arXiv subjects

Eric M. Schlegel

Publications and source records attributed to Eric M. Schlegel.

At least 19 recordsLinked to original sources

arXiv2026

High Resolution X-ray Spectroscopy of the Nova-Like Cataclysmic Variable BZ Cam using Chandra HETG: Diagnosis of the ADAF-like (Advective) Hot Flow

Nova-likes such as BZ Cam are high state Cataclysmic Variables showing hard X-ray emission that can be characterized with advective hot flows in the inner accretion disk. We explore Chandra High Energy Transmission Grating (HETG) observations of BZ~Cam for detailed line diagnosis and ionization conditions in the X-ray regime. We mostly find H- and He-like emission lines of Mg, Si, S, and Fe. All He-like line components of forbidden, intercombination and resonance lines are present. The R ratios of selected lines indicate plasma densities of a few $\times$10$^{12-14}$ cm$^{-3}$ and G ratios reveal temperatures (3-6)$\times$ 10$^6$ K where the Fe lines yield (1-3)$\times$ 10$^7$ K. The H to He line ratios and the R and G ratios show that the plasma is in a nonequilibrium ionization condition, which is consistent with our previous X-ray results and the accretion flow in the X-ray region being an ADAF-like (advective) hot flow. Simultaneous fits of the HEG and MEG spectra or the broadband joint spectra of ROSAT, Chandra zero order and NuSTAR yield temperatures 3.4-6.3 keV using a VNEI model of plasma emission (in XSPEC) or Bremsstrahlung emission. An additional power law is detected above 98\% Confidence Level in the broadband analysis. The orbital variations and the broadband spectra show dipping/veiling of the X-rays and an additional warm absorber model with an ionization parameter log($ξ$) = 2.7 is required at the 3$σ$ level, along with the VNEI model where the HEG and MEG simultaneous fits yield the log($ξ$) = 3.6 .

astro-ph.HE↗

arXiv2024

Multi-Epoch Observations of the Nearby Spiral Galaxy NGC 3938 with the Chandra X-ray Observatory

We present an analysis of two epochs of ACIS observations of the SA(s)c spiral galaxy NGC 3938 with the Chandra X-ray Observatory. The total exposure time of the observations was 95 ksec with a limiting unabsorbed luminosity of approximately 10^{38}$ ergs/sec assuming a distance of 22 Mpc. A total of 47 discrete merged sources from both epochs were detected at the 3sigma level or greater with the D25 radius. We demonstrate that at the time of the Chandra observations, the nucleus was not detected. We connect the detected sources to counterparts in other wavebands to the degree possible. Based on the two epochs, we identify three variable sources and an additional two that may have varied between the two observations. We do not formally detect any of the five historical supernovae that have occurred in NGC 3938. The luminosity function of NGC 3938 is compared to a recent compilation of 38 galaxies and we identify a potentially significant problem with the `known' distance to NGC 3938. Star formation rate and metallicity values are also computed; the star formation rate is highly dependent upon the adopted distance. The metallicity appears to lie in the range of 8.2-9.2, consistent with values from other work. We include in an appendix a short discussion of the sources that lie in Chandra's field-of-view but lie outside of NGC 3938.

astro-ph.GA↗

arXiv2024

X-ray Spectroscopy of the Dwarf Nova Z Chamaeleontis in Quiescence and Outburst Using the XMM-Newton Observatory

