arXiv subjects

Oleg Kargaltsev

Publications and source records attributed to Oleg Kargaltsev.

At least 73 records · Page 4Linked to original sources

arXiv2016

Deep Chandra Observations of the Pulsar Wind Nebula Created by PSR B0355+54

We report on Chandra X-ray Observatory (CXO) observations of the pulsar wind nebula (PWN) associated with PSR B0355+54 (eight observations with a 395 ks total exposure, performed over an 8 month period). We investigated the spatial and spectral properties of the emission coincident with the pulsar, compact nebula (CN), and extended tail. We find that the CN morphology can be interpreted in a way that suggests a small angle between the pulsar spin axis and our line-of-sight, as inferred from the radio data. On larger scales, emission from the 7' (2 pc) tail is clearly seen. We also found hints of two faint extensions nearly orthogonal to the direction of the pulsar's proper motion. The spectrum extracted at the pulsar position can be described with an absorbed power-law + blackbody model. The nonthermal component can be attributed to magnetospheric emission, while the thermal component can be attributed to emission from either a hot spot (e.g., a polar cap) or the entire neutron star surface. Surprisingly, the spectrum of the tail shows only a slight hint of cooling with increasing distance from the pulsar. This implies either a low magnetic field with fast flow speed, or particle re-acceleration within the tail. We estimate physical properties of the PWN and compare the morphologies of the CN and the extended tail with those of other bow shock PWNe observed with long CXO exposures.

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arXiv2016

First Detection of a Pulsar Bow Shock Nebula in Far-UV: PSR J0437-4715

Pulsars traveling at supersonic speeds are often accompanied by cometary bow shocks seen in Halpha. We report on the first detection of a pulsar bow shock in the far-ultraviolet (FUV). We detected it in FUV images of the nearest millisecond pulsar J0437-4715 obtained with the Hubble Space Telescope. The images reveal a bow-like structure positionally coincident with part of the previously detected Halpha bow shock, with an apex at 10" ahead of the moving pulsar. Its FUV luminosity, L(1250-2000 A) ~ 5x10^28 erg/s, exceeds the Halpha luminosity from the same area by a factor of 10. The FUV emission could be produced by the shocked ISM matter or, less likely, by relativistic pulsar wind electrons confined by strong magnetic field fluctuations in the bow shock. In addition, in the FUV images we found a puzzling extended (~3" in size) structure overlapping with the limb of the bow shock. If related to the bow shock, it could be produced by an inhomogeneity in the ambient medium or an instability in the bow shock. We also report on a previously undetected X-ray emission extending for about 5" ahead of the pulsar, possibly a pulsar wind nebula created by shocked pulsar wind, with a luminosity L(0.5-8 keV) ~ 3x10^28 erg/s.

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arXiv2016

Chandra Observations of Outflows from PSR J1509-5850

PSR J1509-5850 is a middle-aged pulsar with the period P ~ 89 ms, spin-down power Edot = 5.1 x 10^35 erg/s, at a distance of about 3.8 kpc. We report on deep Chandra X-ray Observatory observations of this pulsar and its pulsar wind nebula (PWN). In addition to the previously detected tail extending up to 7' southwest from the pulsar (the southern outflow), the deep images reveal a similarly long, faint diffuse emission stretched toward the north (the northern outflow) and the fine structure of the compact nebula (CN) in the pulsar vicinity. The CN is resolved into two lateral tails and one axial tail pointing southwest (a morphology remarkably similar to that of the Geminga PWN), which supports the assumption that the pulsar moves towards the northeast. The luminosities of the southern and northern outflows are about 1 x 10^33 and 4 x 10^32 erg/s, respectively. The spectra extracted from four regions of the southern outflow do not show any softening with increasing distance from the pulsar. The lack of synchrotron cooling suggests a high flow speed or in-situ acceleration of particles. The spectra extracted from two regions of the northern outflow show a hint of softening with distance from the pulsar, which may indicate slower particle propagation. We speculate that the northern outflow is associated with particle leakage from the bow shock apex into the ISM, while the southern outflow represents the tail of the shocked pulsar wind behind the moving pulsar. We estimate the physical parameters of the observed outflows and compare the J1509-5850 PWN with PWNe of other supersonically moving pulsars.

