arXiv subjects

Robert Petre

Publications and source records attributed to Robert Petre.

86 records · Page 5Linked to original sources

arXiv2005

Late-time X-Ray, UV and Optical Monitoring of Supernova 1979C

We present results from observations of SN 1979C with XMM-Newton in X-rays and in the UV, archival X-ray and HST data, and follow-up ground-based optical imaging. The XMM-Newton MOS spectrum shows two-temperature thermal plasma emission characteristics of both the forward (4.1 keV) and reverse shock (0.78 keV) with no intrinsic absorption. The long-term X-ray lightcurve of SN 1979C shows no sign of a decline over 16-23 yrs after its outburst. The high X-ray luminosity (8 x 10^38 ergs/s) is caused by the interaction of the shock with dense circumstellar matter, likely deposited by a strong stellar wind from the progenitor with a high mass-loss rate of 1.5 x 10^-4 M_sun/yr. The X-ray data support a strongly decelerated shock and show a mass-loss rate history which is consistent with a constant progenitor mass-loss rate and wind velocity over the past >16,000 yrs in the evolution of the progenitor. We find a CSM density profile of rho_CSM \propto r^-s with index s<1.7 and high CSM densities (>10^4 cm^-3) out to large radii from the site of the explosion (> 4 x 10^17 cm). Using XMM-Newton Optical Monitor data we further detect optical/UV emission from SN 1979C, with B, U, and UVW1-band lum. of 5,7, and 9 x 10^36 ergs/s. The young stellar cluster in the vicinity of the SN, as imaged by the HST and follow-up ground-based optical imaging, can only provide a fraction of the total observed flux, so that a significant contribution to the output likely arises from the strong interaction of SN 1979C with dense CSM.

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arXiv2004

A Million-Second Chandra View of Cassiopeia A

We introduce a million-second observation of the supernova remnant Cassiopeia A with the Chandra X-ray Observatory. The bipolar structure of the Si-rich ejecta (NE jet and SW counterpart) is clearly evident in the new images, and their chemical similarity is confirmed by their spectra. These are most likely due to jets of ejecta as opposed to cavities in the circumstellar medium, since we can reject simple models for the latter. The properties of these jets and the Fe-rich ejecta will provide clues to the explosion of Cas A.

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arXiv2002

Thermal and Nonthermal Emission from the Forward Shock in Tycho's Supernova Remnant

We present Chandra X-ray images of Tycho's supernova remnant that delineate its outer shock as a thin, smooth rim along the straight northeastern edge and most of the circular western half. The images also show that the Si and S ejecta are highly clumpy, and have reached near the forward shock at numerous locations. Most of the X-ray spectra that we examine along the rim show evidence of line emission from Si and S ejecta, while the continuum is well-represented by either a thermal or nonthermal model. If the continuum is assumed to be thermal, the electron temperatures at the rim are all similar at about 2 keV, while the ionization ages are very low, because of the overall weakness of the line emission. These electron temperatures are substantially below those expected for equilibration of the electron and ion temperatures, assuming shock velocities inferred from radio and X-ray expansion measurements; the electron to mean temperature ratios are <~0.1-0.2, indicating that collisionless heating of the electrons at the shock is modest. The nonthermal contribution to these spectra may be important, but cannot be strongly constrained by these data. It could account for as much as half of the flux in the 4-6 keV energy range, based on an extrapolation of the hard X-ray spectrum above 10 keV.

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arXiv2002

ASCA Observations of the Supernova Remnant IC 443: Thermal Structure and Detection of Overionized Plasma

We present the results of X-ray spatial and spectral studies of the ``mixed-morphology'' supernova remnant IC 443 using ASCA. IC 443 has a center-filled image in X-ray band, contrasting with the shell-like appearance in radio and optical bands. The overall X-ray emission is thermal, not from a synchrotron nebula. ASCA observed IC 443 three times, covering the whole remnant. From the image analysis, we found that the softness-ratio map reveals a shell-like structure. At the same time, its spectra require two (1.0 keV and 0.2 keV) plasma components; the emission of the 0.2 keV plasma is stronger in the region near the shell than the center. These results can be explained by a simple model that IC 443 has a hot (1.0 keV) interior surrounded by a cool (0.2 keV) outer shell. From the emission measures, we infer that the 0.2 keV plasma is denser than the 1.0 keV plasma, suggesting pressure equilibrium between the two. In addition, we found that the ionization temperature of sulfur, obtained from H-like K$α$ to He-like K$α$ intensity ratio, is 1.5 keV, significantly higher than the gas temperature of 1.0 keV suggested from the continuum spectrum. The same can be concluded for silicon. Neither an additional, hotter plasma component nor a multi-temperature plasma successfully accounts for this ratio, and we conclude that the 1.0 keV plasma is overionized. This is the first time that overionized gas has been detected in a SNR. For the gas to become overionized in the absence of a photoionizing flux, it must cool faster than the ions recombine. Thermal conduction from the 1.0 keV plasma to the 0.2 keV one could cause the 1.0 keV plasma to become overionized, which is plausible within an old (3$\times10^4$ yr) SNR.

