Searching for the pulsar in G18.95-1.1: Discovery of an X-ray point source and associated synchrotron nebula with Chandra

Using the Chandra X-ray Observatory, we have pinpointed the location of a faint X-ray point source (CXOUJ182913.1-125113) and an associated diffuse nebula in the composite supernova remnant G18.95-1.1. These objects appear to be the long-sought pulsar and its wind nebula. The X-ray spectrum of the point source is best described by an absorbed powerlaw model with Gamma=1.6 and an N_H of ~1x10^(22) cm^(-2). This model predicts a relatively low unabsorbed X-ray luminosity of about L_X (0.5-8.0keV) = 4.1x10^(31)D_2^2 erg s^(-1), where D_2 is the distance in units of 2kpc. The best-fitted model of the diffuse nebula is a combination of thermal (kT = 0.48keV) and non-thermal (1.4 < Gamma < 1.9) emission. The unabsorbed X-ray luminosity of L_X = 5.4x10^(33)D_2^2 erg s^(-1) in the 0.5-8keV energy band seems to be largely dominated by the thermal component from the SNR, providing 87% of L_X in this band. No radio or X-ray pulsations have been reported for CXOUJ182913.1-125113. If we assume an age of ~5300yr for G18.95-1.1 and use the X-ray luminosity for the pulsar and the wind nebula together with the relationship between spin-down luminosity (via magnetic dipole radiation) and period, we estimate the pulsar's period to be P = 0.4s. Compared to other rotation-powered pulsars, a magnetic field of 2.2x10^(13)G is implied by its location in the P-Pdot diagram, a value which is close to that of the quantum critical field.Comment: 8 pages, 3 Figures, accepted for publication in Ap

Interpretation of the Center-Filled Emission from the Supernova Remnant W44

(Abridged) We have investigated two evolutionary scenarios advanced to explain the centrally-brightened X-ray morphology of the supernova remnant (SNR) W44: (1) a model involving the slow thermal evaporation of clouds engulfed by a supernova blast wave as it propagates though a clumpy interstellar medium (ISM), and (2) a hydrodynamical simulation of a blast wave propagating through a homogeneous ISM, including the effects of radiative cooling. Both models can have their respective parameters tuned to approximate the remnant's morphology. The mean temperature of the hot plasma in W44 (~0.9 keV) as determined by our nonequilibrium ionization X-ray spectral analysis provides the essential key to discriminate between these scenarios. Based on the size (using the well established distance of 3 kpc) and temperature of W44, the dynamical evolution predicted by the cloud evaporation model gives an age for the SNR of merely 6500 yr. We argue that, because this age is inconsistent with the characteristic age (approx. 20000 yr) of the associated PSR 1853+01, this model cannot provide the explanation for the center-filled morphology. We favor the radiative-phase shock model since it can reproduce both the morphology and age of W44 assuming reasonable values for the initial explosion energy in the range 0.7E51 to 0.9E51 ergs and the ambient ISM density of between 3 and 4 cm**-3.Comment: 31 pages, including 4 postscript figs, LaTeX, accepted by Ap.

Guiding the Way to Gamma-Ray Sources: X-ray Studies of Supernova Remnants

Supernova remnants have long been suggested as a class of potential counterparts to unidentified gamma-ray sources. The mechanisms by which such gamma-rays can arise may include emission from a pulsar associated with a remnant, or a variety of processes associated with energetic particles accelerated by the SNR shock. Imaging and spectral observations in the X-ray band can be used to identify properties of the remnants that lead to gamma-ray emission, including the presence of pulsar-driven nebulae, nonthermal X-ray emission from the SNR shells, and the interaction of SNRs with dense surrounding material.Comment: 16 pages, 11 figures, To appear in the proceedings of the workshop: "The Nature of the Unidentified Galactic Gamma-Ray Sources" held at INAOE, Mexico, October 2000, (A.Carraminana, O. Reiner and D. Thompson, eds.