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arXiv2016

Multi-wavelength study of HESS J1741-302

We present the results of two Chandra X-ray Observatory ( CXO) observations of TeV gamma-ray source HESS J1741-302A/B. Our analysis also includes GeV gamma-ray and radio data. We investigate whether there is any connection between HESS J1741-302A/B and the sources seen at lower energies. One of the brightest X-ray sources in the HESS J1741-302B field, CXOU J174112.1-302908, appears to be associated with a low-mass star (possibly representing a quiescent LMXB or CV), hence, it is unlikely to be a source of TeV gamma-rays. In the same field we detected X-rays from WR 98a, which is likely to be a colliding wind binary with massive stars, however, no TeV emission has been reported so far from such systems although a predictions have been made. Finally, Suzaku source J1740.5-3014 (which is not covered by the CXO observations) appears to be a hard X-ray source detected by INTERGAL ISGRI which supports the magnetized CV classification and also makes its association with the TeV emission unlikely. The young pulsar, undetected in X-rays and located near the CV, may be the contributor of relativistic particles responsible for the TeV emission. Alternatively, HESS J1741-302 could be a new type of accelerator, which is dark at lower energies.

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arXiv2015

Pulsar-Wind Nebulae: Recent Progress in Observations and Theory

In this review we describe recent observational and theoretical developments in our understanding of pulsar winds and pulsar-wind nebulae (PWNe). We put special emphasis on the results from observations of well-characterized PWNe of various types (e.g., torus-jet and bowshock-tail), the most recent MHD modeling efforts, and the status of the flaring Crab PWN puzzle.

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arXiv2015

Magnesium-rich Ejecta in the SNR G284.3$-$1.8 Around the High-Mass Gamma-Ray Binary 1FGL J1018.6$-$5856

We present results from two {\it Chandra} observations of the 16.6 d X-ray/$γ$-ray high-mass binary 1FGL J1018.6$-$5856 located at the center of the supernova remnant G284.3$-$1.8. The binary spectra, separated by 0.25 in binary phase, are fit with an absorbed power-law model with $Γ\approx1.7-1.8$ for both observations (the flux during the second observation is a factor of 1.7 smaller). In the high-resolution ACIS-I image we found a hint of extended emission $\approx2''$--$3''$ southeast of the binary, significant at the $3σ$ level. Binary evolution codes reproduce the system's observed properties with two massive stars with an initial 18-day period, undergoing mass transfer and leaving behind a heavy $\approx2$ $\msun$ neutron star. The initial mass of the progenitor star in this scenario is 27 $\pm 4$ $\msun$. \emph{Chandra} and {\it XMM-Newton} images of the remnant show it has a relatively low X-ray surface brightness. The two brightest regions of extended X-ray emission, with luminosities $\sim10^{33}$ erg s$^{-1}$ for $d=5$ kpc, lie in the northern and western portions and show significantly different spectra. The northern patch is consistent with shocked ISM, with a low temperature and long ionization timescale. However, the western patch is dominated by ejecta, and shows significantly enhanced Mg content relative to other ejecta products. The abundance ratios inferred resemble those from the Large Magellanic Cloud remnant N49B. To our knowledge, this is only the second case of such Mg-rich ejecta found in an SNR. Nucleosynthesis models for core-collapse SNe predict Mg-rich ejecta from very massive progenitors of $>25$ $\msun$.

