arXiv subjects

Daniel A. Schwartz

Publications and source records attributed to Daniel A. Schwartz.

15 recordsLinked to original sources

arXiv2026

Chandra X-ray imaging and IC/CMB model for the inner jet of PKS 0637-752

We present the X-ray surface brightness maps of the jet from the quasar PKS 0637-752, using two epochs of archival Chandra observations (1999 and 2017). We present, for the first time, a model of the faint inner jet extending from 3.4 to 7 arcsec from the core, interpreting its X-ray emission as inverse Compton scattering of cosmic microwave background photons by the synchrotron-emitting relativistic electrons. The inner jet contrasts with the bright outer part of the jet, dominated by several discrete knots for which upper limits to the Fermi gamma-ray flux rule out the inverse Compton mechanism. It is the first detailed inverse Compton model published for this inner jet. We examine three physically motivated scenarios, distinguished by the high-energy cutoff of the electron distribution, that can account for the observed radio, millimeter/submillimeter, and X-ray emission while remaining consistent with optical and gamma-ray upper limits. We also report for the first time X-rays from the radio lobe at the end of the receding jet.

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arXiv2026

Speeding up Gravitational Lens Mass Models with Machine Learning: Applications in X-ray Astronomy

Multi-wavelength observations of quadruply lensed quasars constitute a powerful probe of cosmology, dark matter substructure along the line of sight, and the structure of X-ray emitting regions in high-redshift quasars. These investigations are conditional on acquiring an accurate model for the surface mass density of matter lensing these quasars. We propose a simulation-based machine learning method to accelerate parameter inference in real quadruply lensed systems by several orders of magnitude. We simulate a grid of quadruply lensed sources with Singular Isothermal Ellipsoid (SIE) lenses and use the projected positions of the four lensed images to train two fully connected neural networks that predict the mass parameter and ellipticity. For a large fraction of simulated systems, the neural network-initialised mass models converge in time-scales of a few minutes and recover the source position at the <0.''005 level for a broad range of lens masses and ellipticities. We apply our neural networks to seven quadruply lensed quasars, lensed by isolated galaxies or a galaxy-perturber pair, which have archival Chandra observations. The final optimised mass models for each quasar predict the observed lensed image positions in Gaia Data Release 3. These mass models enable the caustic method, which locates the X-ray-to-optical emission regions to milliarcsecond precision in these otherwise unresolvable systems, improving the effective angular resolution of Chandra at high-z by up to two orders of magnitude. Our approach accelerates this mass modelling by supplying informed initial parameters, enabling application to the many new quadruply lensed systems expected from forthcoming surveys.

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arXiv2025

Two Epochs of VLBI Observations of 8 KISSR Seyfert & LINER Galaxies: Suggestions of Fast and Filamentary Outflows

We present here the results from a second epoch of phase-referenced VLBA observations of 8 Seyfert and LINER galaxies from the KISSR sample. These sources were chosen based on the presence of double peaks or asymmetries in their emission lines as observed in SDSS spectra. Parsec-scale radio emission is detected in 7 of the 8 sources in the second epoch. Jet-like features appear to persist over a $\sim4-9$ year timeline in these `radio-quiet' AGN. A few sources like KISSR1494, however, show significantly different structures after a 9 year interval. KISSR102, which was previously suggested to be a binary black hole candidate based on the presence of two compact cores, shows the tentative signatures of superluminal jet motion ($1.05\pm0.45$c). Tentative superluminal motion in a jet knot has been reported in another source, KISSR872 ($1.65\pm0.57$c). We present 1.5 GHz images from the VLA A-array of 4 sources. These images reveal core-lobe or core-halo structures. The alignment of the VLBI jet direction with the kpc-scale spectral index gradient, as well as the mismatch between the star formation rate derived from the radio and H$α$ line emission, support the suggestion that the kpc-scale emission is AGN-jet-related. The jets in KISSR sources appear to lose collimation over spatial scales between 200 parsec and 1 kpc. Overall, the characteristics of the KISSR jets are reminiscent of similar properties observed in VLBI monitoring studies of `radio-loud' AGN jets even as subtle differences related to the compactness and brightness of jet features remain.