We present X-ray spectroscopy of the SU UMa-type dwarf nova (DN) Z Cha using the EPIC and RGS instruments onboard the XMM-Newton Observatory. The quiescent system can be modeled by collisional equilibrium or nonequilibrium plasma models, yielding a kT of 8.2-13.0 keV at a luminosity of (5.0-6.0)$\times$10$^{30}$ erg/s. The spectra yield better reduced $χ^{2}$ using partial covering absorbers of cold and photoionized nature. The ionized absorber has an equivalent N$_H$=(3.4-5.9)$\times$10$^{22}$ cm$^{-2}$ and a log($ξ$)=3.5-3.7 with (50-60)% covering fraction when VNEI model (XSPEC) is used. The line diagnosis in quiescence shows no resonance lines with only detected forbidden lines of Ne, Mg, Si. The H-like C, O, Ne, and Mg are detected. The strongest line is O VIII with (2.7-4.6)$\times$10$^{-14}$ erg/s/cm$^2$. The quiescent X-ray emitting plasma is not collisional and not in ionization equilibrium which is consistent with hot ADAF-like accretion flows. The line diagnosis in outburst shows He-like O, and Ne with intercombination lines being the strongest along with weaker resonance lines. This indicates the plasma is more collisional and denser, but yet not in a collisional equilibrium, revealing ionization timescales of (0.97-1.4)$\times$10$^{11}$ s cm$^{-3}$. The R-ratios in outburst yield electron densities of (7-90)$\times$10$^{11}$ cm$^{-3}$ and the G-ratios yield electron temperatures of (2-3)$\times$10$^{6}$ K. The outburst luminosity is (1.4-2.5)$\times$10$^{30}$ erg/s. The flow is inhomogeneous in density. All detected lines are narrow with widths limited by the resolution of RGS yielding Keplerian rotational velocities $<$1000 km/s. This is too low for boundary layers, consistent with the nature of ADAF-like hot flows.

astro-ph.HE↗

arXiv2022

Characterizing the Advective Hot Flows of Nova-Like Cataclysmic Variables in the X-rays: The case of BZ Cam and V592 Cas

We present a joint spectral analysis of ROSAT PSPC, Swift XRT, and NuSTAR FPMA/B data of the nova-like (NL) cataclysmic variables (CVs), BZ Cam and V592 Cas in the 0.1-78.0 keV band. Plasma models of collisional equilibrium fail to model the 6.0-7.0 iron line complex and continuum with $χ^2_ν$ larger than 2.0. Our results show nonequilibrium ionization (NEI) conditions in the X-ray plasma with temperatures of 8.2-9.4 keV and 10.0-12.9 keV for BZ Cam and V592 Cas, respectively. The centroids of He-like and H-like iron ionization lines are not at their equilibrium values as expected from NEI conditions. We find power law spectral components that reveal the existence of scattering and Comptonization with a photon index of 1.50-1.87. We detect a P Cygni profile in the H-like iron line of BZ Cam translating to outflows of 4500-8700 km s$^{-1}$ consistent with the fast winds in the optical and UV. This is the first time such a fast collimated outflow is detected in the X-rays from an accreting CV. An Iron K$α$ line around 6.2-6.5 keV is found revealing the existence of reflection effects in both sources. We study the broadband noise and find that the optically thick disk truncates in BZ Cam and V592 Cas consistent with transition to an advective hot flow structure. V592 Cas also exhibits a quasi-periodic oscillation at 1.4$^{+2.6}_{-0.3}$ mHz. In general, we find that the two NLs portray spectral and noise characteristics as expected from advective hot accretion flows at low radiative efficiency.

astro-ph.HE↗

arXiv2021

Multiwavelength Observations of the RV Tauri Variable System U Monocerotis: Long-Term Variability Phenomena That Can Be Explained by Binary Interactions with a Circumbinary Disk