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arXiv2015

An extended X-ray object ejected from the PSR B1259-63/LS 2883 binary

We present the analysis of the Chandra X-ray Observatory observations of the eccentric gamma-ray binary PSR B1259-63/LS 2883. The analysis shows that the extended X-ray feature seen in previous observations is still moving away from the binary with an average projected velocity of about 0.07c and shows a hint of acceleration. The spectrum of the feature appears to be hard (photon index of 0.8) with no sign of softening compared to previously measured values. We interpret it as a clump of plasma ejected from the binary through the interaction of the pulsar with the decretion disk of the O-star around periastron passage. We suggest that the clump is moving in the unshocked relativistic pulsar wind (PW), which can accelerate the clump. Its X-ray emission can be interpreted as synchrotron radiation of the PW shocked by the collision with the clump.

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arXiv2015

X-ray analysis of the proper motion and pulsar wind nebula for PSR J1741-2054

We obtained six observations of PSR J1741-2054 using the $Chandra$ ACIS-S detector totaling $\sim$300 ks. By registering this new epoch of observations to an archival observation taken 3.2 years earlier using X-ray point sources in the field of view, we have measured the pulsar proper motion at $μ=109 \pm 10 {\rm mas\ yr}^{-1}$ in a direction consistent with the symmetry axis of the observed H$α$ nebula. We investigated the inferred past trajectory of the pulsar but find no compelling association with OB associations in which the progenitor may have originated. We confirm previous measurements of the pulsar spectrum as an absorbed power law with photon index $Γ$=2.68$\pm$0.04, plus a blackbody with an emission radius of (4.5$^{+3.2}_{-2.5})d_{0.38}$ km, for a DM-estimated distance of $0.38d_{0.38}$ kpc and a temperature of $61.7\pm3.0$ eV. Emission from the compact nebula is well described by an absorbed power law model with a photon index of $Γ$ = 1.67$\pm$0.06, while the diffuse emission seen as a trail extending northeast of the pulsar shows no evidence of synchrotron cooling. We also applied image deconvolution techniques to search for small-scale structures in the immediate vicinity of the pulsar, but found no conclusive evidence for such structures.

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arXiv2015

Optical observations of PSR J2021+3651 in the Dragonfly Nebula with the GTC

PSR J2021+3651 is a 17 kyr old rotation powered pulsar detected in the radio, X-rays, and $γ$-rays. It powers a torus-like pulsar wind nebula with jets, dubbed the Dragonfly, which is very similar to that of the Vela pulsar. The Dragonfly is likely associated with the extended TeV source VER J2019+368 and extended radio emission. We conducted first deep optical observations with the GTC in the Sloan $r'$ band to search for optical counterparts of the pulsar and its nebula. No counterparts were detected down to $r'\gtrsim27.2$ and $\gtrsim24.8$ for the point-like pulsar and the compact X-ray nebula, respectively. We also reanalyzed Chandra archival X-ray data taking into account an interstellar extinction--distance relation, constructed by us for the Dragonfly line of sight using the red-clump stars as standard candles. This allowed us to constrain the distance to the pulsar, $D=1.8^{+1.7}_{-1.4}$ kpc at 90% confidence. It is much smaller than the dispersion measure distance of $\sim$12 kpc but compatible with a $γ$-ray "pseudo-distance" of 1 kpc. Based on that and the optical upper limits, we conclude that PSR J2021+3651, similar to the Vela pulsar, is a very inefficient nonthermal emitter in the optical and X-rays, while its $γ$-ray efficiency is consistent with an average efficiency for $γ$-pulsars of similar age. Our optical flux upper limit for the pulsar is consistent with the long-wavelength extrapolation of its X-ray spectrum while the nebula flux upper limit does not constrain the respective extrapolation.

