arXiv subjects

John P. Hughes

Publications and source records attributed to John P. Hughes.

At least 217 records · Page 12Linked to original sources

arXiv2003

Detection of Magnesium-Rich Ejecta in the Middle-Aged Supernova Remnant N49B

The middle-aged supernova remnant (SNR) N49B in the Large Magellanic Cloud has been observed with the {\it Chandra X-Ray Observatory}. The superb angular resolution of {\it Chandra} resolves the complex structure of X-ray emitting filaments across the SNR. All observed features are soft ($E <$ 3 keV) and we find no evidence for either point-like or extended hard emission within the SNR. Spectral lines from O, Ne, Mg, Si, S, and Fe are present. Equivalent width images for the detected elemental species and spatially-resolved spectral analysis reveal the presence of Mg-rich ejecta within the SNR. We find no such enrichment in O or Ne, which may reflect details of the nucleosynthesis process or the heating and cooling of the ejecta as it evolved. The bright circumferential filaments are emission from the shocked dense interstellar medium (ISM). We detect faint diffuse X-ray emission that extends beyond the X-ray bright filaments toward the west and southeast. These features appear to be the blast wave shock front expanding into lower density portions of the ISM seen in projection. We set an upper limit of $\sim$$2\times 10^{33}$ ergs s$^{-1}$ on the 0.5 $-$ 5 keV band X-ray luminosity of any embedded compact object.

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arXiv2003

An X-Ray Pulsar in the Oxygen-Rich Supernova Remnant G292.0+1.8

We report the discovery of pulsed X-ray emission from the compact object CXOU J112439.1-591620 within the supernova remnant (SNR) G292.0+1.8 using the High Resolution Camera on the Chandra X-ray Observatory. The X-ray period (P=0.13530915 s) is consistent with extrapolation of the radio pulse period of PSR J1124-5916 for a spindown rate of dP/dt=7.6E-13 s/s. The X-ray pulse is single peaked and broad with a FWHM width of 0.23P (83 degrees). The pulse-averaged X-ray spectral properties of the pulsar are well described by a featureless power law model with an absorbing column density, N_H= 3.1E21 atoms/cm^2; photon index, gamma = 1.6; and unabsorbed 0.3-10 keV band luminosity, L_X = 7.2E32 erg/s. We plausibly identify the location of the pulsar's termination shock. Pressure balance between the pulsar wind and the larger synchrotron nebula, as well as lifetime issues for the X-ray-emitting electrons, argues for a particle- dominated PWN that is far from the minimum energy condition. Upper limits on the surface temperature of the neutron star are at, or slightly below, values expected from ``standard'' cooling curves. There is no optical counterpart to the new pulsar; its optical luminosity is at least a factor of 5 below that of the Crab pulsar.

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arXiv2003

The Physics of Supernova Remnant Blast Waves. I. Kinematics of DEM L71 in the Large Magellanic Cloud

We present the results from Fabry-Perot imaging spectroscopy of the Balmer-dominated supernova remnant DEM L71 (0505-67.9) in the LMC. Spectra extracted from the entire circumference of the blast wave reveal the broad and narrow component H-alpha line emission characteristic of non-radiative shocks in partially neutral gas. The new spectra of DEM L71 include portions of the rim that have not been previously observed. We find that the broad component width varies azimuthally along the edge of DEM L71, ranging from 450+/-60 km/s along the eastern edge to values as high as 985 (+210)(-165) km/s along the faint western edge. In part of the faint northern rim the broad component is not detected, possibly indicating a lower density in these regions and/or a broad component width in excess of 1000 km/s. Between the limits of zero and full electron-ion temperature equilibration at the shock front, the allowed range of shock velocities is 430-560 km/s along the east rim and 700-1250 km/s along other parts of the blast wave. The H-alpha broad-to-narrow flux ratios vary considerably around the remnant, ranging from 0.4 to 0.8. These ratios lie below the values predicted by our shock models. We find that narrow component H-alpha emission from a cosmic ray precursor may be the cause of the discrepancy. The least decelerated portions of the blast wave (i.e., regions excluding the brightest filaments) are well characterized by Sedov models with a kinetic energy E_51= (0.37+/-0.06)*D_50**(5/2), where D_50 is the LMC distance in units of 50 kpc. The corresponding age for DEM L71 is (4360+/-290)*D_50 yr. This is the first time that velocity information from the entire blast wave has been utilized to study the global kinematics of a non-radiative SNR at a known distance.