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arXiv2025

Investigating the emission mechanism in the spatially-resolved jet of two z $\approx$ 3 radio-loud quasars

This study focuses on high-redshift, z > 3, quasars where resolved X-ray jets remain underexplored in comparison to nearby sources. Building upon previous work, we identify and confirm extended kpc-scale jets emission in two quasars (J1405+0415, z = 3.215 ; J1610+1811, z = 3.122) through meticulous analysis of Chandra X-ray data. To deepen our understanding, high-resolution radio follow- up observations were conducted to constrain relativistic parameters, providing valuable insights into the enthalpy flux of these high-redshift AGN jets. The investigation specifically aims to test the X-ray emission mechanism in these quasars by exploring the inverse Compton scattering of cosmic microwave background (IC/CMB) photons by synchrotron-emitting electrons. Our novel method uses a prior to make a Bayesian estimate of the unknown angle of the jet to our line of sight, thus breaking the usual degeneracy of the bulk Lorentz factor and the Doppler beaming factor.

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arXiv2023

Relativistic Jet Motion in the Radio-quiet LINER Galaxy KISSR872

We report superluminal jet motion with an apparent speed of $β_\mathrm{app}=1.65\pm0.57$ in the radio-quiet (RQ) low ionisation nuclear emission line region (LINER) galaxy, KISSR872. This result comes from two epoch phase-referenced very long baseline interferometry (VLBI) observations at 5 GHz. The detection of bulk relativistic motion in the jet of this extremely radio faint AGN, with a total 1.4 GHz flux density of 5 mJy in the 5.4 arcsec resolution Very Large Array (VLA) FIRST survey image and 1.5 mJy in the $\sim5$ milli-arcsec resolution Very Long Baseline Array (VLBA) image, is the first of its kind in a RQ LINER galaxy. The presence of relativistic jets in lower accretion rate objects like KISSR872, with an Eddington ratio of 0.04, reveals that even RQ AGN can harbor relativistic jets, and evidentiates their universality over a wide range of accretion powers.

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arXiv2023

The Fundamental Planes of Black Hole Activity for Radio-Loud and Radio-Quiet Quasars

We examine the fundamental plane of black hole activity for correlations with redshift and radio loudness in both radio-loud and radio-quiet quasar populations. Sources are compiled from archival data of both radio-loud and radio-quiet quasars over redshifts $0.1 < z < 5.0$ to produce a sample of 353 sources with known X-ray, radio, and black hole mass measurements. A fundamental plane of accretion activity is fit to a sample of radio-loud and radio-quiet quasars, and we find a dichotomy between radio-loud and radio-quiet sources. The set of best-fit equations that best describe the two samples are $\log{L_{R}} = (1.12 \pm 0.06) \log{L_{X}} - (0.20 \pm 0.07) \log{M} -(5.64 \pm 2.99)$ for our radio-loud sample and $\log{L_{R}} = (0.48 \pm 0.06) \log{L_{X}} + (0.50 \pm 0.08) \log{M} + (15.26 \pm 2.66)$ for our radio-quiet sample. Our results suggest that the average radio-quiet quasar emission is consistent with advection dominated accretion, while a combination of jet and disc emission dominates in radio-loud quasars. We additionally examine redshift trends amongst the radio-loud and radio-quiet samples, and we observe a redshift dependence for the fundamental plane of radio-loud quasars. Lastly, we utilize the fundamental plane as a black hole mass estimation method and determine it useful in studying systems where standard spectral modeling techniques are not viable.

