arXiv subjects

A. Danehkar

Publications and source records attributed to A. Danehkar.

At least 19 recordsLinked to original sources

arXiv2025

IGR J12580+0134: The Nearest Tidal Disruption Event and its Faint Resurrection

Tidal disruption events (TDEs) are X-ray and gamma-ray radiations emerging from the tidal disintegration of a star or substellar object that passes too close to a supermassive black hole (SMBH) at the center of a galaxy. In November 2010, a TDE designated as IGR J12580+0134 occurred in the galaxy NGC 4845, and it was traced by follow-up XMM-Newton observations in January 2011. To identify a further TDE based on the radio outburst cycle, we requested NICER monitoring observations for nearly a year beginning in March 2023, which we studied here along with the previous XMM-Newton observations. We analyzed X-ray brightness changes using hardness analysis and principal component analysis (PCA), and conducted spectral analysis of the source continuum. The NICER observations revealed the presence of some X-ray flares during March-June 2023 that were much fainter than the TDE observed using XMM-Newton in 2011. The PCA component that mainly contributes to the X-ray outbursts during the TDE is a heavily absorbed power-law continuum emission, whereas there is a small contribution from collisionally ionized plasma in the soft excess, likely from a colliding wind or jet. Similarly, the PCA of the NICER data relates the X-ray flares to a power-law spectrum, albeit with a much lower absorbing column, and partially to soft collisional plasma. The faint X-ray flares captured by NICER could be associated with extremely weak accretion onto the SMBH resident in this galaxy and thus potentially a low-luminosity active galactic nucleus.

astro-ph.HE↗

arXiv2025

Accretion-disk reflection in the Seyfert 1 galaxy NGC 3783 as viewed by Chandra grating spectroscopy

In active galactic nuclei, X-ray illumination of the accretion disk around a supermassive black hole (SMBH) results in the production of the K$\alpha$ fluorescent line of iron, which provides insights into accretion physics and SMBH spins. In this work, we studied X-ray reflection from the accretion disk in the Seyfert 1 galaxy NGC 3783 using all the data collected by the Chandra High Energy Transmission Grating Spectrometer. We used hardness-ratio diagrams to distinguish between different spectral states and conducted spectral analysis of all the multi-epoch datasets, as well as the source in the observed spectral states. Our hardness analysis indicates that the source gradually evolved into a harder state (2013-2016) compared to the previous epochs (2000-2001). Our spectral modeling implies that the relativistically broadened iron emission from the innermost accretion disk is associated with a near-maximal SMBH spin ($a=0.98^{+0.02}_{-0.12}$) in all the datasets, even though the hard state was present in 17% of them, and a consistent spin is also found in different spectral states. In addition, the narrow, bright Fe K$\alpha$ line from distant regions has an excess velocity of $620^{+80}_{-70}$ km s$^{-1}$ relative to the rest frame, implying that some distant layers of the disk could be twisted. Our results suggest that, despite long-term changes in the X-ray brightness of NGC 3783, likely caused by eclipsing material, the relativistic reflection can be constrained thanks to the substantial counts provided by multi-epoch observations, while a warped disk structure may be present.

astro-ph.HE↗

arXiv2024

pyEQUIB Python Package, an addendum to proEQUIB: IDL Library for Plasma Diagnostics and Abundance Analysis

The emission lines from ionized nebulae allow us to determine their physical and chemical properties, along with the interstellar extinction. "pyEQUIB" is a pure Python open-source package including several application programming interface (API) functions that can be employed for plasma diagnostics, abundance analysis of collisionally excited lines (CEL) and recombination lines (RL) in nebular astrophysics, and extinction analysis of Balmer lines. This package implements the IDL library "proEQUIB" in Python and couples it with the "AtomNeb" Python package. This package relies on the Python packages NumPy and SciPy. The API functions of this package can be used for studies of ionized nebulae by astronomers who are familiar with the high-level, general-purpose programming language Python.

