arXiv subjects

Oleg Kargaltsev

Publications and source records attributed to Oleg Kargaltsev.

At least 37 records · Page 2Linked to original sources

arXiv2023

Dataset of Classified Chandra Sources in Globular Clusters

We present a collection of classified X-ray sources in Globular Clusters (GCs) observed by the Chandra X-ray Observatory (CXO), including active binaries, cataclysmic variables, millisecond pulsars, and low-mass X-ray binaries. We cross-match the most accurate published positions from multiwavelength observations of these sources to the Chandra Source Catalog (CSC) Release 2.1, and the HST UV Globular Cluster Survey (HUGS) to extract their multiwavelength properties. The dataset can be accessed via an interactive website and used as a training dataset for machine-learning classification of unidentified X-ray sources in GCs.

astro-ph.HE↗

arXiv2023

A dragon out of breath? Monitoring high-velocity outflows from the high-mass gamma-ray binary LS 2883/PSR B1259-63 during the 2017--2021 binary cycle

Observations of the high-mass gamma-ray binary LS 2883/PSR B1259--63 with the Chandra X-ray Observatory during the 2011--2014 and 2014--2017 binary cycles have shown X-ray emitting clumps, presumably ejected from the binary during periastron passages. These clumps traveled at projected velocities of $\sim0.1 c$ and have shown evidence of being accelerated. The clumps also evolved in shape, size, and flux. We monitored this binary with Chandra during the 2017--2021 binary cycle to search for additional X-ray emitting ejections. While we find evidence of extended emission in two of the six observations, it is unlike the clumps observed in the previous three binary cycles. More specifically, the extended emission is not well localized and no bright clump is observed moving away from the binary. It is still unclear what caused the lack of X-ray emitting clump in this orbital cycle, but it may be due to changes in the decretion disk of the Be star.

astro-ph.HE↗

arXiv2023

Classifying Unidentified X-ray Sources in the Chandra Source Catalog Using a Multiwavelength Machine-learning Approach

The rapid increase in serendipitous X-ray source detections requires the development of novel approaches to efficiently explore the nature of X-ray sources. If even a fraction of these sources could be reliably classified, it would enable population studies for various astrophysical source types on a much larger scale than currently possible. Classification of large numbers of sources from multiple classes characterized by multiple properties (features) must be done automatically and supervised machine learning (ML) seems to provide the only feasible approach. We perform classification of Chandra Source Catalog version 2.0 (CSCv2) sources to explore the potential of the ML approach and identify various biases, limitations, and bottlenecks that present themselves in these kinds of studies. We establish the framework and present a flexible and expandable Python pipeline, which can be used and improved by others. We also release the training data set of 2941 X-ray sources with confidently established classes. In addition to providing probabilistic classifications of 66,369 CSCv2 sources (21% of the entire CSCv2 catalog), we perform several narrower-focused case studies (high-mass X-ray binary candidates and X-ray sources within the extent of the H.E.S.S. TeV sources) to demonstrate some possible applications of our ML approach. We also discuss future possible modifications of the presented pipeline, which are expected to lead to substantial improvements in classification confidences.

astro-ph.HE↗

arXiv2023

XMM-Newton and Chandra observations of the candidate Fermi-LAT pulsar 4FGL J1015.5-6030

4FGL J1015.5-6030 is an unidentified Fermi-LAT source hosting a bright, extended X-ray source whose X-ray spectrum is consistent with that of a young pulsar, yet no pulsations have been found. Here we report on XMM-Newton timing and Chandra imaging observations of the X-ray counterpart of 4FGL J1015.5-6030. We find no significant periodicity from the source and place a 3$σ$ upper-limit on its pulsed fraction of 34$\%$. The Chandra observations resolve the point source from the extended emission. We find that the point source's spectrum is well fit by a blackbody model, with temperature $kT=0.205\pm0.009$ keV, plus a weak power-law component, which is consistent with a thermally emitting neutron star with a magnetospheric component. The extended emission spans angular scales of a few arcseconds up to about 30$''$ from the point source and its spectrum is well fit by a power-law model with a photon index $Γ=1.70\pm0.05$. The extended emission's spectrum and 0.5-10 keV luminosity of 4$\times10^{32}$ erg s$^{-1}$ (at a plausible distance of 2 kpc) are consistent with that of a pulsar wind nebula. Based on a comparison to other GeV and X-ray pulsars, we find that this putative pulsar is likely a middle-aged (i.e., $τ\sim 0.1$--1 Myr) radio-quiet pulsar with $\dot{E}\sim10^{34}-10^{35}$ erg s$^{-1}$.

