X-ray Spectral Index Research Articles

A Self-consistent Treatment of the Line-driving Radiation Force for Active Galactic Nuclei Outflows: New Prescriptions for Simulations

Flows driven by photons have been studied for almost a century, and a quantitative description of the radiative forces on atoms and ions is important for understanding a wide variety of systems with outflows and accretion disks, such as active galactic nuclei (AGN). Quantifying the associated forces is crucial to determining how these outflows enable interactive mechanisms within these environments, such as AGN feedback. The total number of spectral lines in any given ion of the outflow material must be tabulated in order to give a complete characterization of this force. Here, we provide calculations of the dimensionless line force multiplier for AGN environments. For a wide array of representative AGN sources, we explicitly calculate the photoionization balance at the proposed wind-launching region above the accretion disk, compute the strength of the line-driving force on the gas, and revisit and formalize the role of the commonly used ionization parameter ξ in ultimately determining the line-driving force. We perform these computations and analyses for a variety of AGN central source properties, such as black hole mass, initial wind velocity, and number density. We find that, while useful, the ionization parameter provides an incomplete description of the overall ionization state of the outflow material. We use these findings to provide an updated method for calculating the strength of the radiative line-driving using both the X-ray spectral index Γ X and the ionization parameter.

Probing the origin of the two-component structure of broad-line region by reverberation mapping of an extremely variable quasar

ABSTRACT The physical origins of quasar components, such as the broad-line region (BLR) and dust torus, remain under debate. To gain insights into them, we focused on changing-state quasars (CSQs) which provide a unique perspective through structural changes associated with accretion disc state transitions. We targeted SDSS J125809.31+351943.0, an extremely variable CSQ, to study its central core structure and kinematics. We conducted reverberation mapping with optical spectroscopy to explore the structure of the BLR and estimate the black hole mass. The results from Hβ reverberation mapping indicated a black hole mass of $10^{9.64^{+0.11}_{-0.20}}\rm {M_\odot }$. Additionally, we analysed variations in the optical to X-ray spectral indices, αox, before and after the state transition, to investigate the accretion disc. These variations in αox and the Eddington ratio (from 0.4 per cent to 2.4 per cent) exhibitied behaviour similar to state transitions observed in X-ray binary systems. Spectral analysis of Hβ revealed a predominantly double-peaked profile during dim periods, transitioning to include a single-peaked component as the quasar brightened, suggesting that Hβ contains a mixture of two components. Each of these components has its distinct characteristics: the first is a double-peaked profile that remains stable despite changes in the accretion rate, while the second is a variable single-peaked profile. Using time-lags from reverberation mapping, we estimated the spatial relationships between these BLR components, the accretion disc, and the dust torus. Our results suggest that the BLR consists of two distinct components, each differing in location and origin.

Statistical X-ray analysis of gamma-ray-emitting radio galaxies

Abstract We systematically analyzed Swift/XRT X-ray data of gamma-ray-emitting radio galaxies (RGs) to study the origin of X-ray radiation. We studied the short-duration variability of spectra by using Swift/XRT data for 20 RGs. We found that gamma-ray-emitting RGs with a higher gamma-ray-to-X-ray flux ratio show a strong harder-when-brighter behavior. In addition, gamma-ray-emitting RGs with soft gamma-ray spectral index tend to have a hard X-ray spectral index and vice versa. Based on these properties, we imply a trend that gamma-ray-emitting RGs could be roughly classified into three types of X-ray emission: jet-dominated type, disk/corona-dominated type, and intermediate type. This classification is quite consistent with the optical and radio classification of RGs. In addition, we also discuss this result in terms of the Eddington luminosity ratio. These interpretations should be confirmed using larger samples of RGs in the future.