astro-ph.IM↗

arXiv2024

Relativistic Reflection Modeling in AGN and Related Variability from PCA: A Brief Review

X-ray observations of active galactic nuclei (AGNs) reveal relativistic reflections from the innermost regions of accretion disks, which contain general-relativistic footprints caused by spinning supermassive black holes (SMBH). We anticipate the spin of a SMBH to be stable over the human timeframe, so brightness changes in the high-energy corona above the SMBH should slightly alter relativistic reflection. In this brief review, we discuss the latest developments in modeling relativistic reflection, as well as the rapid small variation in relativistic emission disclosed by the principal component analysis (PCA) of X-ray variability in AGN. PCA studies of X-ray spectra from AGNs have shown that relativistically blurred reflection has negligible fluctuations over the course of observations, which could originate from rapid (intrahour) intrinsic variations in near-horizon accretion flows and photon rings. The PCA technique is an effective way to disclose relativistic reflection from X-ray observations of AGNs, simplifying the complexity of largely variable X-ray data for automated spectral analysis with machine learning algorithms.

astro-ph.HE↗

arXiv2024

Disentangling the X-ray variability in the Lyman continuum emitter Haro 11

Lyman break analogs in the local Universe serve as counterparts to Lyman break galaxies (LBGs) at high redshifts, which are widely regarded as major contributors to cosmic reionization in the early stages of the Universe. We studied XMM-Newton and Chandra observations of the nearby LBG analog Haro 11, which contains two X-ray-bright sources, X1 and X2. Both sources exhibit Lyman continuum (LyC) leakage, particularly X2. We analyzed the X-ray variability using principal component analysis (PCA) and performed spectral modeling of the X1 and X2 observations made with the Chandra ACIS-S instrument. The PCA component, which contributes to the X-ray variability, is apparently associated with variable emission features, likely from ionized superwinds. Our spectral analysis of the Chandra data indicates that the fainter X-ray source, X2 (X-ray luminosity $L_{\rm X} \sim 4 \times 10^{40} $ erg s$^{-1}$), the one with higher LyC leakage, has a much lower absorbing column ($N_{\rm H} \sim 1.2 \times 10^{21}$ cm$^{-2}$) than the heavily absorbed luminous source X1 ($L_{\rm X} \sim 9 \times 10^{40} $ erg s$^{-1}$ and $N_{\rm H} \sim 11.5 \times 10^{21}$ cm$^{-2}$). We conclude that X2 is likely less covered by absorbing material, which may be a result of powerful superwinds clearing galactic channels and facilitating the escape of LyC radiation. Much deeper X-ray observations are required to validate the presence of potential superwinds and determine their implications for the LyC escape.

astro-ph.HE↗

arXiv2024

Numerical Modeling of Galactic Superwinds with Time-evolving Stellar Feedback

Mass-loss and radiation feedback from evolving massive stars produce galactic-scale superwinds, sometimes surrounded by pressure-driven bubbles. Using the time-dependent stellar population typically seen in star-forming regions, we conduct hydrodynamic simulations of a starburst-driven superwind model coupled with radiative efficiency rates to investigate the formation of radiative cooling superwinds and bubbles. Our numerical simulations depict the parameter space where radiative cooling superwinds with or without bubbles occur. Moreover, we employ the physical properties and time-dependent ionization states to predict emission line profiles under the assumption of collisional ionization and non-equilibrium ionization caused by wind thermal feedback in addition to photoionization created by the radiation background. We see the dependence of non-equilibrium ionization structures on the time-evolving ionizing source, leading to a deviation from collisional ionization in radiative cooling wind regions over time.