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arXiv2003

The Physics of Supernova Remnant Blast Waves. II. Electron-Ion Equilibration in DEM L71 in the Large Magellanic Cloud

We present analysis and modeling of X-ray spectra from the blast wave shock of DEM L71. We have used both Chandra ACIS-S data and optical Fabry-Perot spectra of the blast wave to measure the electron and proton temperatures, respectively. In principle, when combined, these measurements can determine the degree of electron-ion temperature equilibration (g_0 = T_e/T_p) immediately behind the shock front. In our X-ray analysis we fit Chandra spectra of three nested regions behind the blast wave under three different scenarios: (1) a planar, initially unequilibrated shock (g_0 = m_e/m_p), where the downstream electron and proton temperatures equilibrate through Coulomb collisions, (2) a planar, immediately equilibrated shock (g_0 = 1) and (3) a spherical, equilibrated shock under Sedov evolution. Using independent measurements of T_e and T_p we find that the X-ray spectra from the fastest blast wave locations (V_s~700-1000 km/s) are consistent with little or no equilibration at the shock front and are inconsistent with full equilibration. In contrast, spectra from regions showing slower blast wave speeds (V_s~400-600 km/s) allow full equilibration but exclude zero equilibration. In order to directly constrain the initial equilibration, we incorporated our knowledge of the proton temperatures into our X-ray models to build planar shock models that allowed for a variable g_0. This model confirmed and strengthened the above results. Specifically, we found that X-ray spectra from an intermediate velocity shock (V_s~800 km/s) were consistent with intermediate equilibration, excluding both g_0 = m_e/m_p and g_0 = 1 at greater than 1 sigma. Overall, our results support the picture of decreasing electron-ion equilibration with increasing shock speed.

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arXiv2002

Iron-Rich Ejecta in the Supernova Remnant DEM L71

Chandra X-ray observations of DEM L71, a supernova remnant (SNR) in the Large Magellanic Cloud (LMC), reveal a clear double shock morphology consisting of an outer blast wave shock surrounding a central bright region of reverse-shock heated ejecta. The abundances of the outer shock are consistent with LMC values, while the ejecta region shows enhanced abundances of Si, Fe, and other species. However, oxygen is not enhanced in the ejecta; the Fe/O abundance ratio there is more than 5 times the solar ratio. Based on the relative positions of the blast wave shock and the contact discontinuity in the context of SNR evolutionary models, we determine a total ejecta mass of approximately 1.5 solar masses. Ejecta mass estimates based on emission measures derived from spectral fits are subject to considerable uncertainty due to lack of knowledge of the true contribution of hydrogen continuum emission. Maximal mass estimates, i.e., assuming no hydrogen, result in 1.5 solar masses of Fe and 0.24 solar masses of Si. Under the assumption that an equal quantity of hydrogen has been mixed into the ejecta, we estimate 0.8 solar masses of Fe and 0.12 solar masses of Si. These characteristics support the view that in DEM L71 we see Fe-rich ejecta from a Type Ia SN several thousand years after explosion.

