Supernova Remnant IC Research Articles

Isolated X‐Ray–Infrared Sources in the Region of Interaction of the Supernova Remnant IC 443 with a Molecular Cloud

The nature of the extended hard X-ray source XMMU J061804.3+222732 and its surroundings is investigated using XMM-Newton, Chandra, and Spitzer observations. This source is located in an interaction region of the IC 443 supernova remnant with a neighboring molecular cloud. The X-ray emission consists of a number of bright clumps embedded in an extended structured non-thermal X-ray nebula larger than 30 in size. Some clumps show evidence for line emission at ~1.9 keV and ~3.7 keV at the 99% confidence level. Large-scale diffuse radio emission of IC 443 passes over the source region, with an enhancement near the source. An IR source of about 14 x 7 size is prominent in the 24 um, 70 um, and 2.2 um bands, adjacent to a putative Si K-shell X-ray line emission region. The observed IR/X-ray morphology and spectra are consistent with those expected for J/C-type shocks of different velocities driven by fragmented supernova ejecta colliding with the dense medium of a molecular cloud. The IR emission of the source detected by Spitzer can be attributed to both continuum emission from an HII region created by the ejecta fragment and line emission excited by shocks. This source region in IC 443 may be an example of a rather numerous population of hard X-ray/IR sources created by supernova explosions in the dense environment of star-forming regions. Alternative Galactic and extragalactic interpretations of the observed source are also discussed.

Nonthermal Emission from a Radio-bright Shell-Type Supernova Remnant IC 443

By assuming diffusive shock acceleration of the nonthermal particles in the shell, we model a time-dependent nonthermal particle and photon spectra for the radio-bright shell-type supernova remnant (SNR) IC 443 with radio, optical, and X-ray emission concentrated toward the rim, whereas the γ-rays detected by EGRET are located at the center of the SNR, and the very high energy (VHE) γ-rays detected by MAGIC are displaced to the south, in direct correlation with a molecular cloud (MC). In this model, the nonthermal photon emission consists of two components, one comes from the shell evolving in the interstellar medium (ISM) and the other from the shell interacting with an MC. Our results indicate that (1) the emission from radio to soft X-ray bands is dominated by synchrotron radiation in the shell evolving in the ISM and (2) the detected high-energy emission (>10 MeV) from the SNR is from the shell evolving in the MC; i.e., the high-energy photons with energies from 10 MeV to 0.1 TeV are dominated both by bremsstrahlung and by p-p interaction, and the VHE γ-rays are produced mainly via p-p interaction.

A 21 cm SPECTRAL AND CONTINUUM STUDY OF IC 443 USING THE VERY LARGE ARRAY AND THE ARECIBO TELESCOPE

We report 21 cm spectral-line and continuum observations of the Galactic supernova remnant IC 443 using the Very Large Array (VLA) and the Arecibo telescope. By combining the VLA and Arecibo data, both covering the full extent of IC 443, we have achieved an unprecedented combination of sensitivity and angular resolution, over the continuous range of angular scales from ~40'' to ~1°. Our new radio observations not only reveal previously unknown features of IC 443 but also show the details of the remnant more clearly. The radio morphology of IC 443 consists of two nearly concentric shells. Our 21 cm radio continuum data show that the two shells have distinctly different radial intensity distributions. This morphology supports the scenario whereby the western shell is a breakout portion of the remnant into a rarefied medium. We have developed a dynamical model accounting for the breakout, which provides an estimate for the remnant age of ~2 × 104 yr. The southeastern boundary of the remnant shows interesting features, seen in our observations for the first time: a faint radio continuum halo and numerous spurs. These features are mainly found in the region where IC 443 overlaps with another remnant, G189.6+3.3. These features most likely originate from the interactions of IC 443 with the surrounding medium. The H I emission associated with IC 443 appears over the velocity range between −100 km s−1 and 50 km s−1. The strongest absorption is seen around v LSR ~ −5 km s−1, which corresponds to the systemic velocity of IC 443. We identify a broad, extended lane of H I gas near the systemic velocity as preshock gas in the southern part of the remnant. Most of the shocked H I gas is located along the southern supernova remnant (SNR) boundary and is blueshifted. We derive an accurate mass of the shocked H I gas using template HCO+ (1-0) spectra, which is 493 ± 56 M☉. Our high-resolution H I data enable us to resolve the shocked H I in the northeastern region into a few filamentary structures, which are well correlated with radiative filaments, in both position and velocity. This represents the first unambiguous detection of the recombined H I in an atomic shock. The characteristics of the filaments are not consistent with a clumpy shock model where the radiative signature is from dense clumps. Instead we propose that the shock has been propagating through a uniform medium of nH ~ 10 cm−3.