astro-ph.GA↗

arXiv2024

X-ray Variability in the Symbiotic Binary RT Cru: Principal Component Analysis

Hard X-ray-emitting ($\delta$-type) symbiotic binaries, which exhibit a strong hard X-ray excess, have posed a challenge to our understanding of accretion physics in degenerate dwarfs. RT Cru, which is a member of the $\delta$-type symbiotics, shows stochastic X-ray variability. Timing analyses of X-ray observations from XMM-Newton and NuSTAR, which we consider here, indicate hourly fluctuations, in addition to a spectral transition from 2007 to a harder state in 2012 seen with Suzaku observations. To trace the nature of X-ray variability, we analyze the multi-mission X-ray data using principal component analysis (PCA), which determines the spectral components that contribute most to the flickering behavior and the hardness transition. The Chandra HRC-S/LETG and XMM-Newton EPIC-pn data provide the primary PCA components, which may contain some variable emission features, especially in the soft excess. Additionally, the absorbing column (first order with 50%), along with the source continuum (20%), and a third component (9%) - which likely accounts for thermal emission in the soft band - are the three principal components found in the Suzaku XIS1 observations. The PCA components of the NuSTAR data also correspond to the continuum and possibly emission features. Our findings suggest that the spectral hardness transition between the two Suzaku observations is mainly due to changes in the absorbing material and X-ray continuum, while some changes in the thermal plasma emission may result in flickering-type variations.

astro-ph.HE↗

arXiv2023

Silicon K-edge Dust Properties of Neutron Star Low-mass X-ray Binaries

The dust properties of the line-of-sight materials in neutron star low-mass X-ray binaries (LMXBs) can be probed by X-ray observations and laboratory experiments. We use a Markov chain Monte Carlo (MCMC) method to conduct a spectral analysis of Chandra ACIS-S/HETG archival data of a sample of LMXBs, including GX 5-1 and GX 13+1. Our MCMC-based analysis puts constraints on the Si K-edge dust properties of the outflowing disk winds in this sample. Further X-ray observations of other LMXBs will help us better understand the grain features of dense outflows and accretion flows in neutron star binary systems.

astro-ph.HE↗

arXiv2023

Hydrodynamic Simulations and Time-dependent Photoionization Modeling of Starburst-driven Superwinds

Thermal energies deposited by OB stellar clusters in starburst galaxies lead to the formation of galactic superwinds. Multi-wavelength observations of starburst-driven superwinds pointed at complex thermal and ionization structures which cannot adequately be explained by simple adiabatic assumptions. In this study, we perform hydrodynamic simulations of a fluid model coupled to radiative cooling functions, and generate time-dependent non-equilibrium photoionization models to predict physical conditions and ionization structures of superwinds using the MAIHEM atomic and cooling package built on the program FLASH. Time-dependent ionization states and physical conditions produced by our simulations are used to calculate the emission lines of superwinds for various parameters, which allow us to explore implications of non-equilibrium ionization for starburst regions with potential radiative cooling.

astro-ph.GA↗

arXiv2022

Catastrophic Cooling in Superwinds. III. Non-equilibrium Photoionization

Observations of some starburst-driven galactic superwinds suggest that strong radiative cooling could play a key role in the nature of feedback and the formation of stars and molecular gas in star-forming galaxies. These catastrophically cooling superwinds are not adequately described by adiabatic fluid models, but they can be reproduced by incorporating non-equilibrium radiative cooling functions into the fluid model. In this work, we have employed the atomic and cooling module MAIHEM implemented in the framework of the FLASH hydrodynamics code to simulate the formation of radiatively cooling superwinds as well as their corresponding non-equilibrium ionization (NEI) states for various outflow parameters, gas metallicities, and ambient densities. We employ the photoionization program CLOUDY to predict radiation- and density-bounded photoionization for these radiatively cooling superwinds, and we predict UV and optical line emission. Our non-equilibrium photoionization models built with the NEI states demonstrate the enhancement of C IV, especially in metal-rich, catastrophically cooling outflows, and O VI in metal-poor ones.