astro-ph.HE↗

arXiv2023

First ejection from the PSR B1259-63/LS 2883 high mass gamma-ray binary detected during the 2021-2024 binary cycle

LS 2883/PSR B1259-63 is a high mass, eccentric gamma-ray binary that has previously been observed to eject X-ray emitting material. We report the results of recent Chandra observations near binary apastron in which a new X-ray emitting clump of matter was detected. The clump has a high projected velocity of $v_{\perp}\approx 0.07c$ and hard X-ray spectrum, which fits an absorbed power-law model with $Γ=1.1\pm0.3$. Although clumps with similar velocities and spectra were detected in some of the previous binary cycles, no resolved clumps were seen near apastron in the preceding cycle of 2017-2021.

astro-ph.HE↗

arXiv2022

Population of X-ray Sources in the Intermediate-Age Cluster NGC 3532: a Test Bed for Machine-Learning Classification

Open clusters are thought to be the birth place of most stars in the Galaxy. Thus, they are excellent laboratories for investigating stellar evolution, and X-ray properties of various types of stars (including binary stars, evolved stars, and compact objects). In this work, we investigate the population of X-ray sources in the nearby 300-Myr-old open cluster NGC 3532 using Chandra X-ray Observatory and multi-wavelength data from several surveys. We apply a random-forest machine-learning pipeline (MUWCLASS) to classify all confidently detected X-ray sources (S/N$>5$) in the field of NGC 3532. We also perform a more detailed investigation of brighter sources, including their X-ray spectra and lightcurves. Most X-ray sources are confirmed as coronally-active low-mass stars, many of which are confidently identified by MUWCLASS. Several late B or early A-type \textbf{stars} are relatively bright in X-rays, most of which are likely binaries. We do not find any compact objects among X-ray sources reliably associated with NGC 3532, down to the limiting X-ray flux of $\sim 2\times10^{-15}$ erg s$^{-1}$ cm$^{-2}$, corresponding to $L_X\sim 6\times 10^{28}$ erg s$^{-1}$ at the cluster's distance. We also identify several Galactic sources beyond NGC 3532 that differ from typical coronally active stars, and were classified by MUWCLASS as potential compact objects. Detailed investigation reveals that these sources may indeed belong to rarer classes, and deserve follow up observations.

astro-ph.HE↗

arXiv2022

A NuSTAR and Chandra Investigation of the Misaligned Outflow of PSR J1101-6101 and the Lighthouse Pulsar Wind Nebula

PSR J1101-6101 is an energetic young pulsar which powers the remarkable Lighthouse pulsar wind nebula (PWN). The pulsar belongs to the rare type of radio- and gamma-ray-quiet pulsars which are bright in hard X-rays. Moreover, the Lighthouse PWN is remarkable for its misaligned outflow (which gave rise to the PWN's nickname). Also known as "pulsar filaments", these collimated parsec-scale X-ray structures have been recently discovered in the vicinity of a handful of fast-moving pulsars, and appear unaffected by the ram pressure which confines pulsar tails. We report on NuSTAR observations of PSR J1101-6101 and its misaligned outflow -- the first observation of such a structure above ~10 keV. We detect the outflow up to 25 keV, spatially resolve its spectral evolution with distance from the pulsar, find unambiguous evidence of spectral cooling with distance from the pulsar, and infer physical properties of the particles and magnetic field in the outflow. We also reanalzye archival Chandra data and discuss the outflow's small-scale structure. We detect pulsations from PSR J1101-6101 up to 20 keV, present the X-ray pulse profile, confirm its period derivative, and perform phase-resolved spectroscopy. Lastly, we discuss the X-ray source 2CXO J110158.4-605649 = 2XMM J110158.5--605651 (a serendipitously observed blazar) and suggest it may be the X-ray counterpart to the GeV source 4FGL J1102.0--6054.