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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 Thermal X-ray Emitting Shell of LMC Supernova Remnant 0540-693

We use data from the Advanced CCD Imaging Spectrometer (ACIS) on the Chandra X-ray Observatory to image the shell surrounding the pulsar B0540-69, and to measure its spectral properties. Weak emission line features, notably of Fe, Mg and Si, indicate that the shell is composed primarily of ambient material heated by the blast wave. The shell emission in the east is faint and circular in appearance, with a temperature and ionization age of approximately 5 keV and 10^10 cm^-3 s, respectively. The emission in the west is brighter and has a more complex, distorted morphology, and is fitted with significantly lower average temperatures and higher ionization ages. An image with photon energies above 2 keV shows two hard arcs of emission located diametrically opposite each other at the outer boundary of the remnant shell; the spectra from these regions could include a substantial nonthermal component.

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arXiv2000

Detecting X-ray Synchrotron Emission in Supernova Remnants: Implications for Abundances and Cosmic Rays

The 10^51 ergs released in a supernova have far reaching consequences in the galaxy, determining elemental abundances, accelerating cosmic rays, and affecting the makeup of the interstellar medium. Recently the spectra of several supernova remnants have been found to be dominated by nonthermal emission. Separating the thermal and nonthermal components is important not only for the understanding of cosmic-ray acceleration and shock microphysics properties but for accurate assessment of the temperatures and line strengths. New models designed to model spatially resolved synchrotron X-rays from type Ia supernovae can contribute to the understanding of both the thermal physics (dynamics, abundances) and nonthermal physics (shock acceleration, magnetic-field amplification) of supernova remnants. I will describe model fits to SN 1006, emphasizing the physical constraints that can be placed on SNRs, abundances, and the cosmic-ray acceleration process.

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arXiv2000

The X-ray Remnant of SN1987A

We present high resolution Chandra observations of the remnant of SN1987A in the Large Magellanic Cloud. The high angular resolution of the Chandra X-ray Observatory (CXO) permits us to resolve the X-ray remnant. We find that the remnant is shell-like in morphology, with X-ray peaks associated with some of the optical hot spots seen in HST images. The X-ray light curve has departed from the linear flux increase observed by ROSAT, with a 0.5-2.0 keV luminosity of 1.5 x 10^35 erg/s in January 2000. We set an upper limit of 2.3 x 10^34 ergs/s on the luminosity of any embedded central source (0.5 - 2 keV). We also present a high resolution spectrum, showing that the X-ray emission is thermal in origin and is dominated by highly ionized species of O, Ne, Mg, and Si.

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arXiv2000

A Deep ROSAT HRI Observation of NGC 1313

We describe a series of observations of NGC 1313 using the ROSAT HRI with a combined exposure time of 183.5 ksec. The observations span an interval between 1992 and 1998; the purpose of observations since 1994 was to monitor the X-ray flux of SN1978K, one of several luminous sources in the galaxy. No diffuse emission is detected in the galaxy to a level of ~1-2x10^37 ergs/s/arcmin^-2. A total of eight sources are detected in the summed image within the D_25 diameter of the galaxy. The luminosities of five of the eight range from \~6x10^37 to ~6x10^38 erg/s; these sources are most likely accreting X-ray binaries, similar to sources obseved in M31 and M33. The remaining three sources all emit above 10^39 erg/s. We present light curves of the five brightest sources. Variability is detected at the 99.9% level from four of these. We identify one of the sources as an NGC 1313 counterpart of a Galactic X-ray source. The light curve, though crudely sampled, most closely resembles that of a Galactic black hole candidate such as GX339-4, but with considerably higher peak X-ray luminosity. An additional seven sources lie outside of the D_25 diameter and are either foreground stars or background AGN.

