Ravi Sankrit

On-Orbit Performance of the Far Ultraviolet Spectroscopic Explorer Satellite

The launch of the Far Ultraviolet Spectroscopic Explorer (FUSE) has been followed by an extensive period of calibration and characterization as part of the preparation for normal satellite operations. Major tasks carried out during this period include the initial coalignment, focusing, and characterization of the four instrument channels and a preliminary measurement of the resolution and throughput performance of the instrument. We describe the results from this test program and present preliminary estimates of the on-orbit performance of the FUSE satellite based on a combination of these data and prelaunch laboratory measurements.

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 (northeast jet and southwest 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.

Early science with SOFIA, the stratospheric observatory for infrared astronomy

The Stratospheric Observatory For Infrared Astronomy (SOFIA) is an airborne observatory consisting of a specially modified Boeing 747SP with a 2.7 m telescope, flying at altitudes as high as 13.7 km (45,000 ft). Designed to observe at wavelengths from 0.3 μm to 1.6 mm, SOFIA operates above 99.8% of the water vapor that obscures much of the infrared and submillimeter. SOFIA has seven science instruments under development, including an occultation photometer, near-, mid-, and far-infrared cameras, infrared spectrometers, and heterodyne receivers. SOFIA, a joint project between NASA and the German Aerospace Center Deutsches Zentrum fur Luft und-Raumfahrt, began initial science flights in 2010 December, and has conducted 30 science flights in the subsequent year. During this early science period three instruments have flown: the mid-infrared camera FORCAST, the heterodyne spectrometer GREAT, and the occultation photometer HIPO. This Letter provides an overview of the observatory and its early performance.

Hubble Space Telescope Imaging of the Primary Shock Front in the Cygnus Loop Supernova Remnant

We present Hubble Space Telescope WFPC2 narrowband Hα (F656N) and [O III] λ5007 (F502N) imaging of two fields on the northeastern limb of the Cygnus Loop supernova remnant. This region provides an outstanding example of the initial encounter between the primary blast wave and the surrounding interstellar medium. The Hα images show the primary nonradiative shock front, which, when viewed edge-on, is unresolved at WFPC2 resolution. The [O III] images show portions of filaments that are beginning to become radiative, and in these images the filaments are resolved, appearing fuzzy at WFPC2 resolution. The [O III] filament regions are not bounded directly by Hα filaments, indicating that the shock emission from the nascent radiative region is sufficient to fully ionize the local preshock gas. One field, imaged 4 yr earlier in Hα with WFPC2, is used to study the proper motion of the filament and constrain any brightness variations over this time period. In conjunction with improved models of nonradiative shocks, these data are used to place limits on the possible deceleration of the filament and refine the distance to the Cygnus Loop, arriving at a revised value of d = 540 pc (assuming no deceleration). The second field imaged contains examples of coherent Hα filaments with much more dramatic curvatures than identified previously. We discuss the possible reasons for the different morphology of these filaments and conclude that they can be accommodated with relatively modest variations in local density and shock velocity.

Spitzer Space Telescope observations of Kepler's supernova remnant : A detailed look at the circumstellar dust component

We present 3.6-160 ?m infrared images of Keplers supernova remnant (SN 1604) obtained with the IRAC and MIPS instruments on the Spitzer Space Telescope. We also present MIPS SED low-resolution spectra in the 55-95 ?m region. The observed emission in the MIPS 24 ?m band shows the entire shell. Emission in the MIPS 70 ?m and IRAC 8 ?m bands is seen only from the brightest regions of 24 ?m emission, which also correspond to the regions seen in optical H? images. Shorter wavelength IRAC images are increasingly dominated by stars, although faint filaments are discernible. The SED spectrum of shows a faint continuum dropping off to longer wavelengths and confirms that strong line emission does not dominate the mid-IR spectral region. The emission we see is due primarily to warm dust emission from dust heated by the primary blast wave; no excess infrared emission is observed in regions where supernova ejecta are seen in X-rays. We use models of the dust to interpret the observed 70/24 ?m ratio and constrain the allowed range of temperatures and densities. We estimate the current mass of dust in the warm dust component to be 5.4 ? 10-4 M?, and infer an original mass of about 3 ? 10-3 M? before grain sputtering. The MIPS 160 ?m band shows no emission belonging to the remnant. We place a conservative but temperature-dependent upper limit on any cold dust component roughly a factor of 10 below the cold dust mass inferred from SCUBA observations. Finally, we comment on issues relevant to the possible precursor star and the supernova type.

