arXiv subjects

Steven Ehlert

Publications and source records attributed to Steven Ehlert.

10 recordsLinked to original sources

arXiv2025

Testing the Correlations between X-ray Spectral Properties and Polarization for High Synchrotron Peaked Blazars

IXPE has enabled the X-ray polarizations of many blazars to be measured. We perform the first population study for high synchrotron peaked blazars observed using IXPE using a uniform X-ray data analysis. We find a potential statistically significant correlation between the X-ray spectral curvature and the X-ray polarization degree. More data is needed to determine whether this correlation is robust. The lack of any other correlations may imply that there is little connection between the energy distribution of the X-ray emitting electrons and the uniformity of the magnetic field in the X-ray emitting regions of these blazars. These results will inform future theoretical work and potentially help narrow down the acceleration process of the synchrotron electrons.

astro-ph.HE↗

arXiv2024

X-ray Cool Core Remnants Heated by Strong Radio AGN Feedback

Strong AGN heating provides an alternative means for the disruption of cluster cool cores (CCs) to cluster mergers. In this work we present a systematic Chandra study of a sample of 108 nearby ($z<0.1$) galaxy clusters, to investigate the effect of AGN heating on CCs. About 40% of clusters with small offsets between the BCG and the X-ray centre ($\le50$ kpc) have small CCs. For comparison, 14 of 17 clusters with large offsets have small CCs, which suggests that mergers or sloshing can be efficient in reducing the CC size. Relaxed, small CC clusters generally have weak radio AGNs ($P_{1.4\rm GHz}<10^{23}$ W Hz$^{-1}$), and they show a lack of systems hosting a radio AGN with intermediate radio power ($2\times10^{23}<P_{1.4\rm GHz}<2\times10^{24}$ W Hz$^{-1}$). We found that the strongest circumnuclear ($<1$ kpc) X-ray emission only exists in clusters with strong radio AGN. The duty cycle of relaxed, small CC clusters is less than half of that for large CC clusters. It suggests that the radio activity of BCGs is affected by the properties of the surrounding gas beyond the central $\sim10$ kpc, and strong radio AGNs in small X-ray CCs fade more rapidly than those embedded in large X-ray CCs. A scenario is also presented for the transition of large CCs and coronae due to radio AGN feedback. We also present a detailed analysis of galaxy cluster 3C 129.1 as an example of a CC remnant possibly disrupted by radio AGN.

astro-ph.GA↗

arXiv2023

First X-ray polarization measurement confirms the low black-hole spin in LMC X-3

X-ray polarization is a powerful tool to investigate the geometry of accreting material around black holes, allowing independent measurements of the black hole spin and orientation of the innermost parts of the accretion disk. We perform the X-ray spectro-polarimetric analysis of an X-ray binary system in the Large Magellanic Cloud, LMC X-3, that hosts a stellar-mass black hole, known to be persistently accreting since its discovery. We report the first detection of the X-ray polarization in LMC X-3 with the Imaging X-ray Polarimetry Explorer, and find the average polarization degree of 3.2% +- 0.6% and a constant polarization angle -42 deg +- 6 deg over the 2-8 keV range. Using accompanying spectroscopic observations by NICER, NuSTAR, and the Neil Gehrels Swift observatories, we confirm previous measurements of the black hole spin via the X-ray continuum method, a ~ 0.2. From polarization analysis only, we found consistent results with low black-hole spin, with an upper limit of a < 0.7 at a 90% confidence level. A slight increase of the polarization degree with energy, similar to other black-hole X-ray binaries in the soft state, is suggested from the data but with a low statistical significance.

astro-ph.HE↗

arXiv2023

X-ray Polarization of the Black Hole X-ray Binary 4U 1630-47 Challenges Standard Thin Accretion Disk Scenario