AKARI Detection of the Infrared-Bright Supernova Remnant B0104$-$72.3 in the Small Magellanic Cloud

Abstract We present a serendipitous detection of the infrared-bright supernova remnant (SNR) B0104$-$72.3 in the Small Magellanic Cloud by the Infrared Camera (IRC) onboard AKARI. An elongated, partially complete shell has been detected in all four observed IRC bands, covering 2.6–15 $\mu$m. The infrared shell surrounds radio, optical, and X-ray emission associated with the SNR, and is probably a radiative SNR shell. This is the first detection of an SNR shell in this near/mid-infrared waveband in the Small Magellanic Cloud. The IRC color indicates that the infrared emission might be from shocked H$_2$ molecules with some possible contributions from ionic lines. We conclude that B0104$-$72.3 is a middle-aged SNR interacting with molecular clouds, similar to the Galactic SNR IC 443. Our results highlight the potential of AKARI IRC observations in studying SNRs, especially for diagnosing SNR shocks.

SpitzerSpectral Line Mapping of Supernova Remnants. I. Basic Data and Principal Component Analysis

We report the results of spectroscopic mapping observations carried out toward small (1' × 1') regions within the supernova remnants W44, W28, IC 443, and 3C 391 using the Infrared Spectrograph (IRS) of the Spitzer Space Telescope. These observations, covering the 5.2-37 μm spectral region, have led to the detection of a total of 15 fine-structure transitions of Ne+, Ne++, Si+, P+, S, S++, Cl+, Fe+, and Fe++; the S(0)-S(7) pure rotational lines of molecular hydrogen; and the R(3) and R(4) transitions of hydrogen deuteride. In addition to these 25 spectral lines, the 6.2, 7.7, 8.6, 11.3, and 12.6 μm PAH emission bands were also observed. Most of the detected line transitions have proven strong enough to map in several sources, providing a comprehensive picture of the relative distribution of the various line emissions observable in the Spitzer IRS bandpass. A principal component analysis of the spectral-line maps reveals that the observed emission lines fall into five distinct groups, each of which may exhibit a distinct spatial distribution: (1) lines of S and H2(J > 2); (2) the H2 S(0) line; (3) lines of ions with appearance potentials less than 13.6 eV; (4) lines of ions with appearance potentials greater than 13.6 eV, not including S++; (5) lines of S++. Lines of group 1 likely originate in molecular material subject to a slow, nondissociative shock that is driven by the overpressure within the supernova remnant, and lines in groups 3-5 are associated primarily with dissociative shock fronts with a range of (larger) shock velocities. The H2 S(0) line shows a low-density diffuse emission component and, in some sources, a shock-excited component.

Kinematics of the Molecular Hydrogen Associated with the Supernova Remnant IC 443

We present high angular resolution (0.86'') observations of the kinematics of the molecular hydrogen in one of the zones of interaction of a molecular cloud with the Galactic supernova remnant IC 443. This has been done using a scanning Fabry-Pérot interferometer at the near-IR line at 2.12 μm of molecular hydrogen. The kinematics derived from the Fabry-Pérot velocity cubes thus obtained allows us to have a better insight into the type of motions (and thereby shocks) of molecular gas at smaller scales in this zone of interaction, taking into account the complex morphology and kinematics of the knots, diffuse gas, and filaments that comprise the molecular clumps. A combination of J-type and C-type shocks seems to explain the kinematics, in agreement with the conclusions of Snell et al. based on the large amount of data gathered on the line ratios and abundances of several neutral and molecular species.

