and finally increases to ∼9 Hz. Simultaneously with this, the evolution of corona size and the feedback fraction (the fraction of photons up-scattered in the corona that return to the disc) indicates the change of the morphology of the corona. Compared with contemporaneous radio observations, this evolution suggests a possible connection between the corona and the jet when the system is in the hard-intermediate state and about to transit into the soft-intermediate state.

The Origin of The Soft X-Ray Excess in the Seyfert 1.5 Galaxy ESO 362-G18

We review the Seyfert 1.5 Galaxy ESO 362-G18 for exploring the origin of the soft X-ray excess. The Warm Corona and Relativistic Reflection models are two main scenarios to interpret the soft X-ray excess in active galactic nuclei at present. We use the simultaneous X-ray observation data of XMM-Newton and NuSTAR on 2016 September 24 to perform spectral analysis in two steps. First, we analyze the time-average spectra by using Warm Corona and Relativistic Reflection models. Moreover, we also explore the Hybrid model, Double Reflection model and Double Warm Corona model. We find that both the Warm Corona and Relativistic Reflection models can interpret the time-average spectra well but cannot be distinguished easily based on the time-averaged spectra fit statistics. Second, we add the rms and covariance spectra to perform the spectral analysis with time-average spectra. The result shows that the warm corona could reproduce all of these spectra well. The hot, optical thin corona and neutral distant reflection will increase their contribution with the temporal frequency, meaning that the corona responsible for X-ray continuum comes from the inner compact X-ray region and the neutral distant reflection is made of some moderate scale neutral clumps.

A Reflection Model with a Radial Disk Density Profile

Abstract In this paper we present relxilldgrad_nk, a relativistic reflection model in which the electron density of the accretion disk is allowed to have a radial power-law profile. The ionization parameter also has a nonconstant radial profile and is calculated self-consistently from the electron density and the emissivity. We show the impact of the implementation of the electron density gradient in our model by analyzing a NuSTAR spectrum of the Galactic black hole in EXO 1846–031 during its last outburst in 2019 and a putative future observation of the same source with Athena and eXTP. For the NuSTAR spectrum, we find that the new model provides a better fit, but there is no significant difference in the estimation of the model parameters. For the Athena+eXTP simulation, we find that a model without a disk density profile is unsuitable to test the spacetime metric around the compact object in the sense that modeling uncertainties can incorrectly lead to finding a nonvanishing deformation from the Kerr solution.

Spectral analysis of new black hole candidate AT2019wey observed by NuSTAR

AT2019wey is a new galactic X-ray binary that was first discovered as an optical transient by the Australia Telescope Large Area Survey (ATLAS) on December 7, 2019. AT2019wey consists of a black hole candidate as well as a low-mass companion star ($M_{\text {star }} \lesssim 0.8 M_{\odot}$) and is likely to have a short orbital period ($P_{\text {orb }} \lesssim 8$ h). Although AT2019wey began activation in the X-ray band during almost the entire outburst on March 8, 2020, it did not enter the soft state during the entire outburst. In this study, we present a detailed spectral analysis of AT2019wey in the low/hard state during its X-ray outburst on the basis of Nuclear Spectroscopic Telescope Array \emph observations. We obtain tight constraints on several of its important physical parameters by applying the State-of-art \texttt{relxill} relativistic reflection model family. In particular, we determine that the measured inner radius of the accretion disk is most likely to have extended to the innermost stable circular orbit (ISCO) radius, i.e., $R_{\text{in}}=1.38^{+0.23}_{-0.16}~R_{\text{ISCO}}$. Hence, assuming $R_{\text{in}}$=$R_{\text{ISCO}}$, we find the spin of AT2019wey to be $a_{*}\sim$ $0.97$, which is close to the extreme and an inner disk inclination angle of ~$i\sim$ $22 ^{\circ}$. Additionally, according to our adopted models, AT2019wey tends to have a relatively high iron abundance of $A_{\mathrm{Fe}}\sim$ 5 $A_{\mathrm{Fe}, \odot}$ and a high disk ionization state of $\log \xi\sim$ 3.4.

Testing the weak-equivalence principle near black holes

Today we have quite stringent constraints on possible violations of the Weak Equivalence Principle from the comparison of the acceleration of test-bodies of different composition in Earth's gravitational field. In the present paper, we propose a test of the Weak Equivalence Principle in the strong gravitational field of black holes. We construct a relativistic reflection model in which either the massive particles of the gas of the accretion disk or the photons emitted by the disk may not follow the geodesics of the spacetime. We employ our model to analyze the reflection features of a NuSTAR spectrum of the black hole binary EXO 1846-031 and we constrain two parameters that quantify a possible violation of the Weak Equivalence Principle by massive particles and X-ray photons, respectively.