astro-ph.HE↗

arXiv2022

A Quarter Century of Guitar Nebula/Filament Evolution

We have collected a new deep {\it Chandra X-ray Observatory} ({\it CXO}) exposure of PSR B2224+65 and the `Guitar Nebula', mapping the complex X-ray structure. This is accompanied by a new {\it HST} H$α$ image of the head of the Guitar. Comparing the {\it HST} and {\it CXO} structures in 4 epochs over 25 years, we constrain the evolution of the TeV particles that light up the filament. Cross-field diffusion appears to be enhanced, likely by the injected particles, behind the filament's sharp leading edge, explaining the filament width and its evolving surface brightness profile.

astro-ph.HE↗

arXiv2022

Catalog of X-ray Detected Be Stars (XDBS)

We present a catalog of X-ray Detected Be Stars (XDBS) with 161 Be stars from the Be Star Spectra (BeSS) database having X-ray counterparts in the Chandra Source Catalog v2.0, XMM-Newton 4XMM-DR11 Catalog, or Swift 2SXPS Catalog. The multi-wavelength catalog includes accurate optical positions, X-ray properties (fluxes, photon indices and hardness ratios), optical, near-infrared and infrared photometry, source classifications (when available), and other properties including proper motions, effective temperatures, X-ray to optical flux ratios as well. We also provide a convenient graphical user interface which allows for easy visualization of the catalog content.

astro-ph.HE↗

arXiv2022

Search for extended sources in the images from Chandra X-ray Observatory Advanced CCD Imaging Spectrometer

We present a convenient tool (ChaSES) which allows to search for extended structures in Chandra X-ray Observatory Advanced CCD Imaging Spectrometer (ACIS) images. The tool relies on DBSCAN clustering algorithm to detect regions with overdensity of photons compared to the background. Here we describe the design and functionality of the tool which we make publicly available on GitHub. We also provide online extensive examples of its applications to the real data.

astro-ph.HE↗

arXiv2022

"The Goose" Pulsar Wind Nebula of PSR J1016-5857: The Birth of a Plerion

We report the results of X-ray (CXO) and radio (ATCA) observations of the pulsar wind nebula (PWN) powered by the young pulsar PSR J1016--5857, which we dub "the Goose" PWN. In both bands the images reveal a tail-like PWN morphology which can be attributed to pulsar's motion. By comparing archival and new CXO observations, we measure the pulsar's proper motion $μ=28.8\pm7.3$ mas/yr, yielding a projected pulsar velocity $v \approx 440\pm110$ km/s (at d=3.2 kpc); its direction is consistent with the PWN shape. Radio emission from the PWN is polarized, with the magnetic field oriented along the pulsar tail. The radio tail connects to a larger radio structure (not seen in X-rays) which we interpret as a relic PWN (also known as a plerion). The spectral analysis of the CXO data shows that the PWN spectrum softens from $Γ=1.7$ to $Γ\approx2.3-2.5$ with increasing distance from the pulsar. The softening can be attributed to rapid synchrotron burn-off, which would explain the lack of X-ray emission from the older relic PWN. In addition to non-thermal PWN emission, we detected thermal emission from a hot plasma which we attribute to the host SNR. The radio PWN morphology and the proper motion of the pulsar suggest that the reverse shock passed through the pulsar's vicinity and pushed the PWN to one side.