The X‐Ray Structure of the Pulsar Bow Shock G189.22+2.90 in the Supernova Remnant IC 443

Wepresentadeepobservation withtheChandraX-Ray Observatory of theneutron starbowshockG189.22+2.90 in the supernova remnant (SNR) IC 443. Our data confirm the cometary morphology and central point source seen previously, but also reveal considerable new structure. Specifically, we find that the X-ray nebula consists of two distinct components:a‘‘tongue’’of bright emissionclose to theneutron star,enveloped byalarger,fainter ‘‘tail.’’ We interpret the tongue and tail as delineating the termination shock and the postshock flow, respectively, as previously identified also in the pulsar bow shock G359.23� 0.82 (‘‘the Mouse’’). However, for G189.22+2.90 the tongue is much less elongated than for the Mouse, while the tail is much broader. These differences are consistent with the low Mach number,M P2, expected for a neutron star moving through the hot gas in a SNR’s interior, supporting the case for a physical association between G189.22+2.90 and IC 443. We resolve the standoff distance between the star and the head of the bow shock, which allows us to estimate a space velocity for the neutron star of � 230 km s � 1 , independent of distance. We detect thermal emission from the neutron star surface at a temperature of 102 � 22 eV, which is consistent with the age of SNR IC 443 for standard neutron star cooling models. We also identify two compact knots of hard emission located 1 00 Y2 00 north and south of the neutron star. Subject headingg ISM: individual (G189.22+2.90, IC 443) — pulsars: individual (CXOU J061705.3+222127) — stars: neutron — stars: winds, outflows

Green Bank Telescope Observations of IC 443: The Nature of OH (1720 MHz) Masers and OH Absorption

We present spectral line observations of the ground-state transitions of hydroxyl (OH) toward supernova remnant (SNR) IC 443 carried out with the Green Bank Telescope. Weak, extended OH (1720 MHz) maser emission with OH (1667, 1665, 1612 MHz) absorption is detected along the southern extent of the remnant where no bright compact maser sources have been previously observed. These newly detected SNR-type masers are coincident with well-known molecular clumps and a ridge of shocked H2 emission indicative of the SNR shock front interacting with the adjacent molecular cloud. Simultaneous observation of all four ground-state transitions of OH permits us to fit physical conditions of the shocked gas at the interaction site. A simple two-component model for the line profiles yields the physical parameters for detected regions of maser emission, including excitation temperature, OH column density, and filling factor. Observed line profiles suggest the shock is largely propagating toward the line of sight in the region of these newly identified weak masers. The implications of shock geometry and physical parameters in producing spatially extended OH maser emission in SNRs are explored. We also present VLA radio continuum observations at 330 MHz for comparison with OH line observations of the remnant.

S p i t zer Observations of Hydrogen Deuteride

We report the detection of interstellar hydrogen deuteride (HD) toward the supernova remnant IC 443, and the tentative detection of HD toward the Herbig-Haro objects HH 54 and HH 7 and the star-forming region GGD 37 (Cep A West). Our detections are based upon spectral line mapping observations of the R(3) and R(4) rotational lines of HD, at rest wavelengths of 28.502 and 23.034 μm, respectively, obtained using the Infrared Spectrograph on board the Spitzer Space Telescope. The HD R(4)/R(3) line intensity ratio promises to be a valuable probe of the gas pressure in regions where it can be observed. The derived HD/H2 abundance ratios are (1.19) × 10-5, (1.80) × 10-5, and (1.41) × 10-5, respectively (68.3% confidence limits, based upon statistical errors alone) for IC 443 (clump C), HH 54, and HH 7. If HD is the only significant reservoir of gas-phase deuterium in these sources, the inferred HD/H2 ratios are all consistent with a gas-phase elemental abundance [nD/nH]gas ~ 7.5 × 10-6, a factor of 2-3 below the values obtained previously from observations of atomic deuterium in the Local Bubble and the Galactic halo. However, similarly low gas-phase deuterium abundances have been inferred previously for molecular gas clouds in the Orion region and in atomic clouds along sight lines within the Galactic disk to stars more distant than 500 pc from the Sun.