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arXiv2002

X-Ray Emission from Multi-Phase Shock in the Large Magellanic Cloud Supernova Remnant N49

The supernova remnant (SNR) N49 in the Large Magellanic Cloud has been observed with the Advanced CCD Imaging Spectrometer (ACIS) on board the {\it Chandra X-Ray Observatory}. The superb angular resolution of the {\it Chandra}/ACIS images resolves a point source, the likely X-ray counterpart of soft gamma-ray repeater SGR 0526$-$66, and the diffuse filaments and knots across the SNR. These filamentary features represent the blast wave sweeping through the ambient interstellar medium and nearby dense molecular clouds. We detect metal-rich ejecta beyond the main blast wave shock boundary in the southwest of the SNR, which appear to be explosion fragments or ``bullets'' ejected from the progenitor star. The detection of strong H-like Si line emission in the eastern side of the SNR requires multi-phase shocks in order to describe the observed X-ray spectrum, whereas such a multi-phase plasma is not evident in the western side. This complex spectral structure of N49 suggests that the postshock regions toward the east of the SNR might have been reheated by the reverse shock off the dense molecular clouds while the blast wave shock front has decelerated as it propagates into the dense clouds. The X-ray spectrum of the detected point-like source is continuum dominated and can be described with a power law of $Γ$ $\sim$ 3. This provides a confirmation that this point-like X-ray source is the counterpart of SGR 0526$-$66 in the quiescent state.

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arXiv2002

Revealing New Physical Structures in the Supernova Remnant N63A through Chandra Imaging Spectroscopy

We present Chandra X-ray observations of the supernova remnant (SNR) N63A in the Large Magellanic Cloud (LMC). N63A, one of the brightest LMC remnants, is embedded in an H II region and probably associated with an OB association. The optical remnant consists of three lobes of emission contained within the approximately three times larger X-ray remnant. Our Chandra data reveal a number of new physical structures in N63A. The most striking of these are the several ``crescent''-shaped structures located beyond the main shell that resemble similar features seen in the Vela SNR. In Vela, these have been interpreted as arising from high speed clumps of supernova ejecta interacting with the ambient medium. Another distinct feature of the remnant is a roughly triangular ``hole'' in the X-ray emission near the location of the optical lobes and the brightest radio emission. X-ray spectral analysis shows that this deficit of emission is a result of absorption by an intervening dense cloud with a mass of ~450 M_sun that is currently being engulfed by the remnant's blast wave. We also find that the rim of the remnant, as well as the crescent-shaped features, have considerably softer X-ray spectra than the interior. Limits on hard X-ray emission rule out a young, energetic pulsar in N63A, but the presence of an older or less active one, powering a wind nebula with a luminosity less than ~4e10^34 erg/s, is allowed.

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arXiv2002

The Radial Structure of SNR N103B

We report on the results from a Chandra ACIS observation of the young, compact, supernova remnant N103B. The unprecedented spatial resolution of Chandra reveals sub-arcsecond structure, both in the brightness and in spectral variations. Underlying these small-scale variations is a surprisingly simple radial structure in the equivalent widths of the strong Si and S emission lines. We investigate these radial variations through spatially resolved spectroscopy using a plane-parallel, non-equilibrium ionization model with multiple components. The majority of the emission arises from components with a temperature of 1 keV: a fully ionized hydrogen component; a high ionization timescale (n_e*t > 10^12 s cm^-3) component containing Si, S, Ar, Ca, and Fe; and a low ionization timescale (n_e*t ~ 10^{11} s cm^-3) O, Ne, and Mg component. To reproduce the strong Fe Kalpha line, it is necessary to include additional Fe in a hot (> 2 keV), low ionization (n_e*t ~ 10^10.8 s cm^-3) component. This hot Fe may be in the form of hot Fe bubbles, formed in the radioactive decay of clumps of 56Ni. We find no radial variation in the ionization timescales or temperatures of the various components. Rather, the Si and S equivalent widths increase at large radii because these lines, as well as those of Ar and Ca, are formed in a shell occupying the outer half of the remnant. A shell of hot Fe is located interior to this, but there is a large region of overlap between these two shells. In the inner 30% of the remnant, there is a core of cooler, 1 keV Fe. We find that the distribution of the ejecta and the yields of the intermediate mass species are consistent with model prediction for Type Ia events.

