arXiv subjects

Vikram V. Dwarkadas

Publications and source records attributed to Vikram V. Dwarkadas.

At least 19 recordsLinked to original sources

arXiv2026

X-Ray Observation of Type II Supernova 2024ggi

We investigate the Chandra and XMM-Newton X-ray emission from the core-collapse SN 2024ggi. Spectral fitting is carried out using the vapec model. We find X-ray temperatures between 5-7 keV, lower than the value assumed by previous authors by about a factor of 4-6. However the unabsorbed flux is consistent with published values within the error bars. The X-ray luminosity evolution is somewhat steeper than expected for a steady wind, although it could be consistent with a steady wind within the error bars.

astro-ph.HE↗

arXiv2026

Analysis of 14 Years of X-Ray Emission From SN 2011DH

Ejecta from core-collapse supernovae interact with the circumstellar medium shed by the progenitor star, producing X-ray emission. Previous studies analyzed the X-ray spectrum of the Type IIb supernova SN 2011dh up to 500 days after explosion. Long-term monitoring of X-ray emission provides valuable constraints on supernova evolution and progenitor systems, yet such studies remain rare for Type IIb events due to limited data. Here we present the most comprehensive X-ray light curve of SN 2011dh to date, combining all available Chandra and XMM-Newton data with previously published and newly released Swift observations, extending coverage to 5100 days. We measure a luminosity decline proportional to t$^{-0.74 \pm 0.04}$ and infer a mass-loss rate of $(1.0-2.2) \times 10^{-6}$ solar masses per year for $v_w = 10$ km/s, or $(2.0-4.4) \times 10^{-6} $ solar masses per year for $v_w = 20$ km/s. These estimates agree with earlier results, supporting the interpretation that the X-ray emission has been dominated by an adiabatic reverse shock. The consistency of our late-time results with previous studies demonstrates that SN 2011dh has evolved steadily for nearly 14 years.

astro-ph.HE↗

arXiv2025

On the X-ray Emission From Supernovae, and Implications for the Mass-Loss Rates of their Progenitor Stars

We summarize the X-ray emission from young SNe. Having accumulated data on most observed X-ray SNe, we display the X-ray lightcurves of young SNe. We also explore the X-ray spectra of various SN types. The X-ray emission from Type Ib/c SNe is non-thermal. It is also likely that the emission from Type IIP SNe with low mass-loss rates (around 10$^{-7} \, Msun \,$ yr$^{-1}$) is non-thermal. As the mass-loss rate increases, thermal emission begins to dominate. Type IIn SNe have the highest X-ray luminosities, and are clearly thermal. We do not find evidence of non-thermal emission from Type IIb SNe. The aggregated data are used to obtain approximate mass-loss rates of the progenitor stars of these SNe. Type IIP's have progenitors with mass-loss rates $< 10^{-5}\, Msun \,$ yr$^{-1}$, while Type IIn progenitors generally have mass-loss rates $> 10^{-3}\, Msun $ yr$^{-1}$. However, we emphasize that the density of the ambient medium is the important parameter, and if it is due to a non-steady outflow solution, it can not be translated into a mass-loss rate.

astro-ph.HE↗

arXiv2024

X-ray Observations of Two Type Ia Supernovae with an H$α$ Line in their Optical Spectrum

We report on Chandra X-ray observations of SN 2016jae and SN 2018cqj, both low luminosity Type Ia supernova that showed the presence of a H line in their early optical spectrum. No X-ray emission is detected at the location of either SN. Upper limits to the luminosity of up to 2 $\times 10^{40}\,$ erg s$^{-1}$ are calculated for each SN, depending on the assumed spectral model, temperature and column density. This luminosity is comparable to that of another low-luminosity Type Ia SN, SN 2018fhw, that was observed with Chandra. It is generally lower than upper limits calculated for Type Ia-CSM SNe observed in X-rays, and also below that of SN 2012ca, the only Type Ia-CSM SN to have been detected in X-rays. Comparisons are made to other Type Ia SN with a H line observed in X-rays. The observations suggest that while the density into which the SN is expanding may have been high at the time the H$α$ line was detected, it had decreased considerably by the time of X-ray observations.