A Hard Extended X-Ray Source in the IC 443 Supernova Remnant Resolved by Chandra : A Fast Ejecta Fragment or a New Pulsar Wind Nebula?

A Chandra observation of the isolated hard X-ray source XMMU J061804.3+222732, located in the region of the apparent interaction of the supernova remnant IC 443 with a molecular cloud, resolved the complex structure of the source in a few bright clumps embedded in an extended emission of ~30'' in size. The X-ray spectra of the clumps and the extended emission are dominated by a hard power-law component with a photon index of 1.2-1.4. In addition, we see some indications of an optically thin thermal plasma of an ~0.3 keV temperature. The observed X-ray morphology and spectra are consistent with those expected for an isolated supernova ejecta fragment interacting with a dense ambient medium. A possible alternative interpretation is a pulsar wind nebula associated with either IC 443 or another supernova remnant, G189.6+3.3.

Detection of Water in the Shocked Gas Associated with IC 443: Constraints on Shock Models

We have used the Submillimeter Wave Astronomy Satellite (SWAS) to observe the ground-state 110 → 101 transition of ortho-H2O at 557 GHz in three of the shocked molecular clumps associated with the supernova remnant IC 443. We also observed simultaneously the 487 GHz line (3,1 → 3,2) of O2, the 492 GHz line (3P1 → 3P0) of C I, and the 550 GHz line (J = 5 → 4) of 13CO. We detected the H2O, C I, and 13CO lines toward the shocked clumps B, C, and G. In addition, ground-based observations of the J = 1 → 0 transitions of CO and HCO+ were obtained. Assuming that the shocked gas has a temperature of 100 K and a density of 5 × 105 cm-3, we derive SWAS beam-averaged ortho-H2O column densities of 3.2 × 1013, 1.8 × 1013, and 3.9 × 1013 cm-2 in clumps B, C, and G, respectively. Combining the SWAS results with our ground-based observations, we derive a relative abundance of ortho-H2O to CO in the postshock gas of between 2 × 10-4 and 3 × 10-3. On the basis of our results for H2O, published results of numerous atomic and molecular shock tracers, and archival Infrared Space Observatory (ISO) observations, we conclude that no single shock type can explain these observations. However, a combination of fast J-type shocks (~100 km s-1) and slow C-type shocks (~12 km s-1) or, more likely, slow J-type shocks (~12-25 km s-1) can most naturally explain the postshock velocities and the emission seen in various atomic and molecular tracers. Such a superposition of shocks might be expected as the supernova remnant overtakes a clumpy interstellar medium. The fast J-type shocks provide a strong source of ultraviolet radiation, which photodissociates the H2O in the cooling (T ≤ 300 K) gas behind the slow shocks and strongly affects the slow C-type shock structure by enhancing the fractional ionization. At these high ionization fractions, C-type shocks break down at speeds ~10-12 km s-1, while faster flows will produce J-type shocks. Our model favors a preshock gas-phase abundance of oxygen not in CO that is depleted by a least a factor of 2, presumably as water ice on grain surfaces. Both freezeout of H2O and photodissociation of H2O in the postshock gas must be significant to explain the weak H2O emission seen by SWAS and ISO from the shocked and postshock gas.

Observation of non-thermal emission from the supernova remnant IC 443 with RXTE

In this paper we present analysis of X-ray spectra from the supernova remnant IC 443 obtained using the PCA on RXTE. The spectra in the 3–20 keV band are well fit by a two-component model consisting of thermal and non-thermal components. We compare these results with recent results of other X-ray missions and discuss the need for a cut-off in the non-thermal spectrum. Recent Chandra and XMM-Newton observations suggest that much of the non-thermal emission from IC 443 can be attributed to a pulsar wind nebula. We present the results of our search for periodic emission in the RXTE PCA data. We then discuss the origin of the non-thermal component and its possible association with the unidentified EGRET source.