Understanding the very high energy γ-ray excess in nearby blazars using leptonic model

ABSTRACT The availability of simultaneous X-ray and very high energy (VHE) observations of blazars helps to identify the plausible radiative contributors to the VHE emission. Under leptonic scenario, the VHE emission from BL Lacs are attributed to the synchrotron self Compton (SSC) emission. However, many BL Lacerate (BL Lacs) have shown significant hardening at VHE after correction for the extragalactic background light (EBL) attenuation. We study the spectral hardening of two nearby BL Lac objects, Mkn 421 and Mkn 501 having most number of simultaneous X-ray and VHE observations available among all the blazars. These BL Lacs are relatively close and the effect of EBL attenuation is relatively minimal/negligible. We study the scatter plot between the X-ray spectral indices and intrinsic VHE indices to identify the plausible origin of the VHE emission. For Mkn 501, the VHE spectral indices are steeper than X-ray spectra, suggesting the scattering process happening at extreme Klein–Nishina regime. On the other hand, for Mkn 421, the VHE spectra is remarkably harder than the X-ray spectra, which suggests an additional emission mechanism other than the SSC process. We show this hard VHE spectrum of Mkn 421 can be explained by considering the inverse Compton (IC) emission from a broken power-law electron distribution with Maxwellian pileup. The possibility of the hadronic contribution at VHE γ-rays is also explored by modelling the hard spectrum under the photomeson process.

Revisiting the Chandra Observation on the Region of PSR J1J1809–193: Indication of the Existence of an X-Ray Halo and Implication for the Origin of HESS J1809–193

HESS J1809–193 is an extended TeV γ-ray source and the origin of its γ-ray emission remains ambiguous. The pulsar wind nebula (PWN) of PSR J1J1809–193 lying inside the extended γ-ray emission is a possible candidate. Powered by the central pulsar, ultrarelativistic electrons in the PWN can produce radio to X-ray emission through synchrotron and γ-ray emission by inverse Compton (IC) scattering. To check whether this PWN is the counterpart of HESS J1809–193, we analyzed the Chandra X-ray radial intensity profile and spectral index profile of this PWN. We then adopted a one-zone isotropic diffusion model to fit the keV and TeV data. We found diffuse nonthermal X-ray emission extending beyond the PWN, which is likely an X-ray halo radiated by escaping electron/positron pairs from the PWN. A relatively strong magnetic field of ∼20 μG is required to explain the spatial evolution of the X-ray spectrum (i.e., the significant softening of the spectrum with increasing distance from the pulsar), which, however, would suppress the IC radiation of pairs. Our result implies that a hadronic component may be needed to explain HESS J1809–193.

Disk-corona modeling for spectral index and luminosity correlation of tidal disruption events

We present a relativistic disk-corona model for a steady state advective accretion disk to explain the UV to X-ray spectral index $\alpha_{\text{OX}}$ evolution of \textbf{four} tidal disruption event (TDE) sources XMMSL2J1446, XMMSL1J1404, XMMSL1J0740, \textbf{and AT2018fyk}. The viscous stress in our model depends on gas ($P_g$) and total ($P_t$) pressures as $\tau_{r\phi} \propto P_g^{\mu} P_t^{1-\mu}$, where $\mu$ is a constant. We compare various steady and time-dependent sub-Eddington TDE accretion models along with our disk-corona model to the observed $\alpha_{\text{OX}}$ of TDE sources and find that the disk-corona model agrees with the observations better than the other models. We find that $\mu$ is much smaller than unity for TDE sources XMMSL2J1446, XMMSL1J1404, and XMMSL1J0740. We also compare the relativistic model with a non-relativistic disk-corona model. The relativistic accretion dynamics reduce the spectral index relative to the non-relativistic accretion by increasing the energy transport to the corona. We estimate the mass accretion rate for all the sources and find that the observed luminosity follows a nearly linear relation with the mass accretion rate. The ratio of X-ray luminosity from the disk to the corona increases with the mass accretion rate. The observed $\alpha_{\text{OX}}$ shows positive and negative correlations with luminosity. The disk-corona model explains the negative correlation seen in the TDE sources XMMSL1J0740, XMMSL2J1446, and XMMSL1J1404. However, TDE AT2018fyk shows a positive correlation at higher luminosity and shows a better fit when a simple spherical adiabatic outflow model is added to the relativistic disk-corona model. Even though the disk luminosity dominates at a higher mass accretion rate, we show that the accretion models without a corona are unable to explain the observations, and the presence of a corona is essential.