astro-ph.HE↗

arXiv2023

X-Ray Observations of Type Ia Supernova 2018cqj

I report on Chandra X-ray observations of SN 2018cqj, a low luminosity Type Ia supernova that showed an H$α$ line in its optical spectrum. No X-ray emission was detected at the location of the SN, with an upper limit to the X-ray luminosity of 2 $\times 10^{39}$ erg s$^{-1}$.

astro-ph.HE↗

arXiv2023

On the Evolution of, and Hot Gas in, Wind-Blown Bubbles around Massive Stars -- Wind Bubbles Are Not Energy-Conserving

The structure and evolution of wind-blown bubbles (WBBs) around massive stars has primarily been investigated using an energy-conserving model of wind-blown bubbles. While this model is useful in explaining the general properties of the evolution, several problems remain, including inconsistencies between observed wind luminosities and those derived using this formulation. Major difficulties include the low X-ray temperature and X-ray luminosity, compared to the model. In this paper, we re-examine the evolution, dynamics, and kinematics of WBBs around massive stars, using published ionization gasdynamic simulations of wind-blown bubbles. We show that WBBs can cool efficiently due to the presence of various instabilities and turbulence within the bubble. The expansion of WBBs is more consistent with a momentum-conserving solution, rather than an energy-conserving solution. This compares well with the dynamics and kinematics of observed wind bubbles. Despite the cooling of the bubble, the shocked wind temperature is not reduced to the observed values. We argue that the X-ray emission arise mainly from clumps and filaments within the hot shocked wind region, with temperatures just above 10$^6$ K. The remainder of the plasma can contribute to a lesser extent.

astro-ph.HE↗

arXiv2023

The Bright Supernova 1996cr in the Circinus Galaxy Imaged with VLBI: Shell Structure with Complex Evolution

We present broadband radio flux-density measurements supernova (SN) 1996cr, made with MeerKAT, ATCA and ALMA, and images made from very long baseline interferometry (VLBI) observations with the Australian Long Baseline Array. The spectral energy distribution of SN 1996cr in 2020, at age, $t \sim$8700 d, is a power-law, with flux density, $S \propto ν^{-0.588 \pm 0.011}$ between 1 and 34 GHz, but may steepen at $>35$ GHz. The spectrum has flattened since $t = 5370$ d (2010). Also since $t = 5370$ d, the flux density has declined rapidly, with $S_{\rm 9 \, GHz} \propto t^{-2.9}$. The VLBI image at $t = 8859$ d shows an approximately circular structure, with a central minimum reminiscent of an optically-thin spherical shell of emission. For a distance of 3.7 Mpc, the average outer radius of the radio emission at $t = 8859$ d was $(5.1 \pm 0.3) \times 10^{17}$ cm, and SN 1996cr has been expanding with a velocity of $4650 \pm 1060$ km s$^{-1}$ between $t=4307$ and 8859 d. It must have undergone considerable deceleration before $t = 4307$ d. Deviations from a circular shell structure in the image suggest a range of velocities up to $\sim$7000 km s$^{-1}$, and hint at the presence of a ring- or equatorial-belt-like structure rather than a complete spherical shell.

astro-ph.HE↗

arXiv2022

Chandra Fails to Detect X-ray Emission from Type Ia SN 2018fhw/ASASSN-18tb

We report on Chandra X-ray observations of ASASSN-18tb/SN 2018fhw, a low luminosity Type Ia supernova that showed a H line in its optical spectrum. No X-ray emission was detected at the location of the SN. Upper limits to the luminosity of up to 3 $\times 10^{39}$ erg s$^{-1}$ are calculated, depending on the assumed spectral model, temperature and column density. These are compared to two Type Ia-CSM SNe, SN 2005gj and SN 2002ic, that have been observed with Chandra in the past. The upper limits are lower than the X-ray luminosity found for the Type Ia-CSM SN 2012ca, the only Type Ia SN to have been detected in X-rays. Consideration of various scenarios for the H$α$ line suggests that the density of the surrounding medium at the time of H$α$ line detection could have been as high as 10$^8$ cm$^{-3}$, but must have decreased below 5 $\times\, 10^6$ cm$^{-3}$ at the time of X-ray observation. Continual X-ray observations of SNe which show a H line in their spectrum are necessary in order to establish Type Ia SNe as an X-ray emitting class.