Dust destruction in type Ia supernova remnants in the large magellanic cloud

We present first results from an extensive survey of Magellanic Cloud supernova remnants (SNRs) with the Spitzer Space Telescope. We describe IRAC and MIPS imaging observations at 3.6, 4.5, 5.8, 8, 24, and 70 μm of four Balmer-dominated Type Ia SNRs in the Large Magellanic Cloud (LMC): DEM L71 (0505-67.9), 0509-67.5, 0519-69.0, and 0548-70.4. None was detected in the four short-wavelength IRAC bands, but all four were clearly imaged at 24 μm, and two at 70 μm. A comparison of these images with Chandra broadband X-ray images shows a clear association with the blast wave, and not with internal X-ray emission associated with ejecta. Our observations are well described by one-dimensional shock models of collisionally heated dust emission, including grain size distributions appropriate for the LMC, grain heating by collisions with both ions and electrons, and sputtering of small grains. Model parameters are constrained by X-ray, optical, and far-ultraviolet observations. Our models can reproduce observed 70/24 μm flux ratios only by including sputtering, destroying most grains smaller than 0.03-0.04 μm in radius. We infer total dust masses swept up by the SNR blast waves, before sputtering, on the order of 10-2 M☉, several times less than those implied by a dust-to-gas mass ratio of 0.3% as often assumed for the LMC. Substantial dust destruction has implications for gas-phase abundances.

DUST DESTRUCTION IN FAST SHOCKS OF CORE-COLLAPSE SUPERNOVA REMNANTS IN THE LARGE MAGELLANIC CLOUD

We report observations with the Multiband Imaging Photometer for Spitzer of four supernova remnants (SNRs) believed to be the result of core-collapse supernovae: N132D (0525–69.6), N49B (0525–66.0), N23 (0506–68.0), and 0453–68.5. All four of these SNRs were detected in whole at 24 mm and in part at 70 mm. Comparisons with Chandra broadband X-ray images show an association of infrared (IR) emission with the blast wave. We attribute the observed IR emission to dust that has been collisionally heated by electrons and ions in the hot, X-ray–emitting plasma, with grain size distributions appropriate for the LMC and the destruction of small grains via sputtering by ions. As with our earlier analysis of Type Ia SNRs, models can reproduce observed 70 mm/ 24 mm flux ratios only if effects from sputtering are included, destroying small grains. We calculate the mass of dust swept up by the blast wave in these remnants, and we derive a dust-to-gas mass ratio of several times less than the often assumed value of 0.25% for the LMC. We believe that one explanation for this discrepancy could be porous (fluffy) dust grains. Subject headings: dust, extinction — Magellanic Clouds — supernova remnants

Observations of O VI Emission from the Diffuse Interstellar Medium

We report the —rst Far-Ultraviolet Spectroscopic Explorer measurements of diUuse O VI (jj1032, 1038) emission from the general diUuse interstellar medium outside of supernova remnants or superbubbles. We observed a 30@@ ] 30@@ region of the sky centered at and From the observed l \ 315i.0 b \[ 41i.3. intensities (2930 ^ 290 [random] ^ 410 [systematic] and 1790 ^ 260 [random] ^ 250 [systematic] photons cm~2 s~1 sr~1 in jj1032 and 1038, respectively), derived equations, and assumptions about the source location, we calculate the intrinsic intensity, electron density, thermal pressure, and emitting depth. The intensities are too large for the emission to originate solely in the Local Bubble. Thus, we conclude that the Galactic thick disk and lower halo also contribute. High-velocity clouds are ruled out because there are none near the pointing direction. The calculated emitting depth is small, indicating

The preshock gas of sn 1006 from Hubble space telescope advanced camera for surveys observations

We derive the preshock density and scale length along the line of sight for the collisionless shock from a deep HST image that resolves the Hα filament in SN 1006 and updated model calculations. The very deep ACS high-resolution image of the Balmer line filament in the northwest quadrant shows that 0.25 cm-3 ≤ n0 ≤ 0.4 cm-3 and that the scale along the line of sight is about 2 × 1018 cm, while bright features within the filament correspond to ripples with radii of curvature less than 1/10 that size. The derived densities are within the broad range of earlier density estimates, and they agree well with the ionization timescale derived from the Chandra X-ray spectrum of a region just behind the optical filament. This provides a test for widely used models of the X-ray emission from SNR shocks. The scale and amplitude of the ripples are consistent with expectations for a shock propagating through interstellar gas with ~20% density fluctuations on parsec scales as expected from studies of interstellar turbulence. One bulge in the filament corresponds to a knot of ejecta overtaking the blast wave, however. The interaction results from the rapid deceleration of the blast wave as it encounters an interstellar cloud.We derive the pre-shock density and scale length along the line of sight for the collisionless shock from a deep HST image that resolves the H alpha filament in SN1006 and updated model calculations. The very deep ACS high-resolution image of the Balmer line filament in the northwest (NW) quadrant shows that 0.25<n_0<le

Ejecta, Dust, and Synchrotron Radiation in SNR B0540–69.3: A More Crab-Like Remnant than the Crab

0.4 cm-3 and that the scale along the line of sight is about 2 x 10^{18} cm, while bright features within the filament correspond to ripples with radii of curvature less than 1/10 that size. The derived densities are within the broad range of earlier density estimates, and they agree well with the ionization time scale derived from the Chandra X-ray spectrum of a region just behind the optical filament. This provides a test for widely used models of the X-ray emission from SNR shocks. The scale and amplitude of the ripples are consistent with expectations for a shock propagating though interstellar gas with ~ 20% density fluctuations on parsec scales as expected from studies of interstellar turbulence. One bulge in the filament corresponds to a knot of ejecta overtaking the blast wave, however. The interaction results from the rapid deceleration of the blast wave as it encounters an interstellar cloud.