A powerful (and likely young) radio-loud quasar at z = 5.3

We present the discovery of PSO J191.05696+86.43172 (hereafter PSO J191+86), a new powerful radio-loud quasar (QSO) in the early Universe (z = 5.32). We discovered it by cross-matching the NRAO VLA Sky Survey (NVSS) radio catalog at 1.4 GHz with the first data release of the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS PS1) in the optical. With a NVSS flux density of 74.2 mJy, PSO J191+86 is one of the brightest radio QSO discovered at z ∼ 5. The intensity of its radio emission is also confirmed by the very high value of radio loudness (R > 300). The observed radio spectrum of PSO J191+86 shows a possible turnover around ∼1 GHz (i.e., ∼6 GHz in the rest frame), making it a gigahertz-peaked spectrum (GPS) source. However, variability could affect the real shape of the radio spectrum, because the data in hand were taken ∼25 years apart. By assuming a peak in the observed radio spectrum between 1 and 2 GHz (i.e., ∼6 and 13 GHz in the rest-frame) we find a linear size of the source of ∼10–30 pc and a corresponding kinetic age of 150–460 yr. This would make PSO J191+86 a newly born radio source. However, the large X-ray luminosity (5.3×1045 erg s−1), the flat X-ray photon index (ΓX = 1.32), and the optical–X-ray spectral index (αox̃ = 1.329) are typical of blazars. This could indicate that the nonthermal emission of PSO J191+86 is Doppler boosted. Further radio observations (both on arcsec and parsec scales) are necessary to better investigate the nature of this powerful radio QSO.

Advective accretion disc-corona model with fallback for tidal disruption events

ABSTRACT Tidal disruption events (TDEs) show a correlation between the UV to X-ray spectral index and the Eddington ratio, with non-thermal X-ray emission at the low Eddington ratio. We consider the corona surrounding the accretion disc as a non-thermal X-ray source. We construct a time-dependent and non-relativistic advective accretion disc-corona model for TDEs. The infalling debris is assumed to form a seed disc in time tc, that evolves due to the mass gain from the infalling debris at the constant outer radius with a mass fallback rate $\dot{M}_{\rm fb}$ and the mass loss through accretion onto the black hole. The viscous stress in our model depends on gas (Pg) and total (Pt) pressures as $\tau _{r\phi } \propto P_\mathrm{g}^{1-\mu } P_\mathrm{t}^{\mu }$, where μ is a constant. We find that the mass accretion rate $\dot{M}_a$ evolves from Eddington to sub-Eddington accretion with a late-time evolution close to t−5/3, where t is the time. We find that the bolometric disc luminosity follows a late-time evolution close to t−5/3. The ratio of total X-ray luminosity from corona to bolometric disc luminosity increases with μ and increases at late times for μ ≠ 1. We obtain the X-ray blackbody temperature of the disc that agrees with the temperature from X-ray observations (∼ 105 K). We find the radiative efficiency of the disc increases with time and decreases for a disc when the corona is included. We have neglected the outflow, and our model is more applicable for near-to-sub-Eddington accretion and when $\dot{M}_{\rm fb}$ is sub-Eddington.

Correlated variability of the reflection fraction with the X-ray flux and spectral index for Mrk 478

ABSTRACT The X-ray spectrum of Mrk 478 is known to be dominated by a strong soft excess that can be described using relativistic blurred reflection. Using observations from XMM–Newton, AstroSat, and Swift, we show that for the long-term (∼years) and intermediate-term (days to months) variability, the reflection fraction is anticorrelated with the flux and spectral index, which implies that the variability is due to the hard X-ray producing corona moving closer to and further from the black hole. Using flux-resolved spectroscopy of the XMM–Newton data, we show that the reflection fraction has the same behaviour with flux and index on short time-scales of hours. The results indicate that both the long- and short-term variability of the source is determined by the same physical mechanism of strong gravitational light bending causing enhanced reflection and low flux as the corona moves closer to the black hole.

New candidates for magnetar counterparts from a deep search with theHubble Space Telescope

ABSTRACTWe report the discovery of six new magnetar counterpart candidates from deep near-infrared Hubble Space Telescope (HST) imaging. The new candidates are among a sample of 19 magnetars for which we present HST data obtained between 2018 and 2020. We confirm the variability of previously established near-infrared counterparts, and newly identify candidates for PSR J1622−4950, Swift J1822.3−1606, CXOU J171405.7−381031, Swift J1833−0832, Swift J1834.9−0846, and AX J1818.8−1559 based on their proximity to X-ray localizations. The new candidates are compared with the existing counterpart population in terms of their colours, magnitudes, and near-infrared to X-ray spectral indices. We find two candidates for AX J1818 that are both consistent with previously established counterparts. The other new candidates are likely to be chance alignments, or otherwise have a different origin for their near-infrared emission not previously seen in magnetar counterparts. Further observations and studies of these candidates are needed to firmly establish their nature.