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arXiv2014

Multiwavelength study of the northeastern outskirts of the extended TeV source HESS J1809-193

HESS J1809-193 is an extended TeV gamma-ray source in the Galactic Plane. Multiwavelength observations of the HESS J1809-193 field reveal a complex picture. We present results from three CXO and two Suzaku observations of a region in the northeastern outskirts of HESS J1809-193, where enhanced TeV emission has been reported. Our analysis also includes GeV gamma-ray and radio data. One of the X-ray sources in the field is the X-ray binary XTE J1810-189, for which we present the outburst history from multiple observatories and confirm that XTE J1810-189 is a strongly variable type I X-ray burster, which can hardly produce TeV emission. We investigate whether there is any connection between the possible TeV extension of HESS J1809-193 and the sources seen at lower energies. We find that another X-ray binary candidate, Suzaku J1811-1900, and a radio supernova remnant, SNR G11.4-0.1, can hardly be responsible for the putative TeV emission. Our multiwavelength classification of fainter X-ray point sources also does not produce a plausible candidate. We conclude that the northeast extension of HESS J1809-193, if confirmed by deeper observations, can be considered as a dark accelerator - a TeV source without visible counterpart at lower energies.

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arXiv2014

XMM-Newton and Chandra Observations of the Ejecta-Dominated Mixed-Morphology Galactic Supernova Remnant G352.7-0.1

(Abridged) We present a spatial and spectral X-ray analysis of the Galactic supernova remnant (SNR) G352.7-0.1 using archival data from observations made with XMM-Newton and Chandra. Prior X-ray observations of this SNR revealed a thermal center-filled morphology which contrasts with a shell-like radio morphology, thus establishing G352.7$-$0.1 as a mixed-morphology SNR (MMSNRs). Our study confirms that the X-ray emission comes from the SNR interior and must be ejecta-dominated. Spectra obtained with XMM-Newton may be fit satisfactorily with a single thermal component (namely a non-equilibrium ionization component with enhanced abundances of silicon and sulfur). In contrast, spectra extracted by Chandra from certain regions of the SNR cannot always be fit by a single thermal component. For those regions, a second thermal component with solar abundances or two thermal components with different temperatures and thawed silicon and sulfur abundances (respectively) can generate a statistically-acceptable fit. We argue that the former scenario is more physically-plausible: based on parameters of our spectral fits, we calculate physical parameters including X-ray-emitting mass (about 45 solar masses, for solar abundances). We find no evidence for overionization in the X-ray emitting plasma associated with the SNR: this phenomenon has been seen in other MMSNRs. We have conducted a search for a neutron star within the SNR using a hard (2-10 keV) Chandra image but could not identify a firm candidate. We also present (for the first time) the detection of infrared emission from this SNR as detected at 24 micron by MIPS aboard Spitzer. Finally, we discuss the properties of G352.7-0.1 in the context of other ejecta-dominated MMSNRs.

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arXiv2014

Hubble Space Telescope detection of the double pulsar system J0737-3039 in the far-ultraviolet

We report on detection of the double pulsar system J0737-3039 in the far-UV with the ACS/SBC detector aboard HST. We measured the energy flux F = 4.5+/-1.0e-17 erg cm-2s-1 in the 1250-1550 AA band, which corresponds to the extinction-corrected luminosity L~1.5e28 erg s-1 for the distance d=1.1 kpc and a plausible reddening E(B-V)=0.1. If the detected emission comes from the entire surface of one of the neutron stars with a 13 km radius, the surface blackbody temperature is in the range T~2-5e5 K for a reasonable range of interstellar extinction. Such a temperature requires an internal heating mechanism to operate in old neutron stars, or it might be explained by heating of the surface of the less energetic Pulsar B by the relativistic wind of Pulsar A. If the far-UV emission is non-thermal (e.g., produced in the magnetosphere of Pulsar A), its spectrum exhibits a break between the UV and X-rays.