We present X-ray through submillimeter observations of the classical RV Tauri (RVb-type) variable U Mon, a post-AGB binary with a circumbinary disk (CBD). Our SMA observations indicate a CBD diameter of $\lesssim$550 au. Our XMM-Newton observations make U Mon the first RV Tauri variable detected in X-rays. The X-ray emission is characteristic of a hot plasma ($\sim$10 MK), with L$_{X}=5\times10^{30}{\rm erg}~{\rm s}^{-1}$, and we consider its possible origin from U Mon, its companion, and/or binary system interactions. Combining DASCH and AAVSO data, we extend the time-series photometric baseline back to the late 1880s and find evidence that U Mon has secular changes that appear to recur on a timescale of $\sim$60 yr, possibly caused by a feature in the CBD. From literature radial velocities we find that the binary companion is a $\sim$2 M$_{\odot}$ A-type main-sequence star. The orientation of the binary's orbit lies along our line of sight ($ω= 95^\circ$), such that apastron corresponds to photometric RVb minima, consistent with the post-AGB star becoming obscured by the near side of the CBD. In addition, we find the size of the inner-CBD hole ($\sim$4.5-9 au) to be comparable to the binary separation, implying that one or both stars may interact with the CBD at apastron. The obscuration of the post-AGB star implicates the companion as the likely source of the enhanced H$α$ observed at RVb minima and of the X-ray emission that may arise from accreted material.

astro-ph.SR↗

arXiv2020

Multiwavelength Analysis of the Variability of the Blazar 3C 273

We present multiwavelength light curves and polarimetric data of the Flat Spectrum Radio Quasar 3C 273 over 8 years. The wavelength range of our data set extends from radio to gamma-rays. We found that the optical emission in this source is dominated by the accretion disk during the entire time-frame of study. We additionally find that in contrast with the observed behaviour in other blazars, 3C 273 does not show a correlation between the gamma-ray spectral index and the gamma-ray luminosity. Finally, we identified an anti-correlation between the 15 GHz and V-band light curves for the time-range $JD_{245} = 4860 - 5760$, which we speculate is the consequence of the inner part of the accretion disk falling into the black hole, followed by the ejection of a component into the jet.

astro-ph.HE↗

arXiv2020

Physical geometry of the quasispherical Szekeres models

The quasispherical Szekeres metric is an exact solution to Einstein's equations describing an inhomogeneous and anisotropic cosmology. Though its governing equations are well-known, there are subtle, often-overlooked details in how the model's functions relate to its physical layout, including the shapes and relative positions of structures. We present an illustrated overview of the quasispherical Szekeres models and show exactly how the model functions relate to the physical shape and distribution of matter. In particular, we describe a shell rotation effect that has not previously been fully understood. We show how this effect relates to other known properties, and lay out some mathematical tools useful for constructing models and picturing them accurately.

gr-qc↗

arXiv2019

The X-ray-Resolved Supernova Remnant S8 in the Dwarf Irregular Galaxy IC 1613

We conducted an observation of the nearby irregular galaxy IC 1613 with the Chandra X-ray Observatory using the S3 chip of the ACIS with an effective exposure time of 49.9 ksec. The observation primarily targeted the extensive bubble and star formation region in the northeast quadrant of the galaxy. The only known supernova remnant (SNR) in IC 1613, S8, is also the galaxy's most luminous X-ray source (L_X (0.3-8 keV) ~5-6x10^{36} erg/s). We resolve the SNR with Chandra and compare its nearly circular X-ray morphology with Halpha and radio images. We assign an upper limit on the luminosity of any possible associated compact central object of ~4x10^{35}$ erg/s (0.3-8 keV band) and conclude that we would detect a Crab-like pulsar but not a Cas A-like object. We infer an age for S8 of ~3400-5600 years and compare it to other SNRs in the Local Group. We suggest that S8 is a young composite SNR.

astro-ph.HE↗

arXiv2019

CMB dipoles and other low-order multipoles in the quasispherical Szekeres model

Several authors have previously shown that Gpc-scale void based on the spherically symmetric LTB model can provide a good fit to certain cosmological data, including the SNIa data, but it is only consistent with the observed CMB dipole if we are located very close to the center, in violation of the Copernican principle. In this work we investigate the more general quasispherical Szekeres model, which does not include spherical symmetry, in order to determine whether this option may be less constricting. We find that the observer is still constrained to a small region, but it is not as geometrically ``special'' as the center of an LTB void. Furthermore, whereas the quadrupole and octupole near the center of an LTB void are necessarily small, certain Szekeres models can include a significant quadrupole while still being consistent with the observed dipole, hinting that Szekeres models may be able give an explanation for the observed quadrupole/octupole anomalies.