astro-ph.GA↗

arXiv2022

Covariant Evolution of Gravitoelectromagnetism

The long-range gravitational terms associated with tidal forces, frame-dragging effects, and gravitational waves are described by the Weyl conformal tensor, the traceless part of the Riemann curvature that is not locally affected by the matter field. The Ricci and Bianchi identities provide a set of dynamical and kinematic equations governing the matter coupling and evolution of the electric and magnetic parts of the Weyl tensor, so-called gravitoelectric and gravitomagnetic fields. A detailed analysis of the Weyl gravitoelectromagnetic fields can be conducted using a number of algebraic and differential identities prescribed by the 1+3 covariant formalism. In this review, we consider the dynamical constraints and propagation equations of the gravitoelectric/-magnetic fields and covariantly debate their analytic properties. We discuss the special conditions under which gravitational waves can propagate, the inconsistency of a Newtonian-like model without gravitomagnetism, the nonlinear generalization to multi-fluid models with different matter species, as well as observational effects caused by the Weyl fields via the kinematic quantities. The 1+3 tetrad and 1+1+2 semi-covariant methods, which can equally be used for gravitoelectromagnetism, are briefly explained, along with their correspondence with the covariant formulations.

gr-qc↗

arXiv2022

Physical Conditions and Chemical Abundances of the Variable Planetary Nebula IC 4997

The planetary nebula (PN) IC 4997 is one of a few rapidly evolving objects with variable brightness and nebular emission around a hydrogen-deficient star. In this study, we have determined the physical conditions and chemical abundances of this object using the collisionally excited lines (CELs) and optical recombination lines (ORLs) measured from the medium-resolution spectra taken in July 2014 with the FIbre-fed Échelle Spectrograph on the Nordic Optical Telescope at La Palma Observatory. We derived electron densities of $\gtrsim 3 \times 10^4$ cm$^{-3}$ and electron temperatures of $\gtrsim 14,000$ K from CELs, whereas cooler temperatures of $\sim 11,000$ and $\sim 7,000$ K were obtained from helium and heavy element ORLs, respectively. The elemental abundances deduced from CELs point to a metal-poor progenitor with [O/H]$\lesssim-0.75$, whereas the ORL abundances are slightly above the solar metallicity, [O/H]$\approx 0.15$. Our abundance analysis indicates that the abundance discrepancy factors (ADFs$\equiv$ORLs/CELs) of this PN are relatively large: ADF(O$^{2+})\gtrsim 8$ and ADF(N$^{2+})\gtrsim 7$. Further research is needed to find out how the ADFs and variable emissions are formed in this object and whether they are associated with a binary companion or a very late thermal pulse.

astro-ph.SR↗

arXiv2022

Morpho-kinematic properties of Wolf-Rayet planetary nebulae

The majority of planetary nebulae (PNe) show axisymmetric morphologies, whose causes are not well understood. In this work, we present spatially resolved kinematic observations of 14 Galactic PNe surrounding Wolf-Rayet ([WR]) and weak emission-line stars ($wels$) based on the H$α$ and [N II] emission taken with the Wide Field Spectrograph on the Australian National University 2.3-m telescope. Velocity-resolved channel maps and position--velocity diagrams, together with archival Hubble Space Telescope ($HST$) and ground-based images, are employed to construct three-dimensional morpho-kinematic models of 12 objects using the program SHAPE. Our results indicate that these 12 PNe mostly have elliptical morphologies with either open or closed outer ends. The kinematic maps show the on-sky orientations of the interior shells in NGC6578 and NGC6629, as well as the compact ($\leq 6$ arcsec) PNe Pe1-1, M3-15, M1-25, Hen2-142, and NGC6567, in agreement with the elliptically symmetric morphologies seen in high-resolution $HST$ images. Point-symmetric knots in Hb4 exhibit deceleration with distance from the central star, which could be due to shock collisions with the ambient medium. The velocity dispersion maps of Pe1-1 also disclose the shock interaction between its collimated outflows and the interstellar medium. Collimated bipolar outflows are also visible in the position--velocity diagrams of M3-30, M1-32, and M3-15, which are reconstructed by tenuous prolate ellipsoids extending upward from dense equatorial regions in the kinematic models. The formation of aspherical morphologies and collimated outflows in these PNe could be related to the stellar evolution of hydrogen-deficient [WR] and $wels$ nuclei, which require further investigation.