Large energy-dependent X-ray polarization degree is detected by the Imaging X-ray Polarimetry Explorer ({IXPE}) in the high-soft emission state of the black hole X-ray binary 4U 1630--47. The highly significant detection (at $\approx50\sigma$ confidence level) of an unexpectedly high polarization, rising from $\sim6\%$ at $2$ keV to $\sim10\%$ at $8$ keV, cannot be easily reconciled with standard models of thin accretion discs. In this work we compare the predictions of different theoretical models with the {IXPE} data and conclude that the observed polarization properties are compatible with a scenario in which matter accretes onto the black hole through a thin disc, covered by a partially-ionized atmosphere flowing away at mildly relativistic velocities.

astro-ph.HE↗

arXiv2022

A Probabilistic Method of Background Removal for High Energy Astrophysics Data

We present a new statistical method for constructing background subtracted measurements from event list data gathered by X-ray and gamma ray observatories. This method was initially developed specifically to construct images that account for the high background fraction and low overall count rates observed in survey data from the Mikhail Pavlinsky ART-XC telescope aboard the Spektrum R\"{o}ntgen Gamma (SRG) mission, although the mathematical underpinnings are valid for data taken with other imaging missions and analysis applications. This method fully accounts for the expected Poisson fluctuations in both the sky photon and non X-ray background count rates in a manner that does not result in unphysical negative counts. We derive the formulae for arbitrary confidence intervals for the source counts and show that our new measurement converges exactly to the standard background subtraction calculation in the high signal limit. Utilizing these results, we discuss several variants of images designed to optimize different science goals for both pointed and slewing telescopes. Using realistic simulated data of a galaxy cluster as observed by ART-XC we show that our method provides a more significant and robust detection of the cluster emission as compared to a standard background subtraction. We also demonstrate its advantages using real observations of a point source from the ART-XC telescope. These calculations may have widespread applications for a number of source classes observed with high energy telescopes.

astro-ph.IM↗

arXiv2020

Measuring Fluxes of Meteor Showers with the NASA All-Sky Fireball Network

We present an algorithm developed to measure the fluxes of major meteor showers as observed in NASA's All-Sky Fireball Network cameras. Measurements of fluxes from the All-Sky cameras not only improve the Meteoroid Environment Office's (MEO's) ability to provide accurate risk assessments from major showers, but also allows the mass distribution of meteoroids within the shower to be constrained. This algorithm accounts for the shower-specific and event-specific exposure time and collecting area of the sky for nights where sufficiently large samples of shower meteors ($\sim 30$ or more from the shower) are observed. The fluxes derived from the All-Sky Fireball Network for the 2015 Geminid, 2016 Perseid and Quadrantid, 2017 Orionid, and 2018 Leonid shower peaks are calculated. All five of these shower fluxes show excellent agreement with expectations from independent measurements at different mass and luminosity limits. For four of these five showers, the measured mass indices are significantly shallower than what is currently assumed by the NASA Meteoroid Environment Office's (MEO's) annual meteor shower forecast. A direct comparison between forecasted and measured fluxes at limiting masses of $\sim 1 \thinspace \mathrm{g}$ shows good agreement for the three showers for which the observations took place near their peak activity.

astro-ph.EP↗

arXiv2019

NASA's Meteoroid Engineering Model (MEM) 3 and its ability to replicate spacecraft impact rates

Meteoroids pose one of the largest risks to spacecraft outside of low Earth orbit. In order to correctly predict the rate at which meteoroids impact and damage spacecraft, environment models must describe the mass, directionality, velocity, and density distributions of meteoroids. NASA's Meteoroid Engineering Model (MEM) is one such model; MEM 3 is an updated version of the code that better captures the correlation between directionality and velocity and incorporates a bulk density distribution. This paper describes MEM 3 and compares its predictions with the rate of large particle impacts seen on the Long Duration Exposure Facility (LDEF) and the Pegasus II and III satellites.