FRAMEx. II. Simultaneous X-Ray and Radio Variability in Active Galactic Nuclei—The Case of NGC 2992

Abstract Using simultaneous Very Long Baseline Array and Neil Gehrels Swift Observatory X-ray Telescope observations of the active galactic nucleus (AGN) in NGC 2992 over a six-month observing campaign, we observed a large drop in core 5 cm radio luminosity, by a factor of >3, in tandem with a factor of >5 increase in 2–10 keV X-ray luminosity. While NGC 2992 has long been an important object for studies of X-ray variability, our study is the first simultaneous X-ray and radio variability campaign for this object. We observe that the X-ray spectral index does not change over the course of the flare, consistent with a change in the bulk amount of Comptonizing plasma, potentially due to a magnetic reconnection event in the accretion disk. The drop in apparent radio luminosity can be explained by a change in free–free absorption, which we calculate to correspond to an ionized region with a physical extent and electron density consistent with the broad-line region (BLR). Our results are consistent with magnetic reconnection events in the dynamic accretion disk creating outbursts of ionizing material, increasing Compton up-scattering of UV accretion disk photons and feeding material into the BLR. These findings present an important physical picture for the dynamical relationship between X-ray and radio emission in AGNs.

Polycyclic aromatic hydrocarbons and the ionized gas in galaxies with active nuclei

ABSTRACT We present a study for a sample of galaxies with active nuclei to characterize the main type of PAH molecules present in these objects and the local physical conditions of their irradiating sources, as well as the characteristics of the residing ionized gas, by combining optical and infrared data. Photoionization models were built with the cloudy code to reproduce optical emission line ratios in combination with PAH intensity ratios. We find that the species containing 10−82 carbon atoms are the most abundant in the sample. We suggest that family of species with only two or three fused rings of and a nitrogen hanging, such as small aromatic amides are important targets worthy of consideration in future experimental/theoretical as well as observational studies. We find that the AGN photoionization models reproduce most of the observational data in the log (6.2/11.3) versus log ([${\rm N\, {\small II}}$] λ6584/H α) diagram with the optical to X-ray spectral index of αox = −1.4. The flux of small PAH, as well as the flux of ionized PAHs and PANH, decrease as the logarithm of the ionization parameter (log U) increases. The 6.2/11.3 PAH intensity ratio presents an anti correlation between the oxygen abundance and log U.Finally, we found that the ionization degree of PAH species increases with the decreasing of the 11.3/7.7 ratio and the log U, in agreement with the models proposed by Draine & Li.

The origin of X-ray emission in 3CRR sources: Hints from mid-infrared Spitzer observations

Aims. Whether X-ray emission in radio-loud (RL) active galactic nuclei (AGNs) originates from disk coronae or jets is still under debate. For example, the positive relationships in radio-quiet AGNs (such as the optical to X-ray spectral index αOX and Eddington ration λO as well as the X-ray photon index Γ and λO) are not detected in RLAGNs. We intend to further investigate this issue in this work. Methods. A sample of 27 luminous sources (including 16 quasars and 11 high-excitation radio galaxies) was selected from the 3CRR catalog to reinvestigate the origin of X-ray emission in RLAGNs, where the X-ray and mid-infrared fluxes are observed by Chandra/XMM-Newton and Spitzer, respectively. Results. It is found for the first time that there is a significant relationship between the mid-infrared to X-ray spectral index αIX and λIR for whole sample, while there is no relationship between αOX and λO in quasars. There are strong positive relationships between both LR − LX and LUV − LX panels, which can be well fitted by the disk-corona model. However, there is no significant relationship between Γ and λIR. The possible reason is related to the effects of the large-scale magnetic field in RLAGNs. Conclusions. We suggest that the X-ray emission in high-excitation RLAGNs originates from a disk-corona system.