astro-ph.SR↗

arXiv2022

Morphologies of Wolf-Rayet Planetary Nebulae based on IFU Observations

Integral field unit (IFU) spectroscopy of planetary nebulae (PNe) provides a plethora of information about their morphologies and ionization structures. An IFU survey of a sample of PNe around hydrogen-deficient stars has been conducted with the Wide Field Spectrograph (WiFeS) on the ANU 2.3-m telescope. In this paper, we present the H$α$ kinematic observations of the PN M2-42 with a weak emission-line star ($wels$), and the compact PNe Hen 3-1333 and Hen 2-113 around Wolf-Rayet ([WR]) stars from this WiFeS survey. We see that the ring and point-symmetric knots previously identified in the velocity [N II] channels of M2-42 are also surrounded by a thin exterior ionized H$α$ halo, whose polar expansion is apparently faster than the low-ionization knots. The velocity-resolved H$α$ channel maps of Hen3-1333 and Hen2-113 also suggest that the faint multipolar lobes may get to a projected outflow velocity of $\sim 100 \pm 20$ km s$^{-1}$ far from the central stars. Our recent kinematic studies of the WiFeS/IFU survey of other PNe around [WR] and $wels$ mostly hint at elliptical morphologies, while collimated outflows are present in many of them. As the WiFeS does not have adequate resolution for compact ($\leq 6$ arcsec) PNe, future high-resolution spatially-resolved observations are necessary to unveil full details of their morpho-kinematic structures.

astro-ph.SR↗

arXiv2021

3D spatio-kinematic modeling of Abell 48, a planetary nebula around a Wolf-Rayet [WN] star

The planetary nebula (PN) Abell 48 (PN G029.0+00.4) is around a rare Wolf-Rayet [WN5] star whose stellar history is as yet unknown. Using the integral field observations of the H$α$ $λ$6563 and [N II] $λ$6584 line emissions, we conducted a comprehensive spatio-kinematic analysis of this PN. A three-dimensional spatio-kinematic ionization model was developed with the kinematic modeling tool SHAPE to replicate the observed spatially-resolved velocity channels and position--velocity diagrams. According to our kinematic analysis of the H$α$ emission, this object possesses a deformed elliptic toroidal shell with an outer radius of 23 arcsec and a thickness of 15 arcsec associated with an integrated H$α$ emission-line expansion of $\sim 35 \pm 5$ km s$^{-1}$, a maximum poloidal expansion of around $70 \pm 20$ km s$^{-1}$ at an inclination angle of $\sim 30^{\circ}$ with respect to the line of sight, and a position angle of $\sim 130^{\circ}$ measured from east toward north in the equatorial coordinate system. Furthermore, [N II] kinematic modeling reveals the presence of narrow ($\sim 3$ arcsec) exterior low-ionization structures surrounding the main elliptical shell, which could have formed as a result of shock collisions with the interstellar medium. The torus-shaped morphology of this PN could be related to its unusual hydrogen-deficient [WN] nucleus that needs to be inspected further.

astro-ph.SR↗

arXiv2021

Non-equilibrium Photoionization and Hydrodynamic Simulations of Starburst-driven Outflows