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arXiv2021

Discovery of Candidate X-ray Jets in High-Redshift Quasars

We present Chandra X-ray observations of 14 radio-loud quasars at redshifts $3 < z < 4$, selected from a well-defined sample. All quasars are detected in the 0.5-7.0 keV energy band, and resolved X-ray features are detected in five of the objects at distances of 1-12" from the quasar core. The X-ray features are spatially coincident with known radio features for four of the five quasars. This indicates that these systems contain X-ray jets. X-ray fluxes and luminosities are measured, and jet-to-core X-ray flux ratios are estimated. The flux ratios are consistent with those observed for nearby jet systems, suggesting that the observed X-ray emission mechanism is independent of redshift. For quasars with undetected jets, an upper limit on the average X-ray jet intensity is estimated using a stacked image analysis. Emission spectra of the quasar cores are extracted and modeled to obtain best-fit photon indices, and an Fe K emission line is detected from one quasar in our sample. We compare X-ray spectral properties with optical and radio emission in the context of both our sample and other quasar surveys.

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arXiv2021

Multi-scale Radio and X-ray Structure of the High-redshift Quasar PMN J0909+0354

The high-redshift quasar PMN J0909+0354 ($z=3.288$) is known to have a pc-scale compact jet structure, based on global 5-GHz very long baseline interferometry (VLBI) observations performed in 1992. Its kpc-scale structure was studied with the Karl G. Jansky Very Large Array (VLA) in the radio and the Chandra space telescope in X-rays. Apart from the north-northwestern jet component seen in both the VLA and Chandra images at $2.3''$ separation from the core, there is another X-ray feature at $6.48''$ in the northeastern (NE) direction. To uncover more details and possibly structural changes in the inner jet, we conducted new observations at 5 GHz using the European VLBI Network (EVN) in 2019. These data confirm the northward direction of the one-sided inner jet already suspected from the 1992 observations. A compact core and multiple jet components were identified that can be traced up to $\sim0.25$ kpc projected distance towards the north, while the structure becomes more and more diffuse. A comparison with arcsec-resolution imaging with the VLA shows that the radio jet bends by $\sim30^\circ$ between the two scales. The direction of the pc-scale jet as well as the faint optical counterpart found for the newly-detected X-ray point source (NE) favors the nature of the latter as a background or foreground object in the field of view. However, the extended ($\sim160$ kpc) emission around the positions of the quasar core and NE detected by the Wide-field Infrared Survey Explorer (WISE) in the mid-infrared might suggest physical interaction of the two objects.

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arXiv2020

X-ray Properties of Young Radio Quasars at z > 4.5

We present a comprehensive analysis of Chandra X-ray observations of 15 young radio quasars at redshifts $4.5 < z < 5.0$. All sources are detected in the $0.5-7.0$ keV energy band. Emission spectra are extracted, and the average photon index for the sample is measured to be $1.5\pm0.1$. Unabsorbed rest-frame $2-10$ keV luminosities are found to range between $(0.5-23.2) \times 10^{45}$ erg s$^{-1}$. The optical-X-ray power-law spectral index $α_{ox}$ is calculated for each source using optical/UV data available in the literature. The $α_{ox}$-UV relationship is compared with other quasar surveys, and an anticorrelation is observed that agrees with independent estimates. Rest-frame radio and X-ray luminosities are established for the sample, and a correlation between the luminosities is detected. These multiwavelength results reinforce a lack of spectral evolution for quasars over a broad redshift range. We additionally identify three quasars from our multiwavelength analysis that are statistically significant outliers, with one source being a Compton-thick candidate in the early universe, and discuss each in detail.

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arXiv2019

The High-energy emission of jetted AGN

Quasars with flat radio spectra and one-sided, arc-second scale, ~100 mJy GHz radio jets are found to have similar scale X-ray jets in about 60% of such objects, even in short 5 to 10 ks Chandra observations. Jets emit in the GHz band via synchrotron radiation, as known from polarization measurements. The X-ray emission is explained most simply, i.e. with the fewest additional parameters, as inverse Compton (iC) scattering of cosmic microwave background (CMB) photons by the relativistic electrons in the jet. With physics based assumptions, one can estimate enthalpy fuxes upwards of 10^46 erg s$^{-1}$, suffcient to reverse cooling flows in clusters of galaxies, and play a signifcant role in the feedback process which correlates the masses of black holes and their host galaxy bulges. On a quasar-by-quasar basis, we can show that the total energy to power these jets can be supplied by the rotational energy of black holes with spin parameters as low as a = 0.3. For a few bright jets at redshifts less than 1, the Fermi gamma ray observatory shows upper limits at 10 Gev which fall below the fluxes predicted by the iC/CMB mechanism, proving the existence of multiple relativistic particle populations. At large redshifts, the CMB energy density is enhanced by a factor (1+z)^4, so that iC/CMB must be the dominant mechanism for relativistic jets unless their rest frame magnetic field strength is hundreds of micro-Gauss.