Multi-frequency study of spectral indices of BL Lacertae objects and flat-spectrum radio quasars

We present statistical analyses of large homogenous data sample of Fermi-detected blazars thoroughly studied in order to reassess the relationship between flat-spectrum radio quasars (FSRQs) and subclasses of BL Lacertae objects (BL Lacs) blazar populations. We discovered from the average values of γ-ray (αγ−ray) and X-ray (αX−ray) spectral indices that the sequence of distribution is indicative of blazar unification scheme. Analyses of FSRQs and BL Lacs data show difference in the shape of γ-ray and X-ray spectral indices: significant anti-correlation (r> - 0.79) exists between αγ−ray and αX−ray spectral indices. The spectral energy distributions of the blazar subclasses show that FSRQs and BL Lacs have similar spectral properties which can be unified through evolutionary sequence. Nevertheless, there is a significant difference between the shapes of X-ray and γ-ray spectra of blazars suggestive that different mechanisms are responsible for spectral variations in the two energy bands. All these results suggest that there is a form of a unified scheme for all blazars.

Broken Power-law Energy Spectra of the Accelerated Electrons Detected in Radio and Hard X-Rays during the SOL2013-05-13 Event

Abstract Solar flares, resulting from magnetic activity of the Sun, are among the most energetic events in the solar system and in extreme cases directly affect our highly technological society. In this work, we analyze a solar flare detected at millimeter and centimeter wavelengths, as well as X-rays above 1 MeV. Observations of solar flares at these energy bands provide diagnostics of the energetic accelerated electrons and the magnetic fields where the emission is produced. During the SOL2013-05-13 solar flare, radio data were obtained by the telescope system POlarisation Emission of Millimeter Activity at the Sun, which observes the Sun at 45 and 90 GHz with polarization measurements, and at microwaves (1–15 GHz) by the Radio Solar Telescope Network. For the same event, X-ray emission was detected by the RHESSI and Fermi satellites. Spectra at both wavelengths were constructed and fit separately to yield the accelerated electron energy distribution that produced the emission. The optically thin radio spectral index was calculated by fitting the Ramaty model of gyrosynchrotron emission to the observed radio spectrum, whereas the hard X-ray spectral index was obtained from the spectral fit assuming a thermal emission model plus a nonthermal broken power-law distribution. Finally, both spectral indexes were compared and confirmed that the index obtained from the radio spectrum agrees with the index of the X-ray spectrum for energies above the break energy of ∼600 keV. Thus, the hard X-rays more energetic than 600 keV and high radio frequencies of solar flares are emitted by the same population of high-energy accelerated electrons. This result indicates that the accelerated electrons have an energy distribution best represented by a broken power law, with a breakup above energies around 1 MeV.

Redshift evolution of X-ray spectral index of quasars observed by XMM-NEWTON/SDSS

We study the X-ray spectral evolution of quasars with red-shift using a sample of 1280 quasars constructed by cross-referencing the XMM-Newton 3XMM−DR7 and the optical SDSS-DR14 catalogues. The sources were selected to have a minimum detected X-ray counts >600 and which were not affected by high background. The sample redshift distribution ranges from 0-4.9 and peaks around 1. For each source, the XMM-Newton EPIC_PN spectra were generated and fitted with a power-law model with Galactic absorption. Consistent with some earlier results we find a strong correlation between the photon index and redshift. We explicitly test the hypothesis that this is due to the X-ray soft excess and reflection bump being red-shifted away from the XMM band, by adding a black body and reflection component and find that the correlation is no longer significant. Thus, for the first time, we show that there is no evidence for quasar spectral evolution and the correlation between index and red-shift is indeed due to the soft excess and reflection component.

Probing the Disk–Corona Systems and Broad-line Regions of Changing-look Quasars with X-Ray and Optical Observations

Abstract “Changing-look” quasars are a new class of highly variable active galactic nuclei that have changed their spectral type over surprisingly short timescales of just a few years. The origin of this phenomenon is debated, but is likely to reflect some change in the accretion flow. To investigate the disk–corona systems in these objects, we measure optical/UV–X-ray spectral indices ( α OX ) and Eddington ratios ( λ Edd ) of 10 previously discovered changing-look quasars at two or more epochs. By comparing these data with simulated results based on the behavior of X-ray binaries, we find possible similarities in spectral indices below the 1% Eddington ratio. We further investigate the Eddington ratios of changing-look quasars before and after their spectral type changes, and find that changing-look quasars cross the 1% Eddington ratio boundary when their broad emission lines disappear/emerge. This is consistent with the disk-wind model as the origin of broad emission lines.