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arXiv2000

Mapping the X-ray Emitting Ejecta in Cassiopeia A with Chandra

We present X-ray emission line equivalent width images of the bright Galactic supernova remnant Cassiopeia A for the elements Si, S, Ar, Ca, and Fe using a 50,000 s observation with the Advanced CCD Imaging Spectrometer on the Chandra X-ray Observatory. The images essentially identify the bulk of detectable ejecta of these elements over a wide range of surface brightness, and show morphologies distinctly different from that of the broadband X-ray emission and of the 4-6 keV continuum emission. The Si, S, Ar, and Ca maps, while different in turn from those of Fe, show that these X-ray emitting ejecta are distributed similarly to the fast optical ejecta knots, and clearly delineate the X-ray counterpart of the northeast optical jet. Low surface brightness regions just outside the bright shell in the north and west are also shown to have strong line emission. The strong Fe emission is exterior to that of other elements in the east, as previously noted, but is generally coincident elsewhere. The projected interior has relatively little emission traced by high line equivalent widths.

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arXiv2000

Resolving SNR 0540-6944 from LMC X-1 with Chandra

We examine the supernova remnant (SNR) 0540-697 in the Large Magellanic Cloud (LMC) using data from the Chandra ACIS. The X-ray emission from this SNR had previously been hidden in the bright emission of nearby X-ray binary LMC X-1; however, new observations with Chandra can finally reveal the SNR's structure and spectrum. We find the SNR to be a thick-shelled structure about 19 pc in diameter, with a brightened northeast region. Spectral results suggest a temperature of 0.31 keV and an X-ray luminosity (0.3-3.0 keV) of 8.4 x 10^33 erg/s. We estimate an age of 12,000-20,000 yr for this SNR, but note that this estimate does not take into account the possibility of cavity expansion or other environmental effects.

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arXiv1999

The X-ray Line Emission from the Supernova Remnant W49B

The Galactic supernova remnant W49B has one of the most impressive X-ray emission line spectra obtained with the Advanced Satellite for Cosmology and Astronomy (ASCA). We use both plasma line diagnostics and broadband model fits to show that the Si and S emission lines require multiple spectral components. The spectral data do not necessarily require individual elements to be spatially stratified, as suggested by earlier work, although when ASCA line images are considered, it is possible that Fe is stratified with respect to Si and S. Most of the X-ray emitting gas is from ejecta, based on the element abundances required, but is surprisingly close to being in collisional ionization equilibrium. A high ionization age implies a high internal density in a young remnant. The fitted emission measure for W49B indicates a minimum density of 2 cm^-3, with the true density likely to be significantly higher. W49B probably had a Type Ia progenitor, based on the relative element abundances, although a low-mass Type II progenitor is still possible. We find persuasive evidence for Cr and possibly Mn emission in the ASCA spectrum--the first detection of these elements in X-rays from a cosmic source.

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arXiv1999

Supernova Remnants in the Magellanic Clouds III: An X-ray Atlas of LMC Supernova Remnants

We have used archival ROSAT data to present X-ray images of thirty-one supernova remnants (SNRs) in the Large Magellanic Cloud (LMC). We have classified these remnants according to their X-ray morphologies, into the categories of Shell-Type, Diffuse Face, Centrally Brightened, Point-Source Dominated, and Irregular. We suggest possible causes of the X-ray emission for each category, and for individual features of some of the SNRs.

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arXiv1997

The X-ray Iron Emission from Tycho's Supernova Remnant

We present the results of broadband fits to the X-ray spectrum of Tycho's supernova remnant obtained by the Solid-State Imaging Spectrometers on the ASCA Observatory. We use single-temperature, single-ionization-age, nonequilibrium ionization models to characterize the ejecta and the blast-shocked interstellar medium. Based on the Fe K emission at 6.5 keV, previous spectral studies have suggested that the Fe ejecta in this Type Ia remnant are stratified interior to the other ejecta. The ASCA data provide important constraints from the Fe L emission near 1 keV as well as the Fe K emission. We find that the simplest models, with emission from the ejecta and blast wave each at a single temperature and ionization age, severely underestimate the Fe K flux. We show that there is little Fe emission associated with the Si and S ejecta shell. The blast-shocked interstellar medium has abundances roughly 0.3 times the solar value, while the ejecta, with the exception of Fe, have relative abundances that are typical of Type Ia supernovae. The addition of another component of Fe emission, which we associate with ejecta, at a temperature at least two times higher and an ionization age $\sim$ 100 times lower than the Si ejecta, does provide a good fit to the spectrum. This model is consistent with X-ray imaging results. Although fluorescent emission from dust in the remnant may contribute to the Fe K flux, we conclude that it is unlikely to dominate.

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