astro-ph.EP↗

arXiv2019

Dust Production of Comet 21P/Giacobini-Zinner Throughout its 2018 Apparition

We present the results of a long term telescopic observation campaign of comet 21P/Giacobini-Zinner, parent body of the Draconid meteor shower, spanning $\sim 240$ days during its 2018 apparition. Determinations of comet 21P's dust production rate through the $Afρ$ parameter derived from these images show that the comet had a highly asymmetric dust production rate that peaked $\sim 10-30$ days before perihelion, when the comet was at a heliocentric distance of $\sim 1.02 - 1.08 \thinspace \mathrm{AU}$. The single highest $Afρ$ measurement occurred on 2018-Aug-14 (27 days before perihelion), and had a measured value of $Afρ= 1594 \thinspace \mathrm{cm}$. The comet's $Afρ$ profile is well described by a double-exponential model that rises rapidly during ingress and declines even more rapidly during its egress. These results are fully consistent with observations of comet 21P's dust and gas production rates during past apparitions, and suggest that the double-exponential model we have derived provides a reasonable and stable approximation for the comet's activity over the past thirty to forty years.

astro-ph.EP↗

arXiv2014

HST-COS Spectroscopy of the Cooling Flow in Abell 1795 - Evidence for Inefficient Star Formation in Condensing Intracluster Gas

We present far-UV spectroscopy from the Cosmic Origins Spectrograph on the Hubble Space Telescope of a cool, star-forming filament in the core of Abell 1795. These data, which span 1025A - 1700A, allow for the simultaneous modeling of the young stellar populations and the intermediate-temperature (10^5.5 K) gas in this filament, which is far removed (~30 kpc) from the direct influence of the central AGN. Using a combination of UV absorption line indices and stellar population synthesis modeling, we find evidence for ongoing star formation, with the youngest stars having ages of 7.5 +/- 2.0 Myr and metallicities of 0.4 +/- 0.2 Zsun. The latter is consistent with the local metallicity of the intracluster medium. We detect the O VI (1038) line, measuring a flux of 4.0 +/- 0.9 x 10^-17 erg s^-1 cm^-2. The O VI (1032) line is redshifted such that it is coincident with a strong Galactic H2 absorption feature, and is not detected. The measured O VI (1038) flux corresponds to a cooling rate of 0.85 +/- 0.2 (stat) +/- 0.15 (sys) Msun/yr at ~10^5.5 K, assuming that the cooling proceeds isochorically, which is consistent with the classical X-ray luminosity-derived cooling rate in the same region. We measure a star formation rate of 0.11 +/- 0.02 Msun/yr from the UV continuum, suggesting that star formation is proceeding at 13 +/- 3% efficiency in this filament. We propose that this inefficient star formation represents a significant contribution to the larger-scale cooling flow problem.

astro-ph.GA↗

arXiv2010

Extreme AGN Feedback and Cool Core Destruction in the X-ray Luminous Galaxy Cluster MACS J1931.8-2634

We report on a deep, multiwavelength study of the galaxy cluster MACS J1931.8-2634 using Chandra X-ray, Subaru optical, and VLA 1.4 GHz radio data. This cluster (z=0.352) harbors one of the most X-ray luminous cool cores yet discovered, with an equivalent mass cooling rate within the central 50 kpc is approximately 700 solar masses/yr. Unique features observed in the central core of MACSJ1931.8-2634 hint to a wealth of past activity that has greatly disrupted the original cool core. We observe a spiral of relatively cool, dense, X-ray emitting gas connected to the cool core, as well as highly elongated intracluster light (ICL) surrounding the cD galaxy. Extended radio emission is observed surrounding the central AGN, elongated in the east-west direction, spatially coincident with X-ray cavities. The power input required to inflate these `bubbles' is estimated from both the X-ray and radio emission to reside between 4 and 14e45 erg/s, putting it among the most powerful jets ever observed. This combination of a powerful AGN outburst and bulk motion of the cool core have resulted in two X-ray bright ridges to form to the north and south of the central AGN at a distance of approximately 25 kpc. The northern ridge has spectral characteristics typical of cool cores and is consistent with being a remnant of the cool core after it was disrupted by the AGN and bulk motions. It is also the site of H-alpha filaments and young stars. The X-ray spectroscopic cooling rate associated with this ridge is approximately 165 solar masses/yr, which agrees with the estimate of the star formation rate from broad-band optical imaging (170 solar masses/yr). MACS J1931.8-2634 appears to harbor one of most profoundly disrupted low entropy cores observed in a cluster, and offers new insights into the survivability of cool cores in the context of hierarchical structure formation.

astro-ph.CO↗