A Multiwavelength Study of the γ-Ray Binary Candidate HESS J1832–093

Abstract We investigate the nature of the unidentified very-high-energy γ-ray object, HESS J1832–093, in a multiwavelength context. Based on X-ray variability and spectral index ( ), and its broadband spectrum, which was remarkably similar to that of HESS J0632+057—a confirmed γ-ray binary—HESS J1832–093 has been considered a strong γ-ray binary candidate in previous works. In this work, we provide further evidence for this scenario. We obtained a spectrum of its IR counterpart using Gemini/Flamingo, finding absorption lines that are usually seen in massive stars, in particular O stars. We also obtained a rather steep ATCA spectrum ( ), which is consistent with a γ-ray binary. Based on spatial-spectral analysis and variability search, we found that 4FGL J1832.9-0913 is possibly associated with SNR G22.7-0.2 rather than with HESS J1832–093 only.

Hard X-ray properties of radio-selected blazars

Context. Hard X-ray properties of beamed active galactic nuclei have been published in the 105-month Swift/BAT catalog, but there have not been any studies carried out so far on a well-defined, radio-selected sample of low-peaked blazars in the hard X-ray band. Aims. Using the statistically complete MOJAVE-1 sample, we aim to determine the hard X-ray properties of radio-selected blazars, including the enigmatic group of gamma-ray-faint blazars. Additionally, we aim to determine the contribution of radio-selected low-peaked blazars to the diffuse cosmic X-ray background (CXB). Methods. We determined photon indices, fluxes, and luminosities in the range of 20 keV–100 keV of the X-ray spectra of blazars and other extragalactic jets from the MOJAVE-1 sample, derived from the 105-month Swift/BAT survey. We calculated log N–log S distributions and determined the luminosity functions. Results. The majority of the MOJAVE-1 blazars are found to be hard X-ray emitters albeit many at low count rates. The log N–log S distribution for the hard X-ray emission of radio-selected blazars is clearly non-Euclidean, in contrast to the radio flux density distribution. Approximately 0.2% of the CXB in the 20 keV–100 keV band can be resolved into MOJAVE-1 blazars. Conclusions. The peculiar log N–log S distribution disparity might be attributed to different evolutionary paths in the X-ray and radio bands, as tested by luminosity-function modeling. X-ray variability can be ruled out as the dominant contributor. Low-peaked blazars constitute an intrinsically different source population in terms of CXB contribution compared to similar studies of X-ray-selected blazars. The hard X-ray flux and spectral index can serve as a good proxy for the gamma-ray detection probability of individual sources. Future observations combining deep X-ray survey, for example, with eROSITA, and targeted gamma-ray observations with CTA can benefit strongly from the tight connection between these high-energy bands for the different blazar sub-classes.

NuSTAR observation of Ark 564 reveals the variation of coronal temperature with flux

ABSTRACT The hard X-ray spectral index of some active galactic nuclei (AGN) has been observed to steepen with the source flux. This has been interpreted in a Comptonization scenario, where an increase in the soft flux decreases the temperature of the corona, leading to steepening of the photon index. However, the variation of the coronal temperature with flux has been difficult to measure due to the presence of complex reflection component in the hard X-rays and the lack of high-quality data at that energy band. Recently, a 200 ks Nuclear Spectroscopic Telescope Array(NuSTAR) observation of Ark 564 in 3–50 keV band revealed the presence of one of the coolest coronae with temperature kTe ∼ 15 keV in the time-averaged spectrum. Here, we reanalyse the data and examined the spectra in four flux levels. Our analysis shows that the coronal temperature decreased from ∼17 to ∼14 keV as the flux increased. The high energy photon index Γ ∼ 2.3 varied by less than 0.1, implying that the optical depth of the corona increased by about 10 per cent as the flux increased. This first reporting of coronal temperature variation with flux shows that further long observation by NuSTAR of this and other sources would shed light on the geometry and dynamics of the inner regions of the accretion flow.