astro-ph.HE↗

arXiv2022

Advancing the Landscape of Multimessenger Science in the Next Decade

The last decade has brought about a profound transformation in multimessenger science. Ten years ago, facilities had been built or were under construction that would eventually discover the nature of objects in our universe could be detected through multiple messengers. Nonetheless, multimessenger science was hardly more than a dream. The rewards for our foresight were finally realized through IceCube's discovery of the diffuse astrophysical neutrino flux, the first observation of gravitational waves by LIGO, and the first joint detections in gravitational waves and photons and in neutrinos and photons. Today we live in the dawn of the multimessenger era. The successes of the multimessenger campaigns of the last decade have pushed multimessenger science to the forefront of priority science areas in both the particle physics and the astrophysics communities. Multimessenger science provides new methods of testing fundamental theories about the nature of matter and energy, particularly in conditions that are not reproducible on Earth. This white paper will present the science and facilities that will provide opportunities for the particle physics community renew its commitment and maintain its leadership in multimessenger science.

astro-ph.HE↗

arXiv2022

The Future of Gamma-Ray Experiments in the MeV-EeV Range

Gamma-rays, the most energetic photons, carry information from the far reaches of extragalactic space with minimal interaction or loss of information. They bring messages about particle acceleration in environments so extreme they cannot be reproduced on earth for a closer look. Gamma-ray astrophysics is so complementary with collider work that particle physicists and astroparticle physicists are often one in the same. Gamma-ray instruments, especially the Fermi Gamma-ray Space Telescope, have been pivotal in major multi-messenger discoveries over the past decade. There is presently a great deal of interest and scientific expertise available to push forward new technologies, to plan and build space- and ground-based gamma-ray facilities, and to build multi-messenger networks with gamma rays at their core. It is therefore concerning that before the community comes together for planning exercises again, much of that infrastructure could be lost to a lack of long-term planning for support of gamma-ray astrophysics. Gamma-rays with energies from the MeV to the EeV band are therefore central to multiwavelength and multi-messenger studies to everything from astroparticle physics with compact objects, to dark matter studies with diffuse large scale structure. These goals and new discoveries have generated a wave of new gamma-ray facility proposals and programs. This paper highlights new and proposed gamma-ray technologies and facilities that have each been designed to address specific needs in the measurement of extreme astrophysical sources that probe some of the most pressing questions in fundamental physics for the next decade. The proposed instrumentation would also address the priorities laid out in the recent Astro2020 Decadal Survey, a complementary study by the astrophysics community that provides opportunities also relevant to Snowmass.

astro-ph.HE↗

arXiv2021

Thermal and nonthermal emission in the optical-UV spectrum of PSR B0950+08

Based on recent Hubble Space Telescope (HST) observations in far-UV and groundbased observations in optical bands, Pavlov and colleagues have revealed a thermal component in the spectrum of the old pulsar B0950+08 (spin-down age 17.5 Myr) and estimated a neutron star (NS) surface temperature of $(1$--$3)\times 10^5$ K. Our new HST observations in the optical have allowed us to resolve the pulsar from a close-by galaxy and measure the optical fluxes more accurately. Using the newly measured fluxes and a new calibration of the HST's far-UV detector, we fit the optical-UV pulsar's spectrum with a model that consists of a nonthermal power-law ($f_ν\propto ν^α$) and a thermal blackbody components. We obtained the spectral slope $α=-0.3\pm 0.3$, considerably flatter than found from groundbased observations, and the best-fit temperature in the range of $(6$--$12)\times 10^4$ K (as seen by a distant observer), depending on interstellar extinction and NS radius. The temperature is lower than reported previously, but still much higher than predicted by NS passive cooling scenarios for such an old pulsar. This means that some heating mechanisms operate in NSs, e.g., caused by interaction of the faster rotating neutron superfluid with the slower rotating normal matter in the inner crust of the NS.