X-Ray Observations of the supernova remnant G21.5-0.9

We present the analysis of archival X-ray observations of the supernova remnant (SNR) G21.5-0.9. Based on its morphology and spectral properties, G21.5-0.9 has been classified as a Crab-like SNR. In their early analysis of the CHANDRA calibration data, Slane et al. (2000) discovered a low-surface-brightness, extended emission. They interpreted this component as the blast wave formed in the supernova (SN) explosion. In this paper, we present the CHANDRA analysis using a total exposure of ~150 ksec. We also include ROSAT and ASCA observations. Our analysis indicates that the extended emission is non-thermal -- a result in agreement with XMM observations. The entire remnant of radius ~ 2'.5 is best fitted with a power law model with a photon index steepening away from the center. The total unabsorbed flux in the 0.5-10 keV is 1.1E-10 erg/cm2/s with an 85% contribution from the 40" radius inner core. Timing analysis of the High-Resolution Camera (HRC) data failed to detect any pulsations. We put a 16% upper limit on the pulsed fraction. We derive the physical parameters of the putative pulsar and compare them with those of other plerions (such as the Crab and 3C 58). G21.5-0.9 remains the only plerion whose size in X-rays is bigger than in the radio. Deep radio observations will address this puzzle.Comment: 23 pages including 11 figures and 3 tables; accepted by ApJ June 22, 2001; to appear in Oct 20, 2001 issue of Ap

Study of the Composite Supernova Remnant MSH 11-62

We present the analysis of the X-ray data collected during an observation of the supernova remnant (SNR) MSH 11-62 by the Advanced Satellite for Cosmology and Astrophysics (ASCA). We show that MSH 11-62 is a composite remnant whose X-ray emission comes from two distinct contributions. Nonthermal, synchrotron emission, localized to a region of radius (~~)3' (consistent with a point source) dominates the total flux above 2 keV. A second contribution comes from a thermal component, extended up to a radius of (~~)6' and detected only at energies below 2keV. The spatial and spectral analysis imply the presence of a neutron star losing energy at a rate of about (10**36 - 10**37) ergs/s. No pulsed emission is detected and we set a limit on the pulsed fraction of 10%. This is consistent with the lack of a radio pulsar in the remnant, which may indicate that the pulsed emission from the rapidly rotating compact object that should be powering the synchrotron nebula is beamed and our viewing direction is unfavorable. In either event, the central neutron star deposits much of its spin-down energy into the surrounding synchrotron nebula where, through direct imaging with broadband satellites such as ASCA, it is possible to study the energetics and evolution of the compact remnant.Comment: 30 pages, including 5 figures, Latex. To appear in ApJ (May 20, 1998 issue, Vol. 499.

A New ASCA and ROSAT Study of the Supernova Remnant: G272.2-3.2

G272.2-3.2 is a supernova remnant (SNR) characterized by an apparent centrally brightened X-ray morphology and thermally dominated X-ray emission. Because of this combination of Sedov-type (thermal emission) and non-Sedov type (non-shell like morphology) features, the remnant is classified as a ``thermal composite'' SNR. This class of remnant is still poorly understood due in part to the difficulties in modeling accurately all the physical conditions which shape the emission morphology. In this paper we present a combined analysis of data from the ASCA and ROSAT satellites coupled with previous results at other wavelengths. We find that the X-ray emission from G272.2-3.2 is best described by a non-equilibrium ionization (NEI) model with a temperature around 0.70 keV, an ionization timescale of 3200 cm^-3 yr and a relatively high column density (NH about 10^22 atoms/cm^2). We look into the possible explanations for the apparent morphology of G272.2-3.2 using several models (among which both cloud evaporation and thermal conduction models). For each of the models considered we examine all the implications on the evolution of G272.2-3.2.Comment: 26 pages, 8 figures. Accepted for publication in Ap