astro-ph.HE↗

arXiv2022

Seven Years of SN 2014C: a Multi-Wavelength Synthesis of an Extraordinary Supernova

SN 2014C was originally classified as a Type Ib supernova, but at phase ϕ = 127 d post-explosion strong Hα emission was observed. SN 2014C has since been observed in radio, infrared, optical and X-ray bands. Here we present new optical spectroscopic and photometric data spanning ϕ = 947 - 2494 d post-explosion. We address the evolution of the broadened Hα emission line, as well as broad [O III] emission and other lines. We also conduct a parallel analysis of all publicly available multi-wavelength data. From our spectra, we find a nearly constant Hα FWHM velocity width of {\sim}2000 km/s that is significantly lower than that of other broadened atomic transitions ({\sim}3000 - 7000 km/s) present in our spectra ([O I] λ6300; [O III] λλ4959,5007; He I λ7065; [Ca II] λλ7291,7324). The late radio data demand a fast forward shock ({\sim}10,000 km/s at ϕ = 1700 d) in rarified matter that contrasts with the modest velocity of the Hα. We propose that the infrared flux originates from a toroidal-like structure of hydrogen surrounding the progenitor system, while later emission at other wavelengths (radio, X-ray) likely originates predominantly from the reverse shock in the ejecta and the forward shock in the quasi-spherical progenitor He wind. We propose that the Hα emission arises in the boundary layer between the ejecta and torus. We also consider the possible roles of a pulsar and a binary companion.

astro-ph.HE↗

arXiv2022

Ionization-Gasdynamic Simulations of Wind-Blown Nebulae around Massive Stars

Using a code that employs a self-consistent method for computing the effects of photo-ionization on circumstellar gas dynamics, we model the formation of wind-driven nebulae around massive stars. We take into account changes in stellar properties and mass-loss over the star's evolution. Our simulations show how various properties, such as the density and ionization fraction, change throughout the evolution of the star. The multi-dimensional simulations reveal the presence of strong ionization front instabilities in the main-sequence phase, similar to those seen in galactic ionization fronts. Hydrodynamic instabilities at the interfaces lead to the formation of filaments and clumps that are continually being stripped off and mixed with the low density interior. Even though the winds start out as completely radial, the spherical symmetry is quickly destroyed, and the shocked wind region is manifestly asymmetrical. The simulations demonstrate that it is important to include the effects of the photoionizing photons from the star, and simulations that do not include this may fail to reproduce the observed density profile and ionization structure of wind-blown bubbles around massive stars.

astro-ph.SR↗

arXiv2021

A Long-term Study of Ultraluminous X-ray Sources in NGC 891

We perform empirical fits to the \emph{Chandra} and \emph{XMM-Newton} spectra of three ultraluminous X-ray sources (ULXs) in the edge-on spiral galaxy NGC 891, monitoring the region over a seventeen year time window. One of these sources has been visible since the early 1990s with \emph{ROSAT} and has been observed multiple times with \emph{Chandra} and \emph{XMM-Newton}. Another has been visible since 2011. We build upon prior analyses of these sources by analyzing all available data at all epochs. Where possible \emph{Chandra} data is used, since its superior spatial resolution allows for more effective isolation of the emission from each individual source, thus providing a better determination of their spectral properties. We also identify a new transient ULX, CXOU J022230.1+421937, which faded from view over the course of a two month period from Nov 2016 to Jan 2017. Modeling of each source at every epoch was conducted using six different models ranging from thermal bremsstrahlung to accretion disk models. Unfortunately, but as is common with many ULXs, no single model yielded a much better fit than the others. The two known sources had unabsorbed luminosities that remained fairly consistent over five or more years. Various possibilities for the new transient ULX are explored.

astro-ph.HE↗

arXiv2021

Can the Fe K-alpha Line Reliably Predict Supernova Remnant Progenitors?