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arXiv2011

Deep Multiwaveband Observations of the Jets of 0208-512 and 1202-262

We present deep {\it HST, Chandra, VLA} and {\it ATCA} images of the jets of PKS 0208--512 and PKS 1202--262, which were found in a {\it Chandra} survey of a flux-limited sample of flat-spectrum radio quasars with jets (see Marshall et al., 2005). We discuss in detail their X-ray morphologies and spectra. We find optical emission from one knot in the jet of PKS 1202--262 and two regions in the jet of PKS 0208--512. The X-ray emission of both jets is most consistent with external Comptonization of cosmic microwave background photons by particles within the jet, while the optical emission is most consistent with the synchrotron process. We model the emission from the jet in this context and discuss implications for jet emission models, including magnetic field and beaming parameters.

astro-ph.HE↗

arXiv2004

Chandra measurement of the X-ray spectrum of a quasar at z=5.99

We report the first measurement of the X-ray spectrum of the z = 5.99 quasar SDSSp J130608.26+035626.3 from a 120 ks observation by the Chandra ACIS-S instrument. Between 0.5 and 7 keV, corresponding to 3.5--49 keV in the quasar rest frame, we find an energy index of 0.86 +/- 0.2, consistent with the typical indices found for radio quiet quasars at lower redshifts, and inconsistent with the index required to match the diffuse X-ray background. We have a weak indication of a redshifted Fe-K line. In comparing the counting rate between an earlier, short observation and the longer observation reported here, we find evidence for source variability at the 99.9% confidence level. We note that other nearby X-ray sources would bias the measured alpha_{ox} = 1.70 by -0.09 if the X-ray flux were determined from within a 60" extraction circle. Our results for the energy index and the alpha_{ox} are consistent with no strong evolution in the active galactic nucleus emission mechanism with redshift out to z ~ 6, and therefore with the picture that massive black holes have already formed less than 1 Gyr after the big bang.

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arXiv2004

An X-ray View of Radio Sources

We review recent examples where the synergy between radio and X-ray observations has led to substantial progress in understanding astronomical systems. The sub-arcsecond imaging capabilities of the Chandra X-ray observatory provides a 100-fold improvement for comparing X-ray and radio structures. We specifically discuss examples which provide insight into the outflow of material and energy from pulsars and supernovae, the centers of clusters of galaxies, and the nuclei of quasars.

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arXiv2004

The Development and Scientific Impact of the Chandra X-Ray Observatory

I review the operational capabilities of the Chandra X-ray Observatory, including some of the spectacular results obtained by the general observer community. A natural theme of this talk is that Chandra is revealing outflows of great quantities of energy that were not previously observable. I highlight the Chandra studies of powerful X-ray jets. This subject is only possible due to the sub-arcsecond resolution of the X-ray telescope.

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arXiv2002

A Chandra Search for X-Ray Jets in Redshift 6 Quasars

We have searched for X-ray jets in the recent Chandra observations of three Sloan Digital Sky Survey (SDSS) quasars at redshift z ~ 6. All 3 quasars were detected in X-rays in these relatively short observations. SDSS 1030+0524 is not consistent with a point source, and may be a gravitationally lensed system. We find a possible jet-like feature 23" from the quasar SDSS 1306+0356. We can explain the emission by inverse Compton scattering off the Cosmic Microwave Background, assuming that the intrinsic properties of the system are similar to other X-ray jets at z<1. Deeper observations to confirm the interpretation as a jet will be practical.

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