Starburst-driven galactic outflows in star-forming galaxies have been observed to contain complex thermal structures and emission line features that are difficult to explain by adiabatic fluid models and plasmas in photoionization equilibrium (PIE) and collisional ionization equilibrium (CIE). We previously performed hydrodynamic simulations of starburst-driven outflows using the MAIHEM module for non-equilibrium atomic chemistry and radiative cooling functions in the hydrodynamics code FLASH, and calculated emission lines in combined CIE and PIE conditions. In the present study, we consider time-dependent non-equilibrium ionization (NEI) states produced by our MAIHEM simulations. Through extensive CLOUDY calculations made with the NEI states from our hydrodynamic simulations, we predict the UV and optical emission line profiles for starburst-driven outflows in time-evolving non-equilibrium photoionization conditions. Our hydrodynamic results demonstrate applications of non-equilibrium radiative cooling for H II regions in starburst galaxies hosting cool outflows.

astro-ph.GA↗

arXiv2021

Physical and chemical properties of Wolf-Rayet planetary nebulae

Wolf-Rayet ([WR]) and weak emission-line ($wels$) central stars of planetary nebulae (PNe) have hydrogen-deficient atmospheres, whose origins are not well understood. In the present study, we have conducted plasma diagnostics and abundance analyses of 18 Galactic PNe surrounding [WR] and $wels$ nuclei, using collisionally excited lines (CELs) and optical recombination lines (ORLs) measured with the Wide Field Spectrograph on the ANU 2.3-m telescope at the Siding Spring Observatory complemented with optical archival data. Our plasma diagnostics imply that the electron densities and temperatures derived from CELs are correlated with the intrinsic nebular H$β$ surface brightness and excitation class, respectively. Self-consistent plasma diagnostics of heavy element ORLs of N${}^{2+}$ and O${}^{2+}$ suggest that a small fraction of cool ($\lesssim 7000$ K), dense ($\sim 10^4-10^5$ cm$^{-3}$) materials may be present in some objects, though with large uncertainties. Our abundance analyses indicate that the abundance discrepancy factors (ADF$\equiv$ORLs/CELs) of O${}^{2+}$ are correlated with the dichotomies between forbidden-line and He I temperatures. Our results likely point to the presence of a tiny fraction of cool, oxygen-rich dense clumps within the diffuse warm ionized nebulae. Moreover, our elemental abundances derived from CELs are mostly consistent with AGB models in the range of initial masses from 1.5 to 5M$_{\odot}$. Further studies are necessary to understand better the origins of abundance discrepancies in PNe around [WR] and $wels$ stars.

astro-ph.SR↗

arXiv2020

Long-term X-ray Variability of the Symbiotic System RT Cru based on Chandra Spectroscopy

RT Cru belongs to the rare class of hard X-ray emitting symbiotics, whose origin is not yet fully understood. In this work, we have conducted a detailed spectroscopic analysis of X-ray emission from RT Cru based on observations taken by the Chandra Observatory using the Low Energy Transmission Grating (LETG) on the High-Resolution Camera Spectrometer (HRC-S) in 2015 and the High Energy Transmission Grating (HETG) on the Advanced CCD Imaging Spectrometer S-array (ACIS-S) in 2005. Our thermal plasma modeling of the time-averaged HRC-S/LETG spectrum suggests a mean temperature of $kT \sim 1.3$ keV, whereas $kT \sim 9.6$ keV according to the time-averaged ACIS-S/HETG. The soft thermal plasma emission component ($\sim1.3$ keV) found in the HRC-S is heavily obscured by dense materials ($> 5 \times 10^{23}$ cm$^{-2}$). The aperiodic variability seen in its light curves could be due to changes in either absorbing material covering the hard X-ray source or intrinsic emission mechanism in the inner layers of the accretion disk. To understand the variability, we extracted the spectra in the "low/hard" and "high/soft" spectral states, which indicated higher plasma temperatures in the low/hard states of both the ACIS-S and HRC-S. The source also has a fluorescent iron emission line at 6.4 keV, likely emitted from reflection off an accretion disk or dense absorber, which was twice as bright in the HRC-S epoch compared to the ACIS-S. The soft thermal component identified in the HRC-S might be an indication of a jet that deserves further evaluations using high-resolution imaging observations.

astro-ph.HE↗