astro-ph.HE↗

arXiv2021

X-ray imaging observations of the high-mass gamma-ray binary HESS J0632+057

The Chandra X-ray Observatory (CXO) imaged the high-mass gamma-ray binary (HMGB) HESS J0632+057 with the Advanced CCD Imaging Spectrometer (ACIS). We analyzed the CXO data together with 967 ks of archival Swift-XRT observations. On arcsecond scales we find a hint of asymmetric extended emission. On arcminute scales, a region of extended emission ("the blob"), located $\sim$5.5' east of the binary, is seen in both the CXO-ACIS and the Swift-XRT images. The blob does not have a counterpart in the radio, NIR, IR, or optical images. The ACIS spectrum of the blob fits either an absorbed power-law model with photon index $Γ\simeq2.6$, or a thermal plasma model with $kT\simeq3$ keV. Since the blob's absorbing column, $N_{\rm H}$, is significantly larger than that of the binary we conclude that the blob and binary are not directly related. The somewhat larger very deep XRT image suggests that the binary may be located within a shell (or cavity). The four ACIS spectra taken within the $\sim$20 day interval near the light curve minimum suggests that the $N_{\rm H}$ is varying on timescales of days, possibly due to the inhomogeneous circumbinary environment. The XRT spectra extracted from the wider orbital phase intervals support significant changes in $N_{\rm H}$ near the light curve maximum/minimum, which may be responsible for the substantial systematic residuals seen near 1 keV, and provide tentative evidence for a Fe line at 6.4 keV. We find no significant periodic signal in the ACIS data up to 0.156 Hz.

astro-ph.HE↗

arXiv2021

NuSTAR observation of LS 5039

LS 5039 is a high-mass gamma-ray binary hosting a compact object of unknown type. NuSTAR observed LS 5039 during its entire 3.9 day binary period. We performed a periodic signal search up to 1000 Hz which did not produce credible period candidates. We do see the 9.05 s period candidate, originally reported by Yoneda et al. 2020 using the same data, in the Fourier power spectrum, but we find that the statistical significance of this feature is too low to claim it as a real detection. We also did not find significant bursts or quasi-periodic variability. The modulation with the orbital period is clearly seen and remains unchanged over a decade long timescale when compared to the earlier Suzaku light curve. The joint analysis of the NuSTAR and Suzaku XIS data shows that the 0.7-70 keV spectrum can be satisfactory described by a single absorbed power-law model with no evidence of cutoff at higher energies. The slope of the spectrum anti-correlates with the flux during the binary orbit. Therefore, if LS 5039 hosts a young neutron star, its X-ray pulsations appear to be outshined by the intrabinary shock emission. The lack of spectral lines and/or an exponential cutoff at higher energies suggests that the putative neutron star is not actively accreting. Although a black hole scenario still remains a possibility, the lack of variability or Fe K$α$ lines, which typically accompany accretion, makes it less likely.

astro-ph.HE↗

arXiv2021

A Search for TeV Gamma-ray Emission from Pulsar Tails by VERITAS

We report on the search for very-high-energy gamma-ray emission from the regions around three nearby supersonic pulsars (PSR B0355+54, PSR J0357+3205 and PSR J1740+1000) that exhibit long X-ray tails. To date there is no clear detection of TeV emission from any pulsar tail that is prominent in X-ray or radio. We provide upper limits on the TeV flux, and luminosity, and also compare these limits with other pulsar wind nebulae detected in X-rays and the tail emission model predictions. We find that at least one of the three tails is likely to be detected in observations that are a factor of 2-3 more sensitive. The analysis presented here also has implications for deriving the properties of pulsar tails, for those pulsars whose tails could be detected in TeV.

astro-ph.HE↗

arXiv2021

Visualizing Multiwavelength Properties of Classified X-ray Sources from Chandra Source Catalog

We present a simple but informative online tool to visualize the multiwavelength properties of about 2,700 X-ray sources from Chandra Source Catalog version 2.0 with literature verified classifications. Here we describe the catalogs that we used to collect the classifications and extract the MW properties of these sources, and the properties themselves. We also describe the design and functionality of the tool.

astro-ph.HE↗