astro-ph.CO↗

arXiv2019

The {\it K2} Light Curves and Stunted Outbursts of AC Cnc

We describe two observations of the nova-like cataclysmic variable AC Cnc obtained with {\it Kepler} during its revamped second mission ({\it K2}). Using the {\it K2} 1-minute cadence mode, the data were obtained during Campaigns 5 and 18. Campaign 5 (C05) lasted from ${\sim}$2015 Apr 27 to ${\sim}$2015 July 10, a total of 74.8 days, and yielded ${\sim}$106,000 measurements. Campaign 18 (C18) lasted from ${\sim}$2018 May 13 to ${\sim}$2018 July 2, a total of 50.7 days, yielding $\sim$72,000 measurements. The C05 light curve reveals two `stunted outbursts' having properties consistent with stunted bursts observed from the ground; a stunted burst was underway during the C18 observation when it ended. During a stunted outburst, the primary eclipse is found to increase in depth but the residual brightness at mid-primary-eclipse remains nearly constant. By contrast, the secondary eclipse retains the same depth but the brightness at mid-secondary eclipse increases during outburst, following the orbital variations of the out-of-eclipse light. The eclipse ephemeris is statistically consistent with historical ephemerides and shows only marginal evidence for a period change. On the basis of the {\it K2} data, we can not confirm a previously-reported non-orbital periodicity in AC~Cnc.

astro-ph.SR↗

arXiv2018

Time Evolution of Pulsar Magnetosphere I - An Implicit Approach

We apply a computationally efficient technique to validate the global structure of the pulsar magnetosphere. In this first of a series of studies, a 3D, computationally intensive, implicit Crank-Nicolson finite-difference scheme is developed. The region of magnetic influence is evolved under the approximation of force-free electrodynamics. The main objective of this paper is to present our code and use it to demonstrate and verify the now widely - accepted global features of a pulsar magnetosphere. Our results qualitatively agree with previously developed time-dependent models for an oblique rotator. In line with earlier studies, we also demonstrate that our simulations can run for many stellar rotations. Once we extend our code, we believe that our implicit approach can be extremely useful to investigate magnetospheres filled with resistive plasma, develop better resolution current sheets and investigate small scale microphysics of pair creation using particle-in-cell techniques.

astro-ph.HE↗

arXiv2018

A Multi-Frequency Study of the Milky Way-like Spiral Galaxy NGC 6744

We present a multi-frequency study of the intermediate spiral SAB(r)bc type galaxy NGC 6744, using available data from the Chandra X-Ray telescope, radio continuum data from the Australia Telescope Compact Array and Murchison Widefield Array, and Wide-field Infrared Survey Explorer infrared observations. We identify 117 X-ray sources and 280 radio sources. Of these, we find nine sources in common between the X-ray and radio catalogues, one of which is a faint central black hole with a bolometric radio luminosity similar to the Milky Way's central black hole. We classify 5 objects as supernova remnant candidates, 2 objects as likely supernova remnants, 17 as HII regions, 1 source as an AGN; the remaining 255 radio sources are categorised as background objects and one X-ray source is classified as a foreground star. We find the star-formation rate (SFR) of NGC 6744 to be in the range 2.8 - 4.7 $\rm{M_{\odot}~yr^{-1}}$ signifying the galaxy is still actively forming stars. The specific SFR of NGC 6744 is greater than that of late-type spirals such as the Milky Way, but considerably less that that of a typical starburst galaxy.

astro-ph.GA↗

arXiv2016

NGC 5195 in M51: Feedback `Burps' after a Massive Meal?