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arXiv2001

The Structure of the Oxygen-rich Supernova Remnant G292.0+1.8 from Chandra X-ray Images: Shocked Ejecta and Circumstellar Medium

We present results from the observation of the young Galactic supernova remnant (SNR) G292.0+1.8 with the Advanced CCD Imaging Spectrometer (ACIS) on board the {\it Chandra X-ray Observatory}. In the 0.3 $-$ 8 keV band, the high resolution ACIS images reveal a complex morphology consisting of knots and filaments, as well as the blast wave around the periphery of the SNR. We present equivalent width (EW) maps for the elemental species O, Ne, Mg, and Si, which allow us to identify regions of enhanced metallicity in the SNR. G292.0+1.8 is bright in O, Ne, and Si; weaker in S and Ar; with little Fe. The EW and broad-band images indicate that the metal-rich ejecta are distributed primarily around the periphery of the SNR. The central belt-like structure has normal solar-type composition, strongly suggesting that it is primarily emission from shocked circumstellar medium rather than metal-rich ejecta. We propose that the belt traces its origin to enhanced mass loss in the star's equatorial plane during the slow, red supergiant phase. We also identify thin filaments with normal composition, centered on and extending nearly continuously around the outer boundary of the SNR. These may originate in a shell caused by the stellar winds from the massive progenitor in the red/blue supergiant phases, over-run by the blast wave.

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arXiv2001

An X-Ray Study of the Supernova Remnant G290.1-0.8

G290.1-0.8 (MSH 11-61A) is a supernova remnant (SNR) whose X-ray morphology is centrally bright. However, unlike the class of X-ray composite SNRs whose centers are dominated by nonthermal emission, presumably driven by a central pulsar, we show that the X-ray emission from G290.1-0.8 is thermal in nature, placing the remnant in an emerging class which includes such remnants as W44, W28, 3C391, and others. The evolutionary sequence which leads to such X-ray properties is not well understood. Here we investigate two scenarios for such emission: evolution in a cloudy interstellar medium, and early-stage evolution of a remnant into the radiative phase, including the effects of thermal conduction. We construct models for these scenarios in an attempt to reproduce the observed center-filled X-ray properties of G290.1-0.8, and we derive the associated age, energy, and ambient density conditions implied by the models. We find that for reasonable values of the explosion energy, the remnant age is of order (1 - 2) x 10^{4} yr. This places a fairly strong constraint on any association between G290.1-0.8 and PSR J1105-610, which would require an anomalously large velocity for the pulsar.

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arXiv2001

The Small-scale Structure of N103B - Nature of Nuture?

We present new results from a 40.8 ks Chandra ACIS observation of the young supernova remnant (SNR) N103B located in the Large Magellanic Cloud. The high resolution Chandra image reveals structure at the sub-arcsecond level, including several bright knots and filaments. Narrow-band imaging and spatially resolved spectroscopy reveal dramatic spectral variations in this remnant as well. In this paper we discuss whether these variations are due to inhomogeneities in the surrounding environment or were generated in the explosion which created the SNR.