The centroid energy of the Fe K$α$ line has been used to identify the progenitors of supernova remnants (SNRs). These investigations generally considered the energy of the centroid derived from the spectrum of the entire remnant. Here we use {\it XMM-Newton} data to investigate the Fe K$α$ centroid in 6 SNRs: 3C~397, N132D, W49B, DEM L71, 1E 0102.2-7219, and Kes 73. In Kes 73 and 1E 0102.2-7219, we fail to detect any Fe K$α$ emission. We report a tentative first detection of Fe K$α$ emission in SNR DEM L71, with a centroid energy consistent with its Type Ia designation. In the remaining remnants, the spatial and spectral sensitivity is sufficient to investigate spatial variations of the Fe K$α$ centroid. We find in N132D and W49B that the centroids in different regions are consistent with that derived from the overall spectrum, although not necessarily with the remnant type identified via other means. However, in SNR 3C~397, we find statistically significant variation in the centroid of up to 100 eV, aligning with the variation in the density structure around the remnant. These variations span the intermediate space between centroid energies signifying core-collapse and Type Ia remnants. Shifting the dividing line downwards by 50 eV can place all the centroids in the CC region, but contradicts the remnant type obtained via other means. Our results show that caution must be used when employing the Fe K$α$ centroid of the entire remnant as the sole diagnostic for typing a remnant.

astro-ph.HE↗

arXiv2021

X-ray Luminosity of Decades-Old Supernovae

The transition from supernovae (SNe) to supernova remnants (SNRs) remains poorly understood, given the age gap between well-studied examples of the two. In order to bridge this gap, we analysed archival Chandra data for some of the oldest supernovae detected in X-rays, in order to extend their light curves out to late times. We fitted the spectra with thermal models. All the SNe with multiple X-ray data points were found to have similar X-ray luminosity, which was decreasing with time. The X-ray luminosity will likely continue to decrease while the SNe are evolving in a wind medium, but is anticipated to increase in the Sedov phase when the SNe are interacting with a constant density interstellar medium, bringing it in line with observed SNRs.

astro-ph.HE↗

arXiv2021

Two Decades of Education and Public Outreach with Chicago Public Schools

Over the past two decades, I have been actively involved in teaching astronomy and astrophysics to Chicago Public School (CPS) students and their teachers, in collaboration with various groups as well as by myself. Valuable resources that we have created for schools include the Multiwavelength Astronomy Website, with modules for infrared, optical, ultraviolet, X-ray and gamma-ray astronomy. The content of each lesson is derived from interviews with scientists, archived oral histories, and/or memoirs. Lessons are evaluated by a science educator and at least one subject matter expert before being produced for the web. They are supplemented by NASA media, archival material from the University of Chicago Library and other archives, and participant contributed photographs, light curves, and spectra. Summer programs provided training to CPS teachers to use the resources in their classrooms. Currently, I lead the Chicago Area Research Mentoring (CHARM) initiative. In the past academic year I worked with a class of 17 diverse 11th grade honors students at the University of Chicago Charter School, Woodlawn. Through frequent lectures ($\sim$ every 4 weeks), these students were exposed to astrophysical topics and concepts not normally covered in a school curriculum. CHARM aims to develop the student's critical thinking, introduce them to astrophysical research methods and techniques, and prepare them for a career in science, technology, engineering and mathematics (STEM), particularly a research-oriented one. In this article, I highlight some projects, educational resources, results achieved, and lessons learned along the way.

physics.ed-ph↗

arXiv2020

Supernova X-Ray Database (SNaX) Updated to Ensure Long-term Stability

The Supernova X-Ray Database (SNaX) was established a few years ago to make X-ray data on supernovae (SNe) publicly available via an elegant searchable web interface. The database has recently been updated to PhP7, had security updates done, and moved to a new server, ensuring its long-term stability. We urge astronomers to continue to download the data as needed for their work. Those with X-ray data on SNe are requested to upload it to the database via the easily fillable spreadsheet, making it accessible to everyone.