We describe a double-arc-like X-ray structure lying ~15-30" (~0.8-1.7 kpc) south of the NGC 5195 nucleus visible in the merged exposures of long Chandra pointings of M51. The curvature and orientation of the arcs argues for a nuclear origin. The arcs are radially separated by ~15" (~1$ kpc), but are rotated relative to each other by ~30 deg. From an archival image, we find a slender Halpha-emitting region just outside the outer edge of the outer X-ray arc, suggesting that the X-ray-emitting gas plowed up and displaced the Halpha-emitting material from the galaxy core. Star formation may have commenced in that arc. Halpha emission is present at the inner arc, but appears more complex in structure. In contrast to an explosion expected to be azimuthally symmetric, the X-ray arcs suggest a focused outflow. We interpret the arcs as episodic outbursts from the central super-massive black hole (SMBH). We conclude that NGC 5195 represents the nearest galaxy exhibiting on-going, large-scale outflows of gas, in particular, two episodes of a focused outburst of the SMBH. The arcs represent a clear demonstration of feedback.

astro-ph.GA↗

arXiv2015

The 2010 Eruption of the Recurrent Nova U Scorpii: The Multi-Wavelength Light Curve

The recurrent nova U Scorpii most recently erupted in 2010. Our collaboration observed the eruption in bands ranging from the Swift XRT and UVOT w2 (193 nm) to K-band (2200 nm), with a few serendipitous observations stretching down to WISE W2 (4600 nm). Considering the time and wavelength coverage, this is the most comprehensively observed nova eruption to date. We present here the resulting multi-wavelength light curve covering the two months of the eruption as well as a few months into quiescence. For the first time, a U Sco eruption has been followed all the way back to quiescence, leading to the discovery of new features in the light curve, including a second, as-yet-unexplained, plateau in the optical and near-infrared. Using this light curve we show that U Sco nearly fits the broken power law decline predicted by Hachisu & Kato, with decline indices of -1.71 +/- 0.02 and -3.36 +/- 0.14. With our unprecedented multi-wavelength coverage, we construct daily spectral energy distributions and then calculate the total radiated energy of the eruption, E_rad=6.99 (+0.83)(-0.57) * 10^44 erg. From that, we estimate the total amount of mass ejected by the eruption to be m_ej=2.10 (+0.24)(-0.17) * 10^-6 M_solar. We compare this to the total amount of mass accreted by U Sco before the eruption, to determine whether the white dwarf undergoes a net mass loss or gain, but find that the values for the amount of mass accreted are not precise enough to make a useful comparison.

astro-ph.SR↗

arXiv2012

Evidence for Type Ia Supernova Diversity from Ultraviolet Observations with the Hubble Space Telescope

We present ultraviolet (UV) spectroscopy and photometry of four Type Ia supernovae (SNe 2004dt, 2004ef, 2005M, and 2005cf) obtained with the UV prism of the Advanced Camera for Surveys on the Hubble Space Telescope. This dataset provides unique spectral time series down to 2000 Angstrom. Significant diversity is seen in the near maximum-light spectra (~ 2000--3500 Angstrom) for this small sample. The corresponding photometric data, together with archival data from Swift Ultraviolet/Optical Telescope observations, provide further evidence of increased dispersion in the UV emission with respect to the optical. The peak luminosities measured in uvw1/F250W are found to correlate with the B-band light-curve shape parameter dm15(B), but with much larger scatter relative to the correlation in the broad-band B band (e.g., ~0.4 mag versus ~0.2 mag for those with 0.8 < dm15 < 1.7 mag). SN 2004dt is found as an outlier of this correlation (at > 3 sigma), being brighter than normal SNe Ia such as SN 2005cf by ~0.9 mag and ~2.0 mag in the uvw1/F250W and uvm2/F220W filters, respectively. We show that different progenitor metallicity or line-expansion velocities alone cannot explain such a large discrepancy. Viewing-angle effects, such as due to an asymmetric explosion, may have a significant influence on the flux emitted in the UV region. Detailed modeling is needed to disentangle and quantify the above effects.