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arXiv2001

A Pulsar Wind Nebula in the Oxygen-Rich Supernova Remnant G292.0+1.8

Using the Chandra X-ray Observatory we have discovered a diffuse, center-filled region of hard X-ray emission within the young, oxygen-rich supernova remnant (SNR) G292.0+1.8. Near the peak of this diffuse emission lies a point-like source of X-ray emission that is well described by an absorbed power-law spectrum with photon index $1.72\pm0.09$. This source appears to be marginally extended; its extent of 1.3$^{\prime\prime}$ (FWHM) is greater than that of a nearby serendipitous X-ray source with FWHM = $1.1^{\prime\prime}$. This is strong evidence for the presence within SNR G292.0+1.8 of a young rapidly-rotating pulsar and its associated pulsar wind nebula. From the unabsorbed, 0.2-4 keV band X-ray luminosity of the pulsar wind nebula ($L_X\sim 4\times 10^{34} \rm ergs s^{-1}$), we infer a spin-down energy loss rate of $\dot E \sim 7\times 10^{36} \rm ergs s^{-1}$ for the still undetected pulsar. The pulsar candidate is $0.9^\prime$ from the geometric center of the SNR which implies a transverse velocity of $\sim$$770 (D/4.8 {\rm kpc}) (t/1600 {\rm yr})^{-1} \rm km s^{-1}$ assuming currently accepted values for the distance and age of G292.0+1.8.

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arXiv2001

Young Supernova Remnants in the Magellanic Clouds

There are a half-dozen or so young supernova remnants in the Magellanic Clouds that display one or more of the following characteristics: high velocity (>1000 km/s) emission, enhanced metallicity, or a rapidly rotating pulsar. I summarize the current state of knowledge of these remnants and present some recent results mostly from the new X-ray astronomy satellites.

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arXiv2000

RXJ 0852.0-0462: Another Nonthermal Shell-Type SNR (G266.2-1.2)

The newly discovered supernova remnant G266.2-1.2 (RX J0852.0--0462), along the line of sight to the Vela SNR, was observed with ASCA for 120 ks. We find that the X-ray spectrum is featureless, and well described by a power law, extending to three the class of shell-type SNRs dominated by nonthermal X-ray emission. Like G347.3-0.5, this low-latitude remnant displays discrete regions of enhanced emission along the rim as well as faint nonthermal emission from the interior. We derive limits on the thermal content of the remnant emission, although the presence of the Vela SNR compromises our ability to seriously constrain a low temperature component. Limits placed on the amount of Sc-K emission are compared with the expected flux based upon the reported 44Ti emission from G2662.-1.2. We also report on an unresolved X-ray source surrounded by diffuse emission near the center of the remnant. The properties of the source are not well determined, but appear consistent with the interpretation that the source is a neutron star surrounded by a synchrotron nebula. Alternatively, the source may be associated with one of two stars located within the positional error circle, but this appears somewhat unlikely.

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arXiv2000

The Expansion of the X-ray Remnant of Tycho's Supernova (SN1572)

Two ROSAT high resolution images separated by nearly five years have been used to determine the expansion of the X-ray remnant of Tycho's supernova (SN1572). The current expansion rate averaged over the entire remnant is $0.124\pm0.011$ % per yr, which, when combined with the known age of the remnant, determines the mean expansion parameter $m$, defined as $R\propto t^m$, to be $0.54\pm0.05$. There are significant radial and azimuthal variations of the X-ray expansion rate. The radial expansion in particular shows highly significant evidence for the more rapid expansion of the forward blast wave as compared to the reverse-shocked ejecta, an effect that has not been seen previously. The expansion parameter varies from $m=0.71\pm0.06$ at the outermost edge of Tycho's supernova remnant (SNR) to a value of $m=0.34\pm0.10$ on the inside edge of the bright rim of emission. These values are consistent with the rates expected for a remnant with constant density ejecta evolving into a uniform interstellar medium during the ejecta-dominated phase of evolution. Based on the size, age, and X-ray expansion rates, I obtain values for the explosion energy and ambient density of $E\approx 4-5\times 10^{50} \rm ergs$ and $n_0 \approx 0.35-0.45 \rm cm^{-3}$. As is also the case for Cas A and Kepler's SNR, the X-ray expansion rate of Tycho's SNR appears to be significantly higher than the radio expansion rate. In the case of Tycho's SNR, however, the difference between radio and X-ray expansion rates is clearly associated with the motion of the forward shock.