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arXiv2013

The dynamic X-ray nebula powered by the pulsar B1259-63

We present observations of the eccentric gamma-ray binary B1259-63/LS2883 with the Chandra X-ray Observatory. The images reveal a variable, extended about 4'', or about 1000 times the binary orbit size) structure, which appears to be moving away from the binary with the velocity of 0.05 of the speed of light. The observed emission is interpreted as synchrotron radiation from relativistic particles supplied by the pulsar. However, the fast motion through the circumbinary medium would require the emitting cloud to be loaded with a large amount of baryonic matter. Alternatively, the extended emission can be interpreted as a variable extrabinary shock in the pulsar wind outflow launched near binary apastron. The resolved variable X-ray nebula provides an opportunity to probe pulsar winds and their interaction with stellar winds in a previously inaccessible way.

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arXiv2013

Chandra Imaging of Gamma-Ray Binaries

We review the multiwavelength properties of the few known gamma-ray binaries, focusing on extended emission recently resolved with Chandra. We discuss the implications of these findings for the nature of compact objects and for physical processes operating in these systems.

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arXiv2013

Gamma-ray and X-ray Properties of Pulsar Wind Nebulae and Unidentified Galactic TeV Sources

We review properties of Galactic VHE sources detected at TeV energies. The number of associations between the VHE sources and pulsars has grown in recent years, making pulsar-wind nebulae the dominant population, although there is still a substantial number of VHE sources which remain to be identified. Among the latter there are several "dark" sources which do not have plausible counterparts at any other wavelengths. In this review we compile and compare the TeV and X-ray properties of pulsar wind nebulae (PWNe), PWN candidates, and unidentified TeV sources.

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arXiv2012

The helical jet of the Vela Pulsar

We have studied the fascinating dynamics of the nearby Vela pulsar's nebula in a campaign comprising eleven 40ks observations with Chandra X-ray Observatory (CXO). The deepest yet images revealed the shape, structure, and motion of the 2-arcminute-long pulsar jet. We find that the jet's shape and dynamics are remarkably consistent with that of a steadily turning helix projected on the sky. We discuss possible implications of our results, including free precession of the neutron star and MHD instability scenarios.

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arXiv2012

Absorption Features in the X-ray Spectrum of an Ordinary Radio Pulsar

The vast majority of known non-accreting neutron stars (NSs) are rotation-powered radio and/or gamma-ray pulsars. So far, their multiwavelength spectra have all been described satisfactorily by thermal and non-thermal continuum models, with no spectral lines. Spectral features have, however, been found in a handful of exotic NSs and thought to be a manifestation of their unique traits. Here we report the detection of absorption features in the X-ray spectrum of an ordinary rotation-powered radio pulsar, J1740+1000. Our findings bridge the gap between the spectra of pulsars and other, more exotic, NSs, suggesting that the features are more common in the NS spectra than they have been thought so far.

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arXiv2012

XMM-Newton view of Swift J1834.9-0846 and its Magnetar Wind Nebula

We report on the analysis of two XMM-Newton observations of the recently discovered soft gamma repeater Swift J1834.9-0846, taken in September 2005 and one month after the source went into outburst on 2011 August 7. We performed timing and spectral analyses on the point source as well as on the extended emission. We find that the source period is consistent with an extrapolation of the Chandra ephemeris reported earlier and the spectral properties remained constant. The source luminosity decreased to a level of 1.6x10^34 erg s^-1 following a decay trend of $\propto t^{-0.5}$. Our spatial analysis of the source environment revealed the presence of two extended emission regions around the source. The first (Region A) is a symmetric ring around the point source, starting at 25arcsec and extending to ~50arcsec. We argue that Region A is a dust scattering halo. The second (Region B) has an asymmetrical shape extending between 50arcsec and 150arcsec, and is detected both in the pre- and post-outburst data. We argue that this region is a possible magnetar wind nebula (MWN). The X-ray efficiency of the MWN with respect to the rotation energy loss is substantially higher than those of rotation powered pulsars: $η_{\rm X}\equiv L_{\rm MWN,0.5-8 keV}/\dot{E}_{\rm rot}\approx0.7$. The higher efficiency points to a different energy source for the MWN of Swift J1834.9-0846, most likely bursting activity of the magnetar, powered by its high magnetic field, B=1.4x10^14 G.

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