astro-ph.HE↗

arXiv2011

A Chandra Observation of the Nearby Sculptor Group Sd Galaxy NGC 7793

(Abridged) We conducted a Chandra ACIS observation of the nearby Sculptor Group Sd galaxy NGC 7793. At the assumed distance to NGC 7793 of 3.91 Mpc, the limiting unabsorbed luminosity of the detected discrete X-ray sources (0.2-10.0 keV) is approximately 3x10^36 ergs s^-1. A total of 22 discrete sources were detected at the 3-sigma level or greater including one ultra-luminous X-ray source (ULX). Based on multiwavelength comparisons, we identify X-ray sources coincident with one SNR, the candidate microquasar N7793-S26, one HII region and two foreground Galactic stars. We also find that the X-ray counterpart to the candidate radio SNR R3 is time-variable in its X-ray emission: we therefore rule out the possibility that this source is a single SNR. A marked asymmetry is seen in the distribution of the discrete sources with the majority lying in the eastern half of this galaxy. All of the sources were analyzed using quantiles to estimate spectral properties and spectra of the four brightest sources (including the ULX) were extracted and analyzed. We searched for time-variability in the X-ray emission of the detected discrete sources using our measured fluxes along with fluxes measured from prior Einstein and ROSAT observations. From this study, three discrete X-ray sources are established to be significantly variable. A spectral analysis of the galaxy's diffuse emission is characterized by a temperature of kT = 0.19-0.25 keV. The luminosity function of the discrete sources shows a slope with an absolute value of Gamma = -0.65+/-0.11 if we exclude the ULX. If the ULX is included, the luminosity function has a long tail to high L_X with a poor-fitting slope of Gamma = -0.62+/-0.2. The ULX-less slope is comparable to the slopes measured for the distributions of NGC 6946 and NGC 2403 but much shallower than the slopes measured for the distributions of IC 5332 and M83.

astro-ph.HE↗

arXiv2010

An X-ray upper limit on the presence of a Neutron Star for the Small Magellanic Cloud and Supernova Remnant 1E0102.2-7219

We present Chandra X-ray Observatory archival observations of the supernova remnant 1E0102.2-7219, a young Oxygen-rich remnant in the Small Magellanic Cloud. Combining 28 ObsIDs for 324 ks of total exposure time, we present an ACIS image with an unprecedented signal-to-noise ratio (mean S/N ~ sqrt(S) ~6; maximum S/N > 35) . We search within the remnant, using the source detection software {\sc wavdetect}, for point sources which may indicate a compact object. Despite finding numerous detections of high significance in both broad and narrow band images of the remnant, we are unable to satisfactorily distinguish whether these detections correspond to emission from a compact object. We also present upper limits to the luminosity of an obscured compact stellar object which were derived from an analysis of spectra extracted from the high signal-to-noise image. We are able to further constrain the characteristics of a potential neutron star for this remnant with the results of the analysis presented here, though we cannot confirm the existence of such an object for this remnant.

astro-ph.GA↗

arXiv2009

A Determination of the Spin of the Black Hole Primary in LMC X-1

The first extragalactic X-ray binary, LMC X-1, was discovered in 1969. In the 1980s, its compact primary was established as the fourth dynamical black-hole candidate. Recently, we published accurate values for the mass of the black hole and the orbital inclination angle of the binary system. Building on these results, we have analyzed 53 X-ray spectra obtained by RXTE and, using a selected sample of 18 of these spectra, we have determined the dimensionless spin parameter of the black hole to be a* = 0.92(-0.07,+0.05). This result takes into account all sources of observational and model-parameter uncertainties. The standard deviation around the mean value of a* for these 18 X-ray spectra, which were obtained over a span of several years, is only 0.02. When we consider our complete sample of 53 RXTE spectra, we find a somewhat higher value of the spin parameter and a larger standard deviation. Finally, we show that our results based on RXTE data are confirmed by our analyses of selected X-ray spectra obtained by the XMM-Newton, BeppoSAX and Ginga missions.

astro-ph.HE↗