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arXiv2000

Two New Ejecta-Dominated Galactic Supernova Remnants: G337.2-0.7 and G309.2-0.6

We present the analysis of new X-ray observations of two Galactic radio supernova remnants (SNRs), G337.2-0.7 and G309.2-0.6. Both remnants exhibit line-rich thermal spectra that require highly non-solar elemental abundance ratios. In each case, Si and S are unequivocally overabundant compared to solar based purely on the measured equivalent width of the K$α$ line complex. Detailed nonequilibrium ionization spectral analysis of these remnants, using a single temperature, single ionization age model, confirm the overabundances, yielding Si, S and Ar abundances many times their solar values: 3 to 5 times solar for G337.2-0.7 and greater than 3 to 10 times solar for G309.2-0.6. We also find that for both remnants, Ne and Mg are underabundant relative to Si, and for G309.2-0.6, Ca and Fe as well are relatively underabundant. This is the first detection of metal-rich, non-solar abundance material in these remnants, identifying them as young ejecta-dominated SNRs. Further support for their youth comes from the distances that we estimate from the fitted column densities (less than 15 kpc for G337.2-0.7, and $4 \pm 2$ kpc for G309.2-0.6). From the implied sizes of the remnants at these distances we infer relatively young ages (2000 to 4500 years for G337.2-0.7, and 700 to 4000 years for G309.2-0.6). Finally we note that no compact object has yet been found within G337.2-0.7, but an X-ray point source is evident within the radio shell of G309.2-0.6, although its nature remains unknown.

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arXiv2000

ASCA Observations of the Twin Supernova Remnants in the Large Magellanic Cloud, DEM L316

We report results from an ASCA X-ray study of DEM L316, an emission nebula in the Large Magellanic Cloud (LMC) consisting of two closely-spaced supernova remnants (SNRs). The SIS image shows separate X-ray sources located at the positions of the two radio- and optically-emitting SNR shells, 0547$-$69.7A and 0547$-$69.7B (hereafter, shell A and B). The individual X-ray spectrum of each shell is well described by optically-thin thermal emission, although the characteristics of the emission differ in important details between them. Shell A exhibits strong iron L emission, which we attribute to the presence of iron-rich ejecta leading to the suggestion that this SNR originates from a Type Ia SN, an explosion of a moderate mass progenitor. Shell B, on the other hand, has a chemical composition similar to that of the interstellar medium of the LMC and so its X-ray emission is dominated by swept-up matter. The different spectral features strongly disfavor the hypothesis that the two shells are due to one supernova explosion into an interconnected bubble. We could not obtain the evidence which positively supports the collision between two SNRs.

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arXiv2000

Electron Heating and Cosmic Rays at a Supernova Shock from Chandra X-ray Observations of E0102.2-7219

In this Letter we use the unprecedented spatial resolution of the Chandra X-ray Observatory to carry out, for the first time, a measurement of the post-shock electron temperature and proper motion of a young SNR, specifically to address questions about the post-shock partition of energy among electrons, ions, and cosmic rays. The expansion rate, 0.100 +/- 0.025 percent per yr, and inferred age, ~1000 yr, of E0102.2-7219, from a comparison of X-ray observations spanning 20 years, are fully consistent with previous estimates based on studies of high velocity oxygen-rich optical filaments in the remnant. With a radius of 6.4 pc for the blast wave estimated from the Chandra image, our expansion rate implies a blast wave velocity of ~6000 km/s and a range of electron temperatures 2.5 - 45 keV, dependent on the degree of collisionless electron heating. Analysis of the Chandra ACIS spectrum of the immediate post-shock region reveals a thermal plasma with abundances and column density typical of the Small Magellanic Cloud and an electron temperature of 0.4-1 keV. The measured electron temperature is significantly lower than the plausible range above, which can only be reconciled if we assume that a significant fraction of the shock energy, rather than contributing to the heating of the post-shock electrons and ions, has gone into generating cosmic rays.

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