astro-ph.HE↗

arXiv2020

Analysis of XMM-Newton Observations of Supernova Remnant W49B and Clues to the Progenitor

W49B is a supernova remnant (SNR) discovered over 60 years ago in early radio surveys. It has since been observed over the entire wavelength range, with the X-ray morphology resembling a centrally-filled SNR. The nature of its progenitor star is still debated. Applying Smoothed Particle Inference techniques to analyze the X-Ray emission from W49B, we characterize the morphology and abundance distribution over the entire remnant. We also infer the density structure and derive the mass of individual elements present in the plasma. The morphology is consistent with an interaction between the remnant and a dense medium along the eastern edge, and some obstruction towards the west. We find a total mass of 130 $(\pm 16)$ M$_{\odot}$ and an estimated ejecta mass of 1.2 $(\pm 0.2)$ M$_{\odot}$. Comparison of the inferred abundance values and individual element masses with a wide selection of SN models suggests that deflagration-to-detonation (DDT) Type Ia models are the most compatible, with Fe abundance being the major discriminating factor. The general agreement between our abundance measurements and those from previous studies suggests that disagreement between various authors is more likely due to the choice of models used for comparison, rather than the abundance values themselves. While our abundance results lean toward a Type Ia origin, ambiguities in the interpretation of various morphological and spectral characteristics of W49B do not allow us to provide a definitive classification.

astro-ph.HE↗

arXiv2020

From Supernova to Remnant: Tracking the Evolution of the Oldest Known X-ray Supernovae

Core-collapse supernovae (SNe) expand into a medium created by winds from the pre-SN progenitor. The SN explosion and resulting shock wave(s) heat up the surrounding plasma, giving rise to thermal X-ray emission, which depends on the density of the emitting material. Tracking the variation of the X-ray luminosity over long periods of time thus allows for investigation of the kinematics of the SN shock waves, the structure of the surrounding medium, and the nature of the progenitor star. In this paper X-ray observations of five of the oldest known X-ray supernovae - SN 1970G, SN 1968D, SN 1959D, SN 1957D and SN 1941C - are analyzed, with the aim of reconstructing their light curves over several decades. For those supernovae for which we can extract multi-epoch data, the X-ray luminosity appears to decline with time, although with large error bars. No increase in the X-ray emission from SN 1970G is found at later epochs, contrary to previous reports. All five SNe show X-ray luminosities that are of comparable magnitude. We compare the late-time X-ray luminosities of these SNe to those of supernova remnants (SNRs) in the Galaxy which are a few hundred years old, and find that when the tentative decline is taken into account, the luminosity of the old SNe studied herein could fall below the luminosity of some of the younger SNRs within a few hundred years. However, the X-ray luminosity should begin to increase as the SNe expand in the Sedov phase, thus reaching that of the observed SNRs.

astro-ph.HE↗

arXiv2020

Time-dependent high-energy gamma-ray signal from accelerated particles in core-collapse supernovae: the case of SN 1993J

Some core-collapse supernovae are likely to be efficient cosmic-ray accelerators up to the PeV range, and therefore, to potentially play an important role in the overall Galactic cosmic-ray population. The TeV gamma-ray domain can be used to study particle acceleration in the multi-TeV and PeV range. This motivates the study of the detectability of such supernovae by current and future gamma-ray facilities. The gamma-ray emission of core-collapse supernovae strongly depends on the level of the two-photon annihilation process: high-energy gamma-ray photons emitted at the expanding shock wave following the supernova explosion can interact with soft photons from the supernova photosphere through the pair production channel, thereby strongly suppressing the flux of gamma rays leaving the system. In the case of SN 1993J, whose photospheric and shock-related parameters are well measured, we calculate the temporal evolution of the expected gamma-ray attenuation by accounting for the temporal and geometrical effects. We find the attenuation to be of about $10$ orders of magnitude in the first few days after the SN explosion. The probability of detection of a supernova similar to SN 1993J with the Cherenkov Telescope Array is highest if observations are performed either earlier than 1 day, or later than 10 days after the explosion, when the gamma-ray attenuation decreases to about $2$ orders of magnitude.

astro-ph.HE↗