The Sizes of OH (1720 MHz) Supernova Remnant Masers: MERLIN and Very Long Baseline Array Observations of IC 443

MERLIN and Very Long Baseline Array (VLBA) observations of the 1720 MHz maser emission from the OH molecule in the supernova remnant IC 443 are presented. Based on MERLIN data with a resolution of 160 mas, the deconvolved sizes of the maser sources are in the range 90-180 mas (135-270 AU). The 12 mas resolution VLBA images show compact cores with sizes in the range of 15-55 mas. The maser brightness temperatures are (2-34) × 106 K for the MERLIN sources and (5-19) × 108 K for the VLBA cores, in agreement with theory. Unlike the Zeeman Stokes V profiles observed in other OH (1720 MHz) SNR masers, single-handed circular polarization line profiles are observed in IC 443 on all angular scales from 1000 to 10 mas resolution. For one line component, the observed line width is 0.24 ± 0.07 km s-1, compared to an estimated Doppler width of 0.49 km s-1. This discrepancy in line widths can be accounted for if the maser emission arises from an elongated ellipsoidal region of masing gas.

ASCAObservations 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$\alpha$ to He-like K$\alpha$ 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.

A Bow Shock Nebula around a Compact X-Ray Source in the Supernova Remnant IC 443

We present Chandra spectra and high-resolution images of the hard X-ray feature in the southern edge of the supernova remnant (SNR) IC 443 that reveal a comet-shaped nebula of hard emission that contains a softer point source at its apex. We also present 20, 6, and 3.5 cm Very Large Array maps that clearly show the cometary nebula. Based on the radio and X-ray morphology and spectrum, and the radio polarization properties, we argue that this object is a synchrotron nebula powered by the compact source that is physically associated with IC 443. The spectrum of the soft point source is adequately but not uniquely fitted by a blackbody model [kT = 0.71 ± 0.08 keV, L = (6.5 ± 0.9) × 1031 ergs s-1]. The morphology of the nebula can be explained by the supersonic motion of the neutron star (VNS ≃ 250 ± 50 km s-1), which causes the relativistic wind of the pulsar to terminate in a bow shock and trail behind as a synchrotron tail. This velocity is consistent with an age of 30,000 yr for the SNR and its associated neutron star.

Near‐Infrared Imaging and [Oi] Spectroscopy of IC 443 using Two Micron All Sky Survey andInfrared Space Observatory

We present near-infrared J (1.25 mum), H (1.65 mum), and K-s (2.17 mum) imaging of the entire supernova remnant IC 443 from the Two Micron All Sky Survey (2MASS), and Infrared Space Observatory (ISO) LWS observations of [O I] for 11 positions in the northeast.

Nonthermal Emission from a Supernova Remnant in a Molecular Cloud

In evolved supernova remnants (SNRs) interacting with molecular clouds, such as IC 443, W44, and 3C391, a highly inhomogeneous structure consisting of a forward shock of moderate Mach number, a cooling layer, a dense radiative shell and an interior region filled with hot tenuous plasma is expected. We present a kinetic model of nonthermal electron injection, acceleration and propagation in that environment and find that these SNRs are efficient electron accelerators and sources of hard X- and gamma-ray emission. The energy spectrum of the nonthermal electrons is shaped by the joint action of first and second order Fermi acceleration in a turbulent plasma with substantial Coulomb losses. Bremsstrahlung, synchrotron, and inverse Compton radiation of the nonthermal electrons produce multiwavelength photon spectra in quantitative agreement with the radio and the hard emission observed by ASCA and EGRET from IC 443. We distinguish interclump shock wave emission from molecular clump shock wave emission accounting for a complex structure of molecular cloud. Spatially resolved X- and gamma- ray spectra from the supernova remnants IC 443, W44, and 3C391 as might be observed with BeppoSAX, Chandra XRO, XMM, INTEGRAL and GLAST would distinguish the contribution of the energetic lepton component to the gamma-rays observed by EGRET.

No Water Masers Associated with Supernova Remnants

We have used the Very Large Array to search for water masers in the three supernova remnants W28, W44, and IC 443. Only one candidate maser was detected, in W44. Follow-up observations using the VLA at 1612 MHz demonstrate that the water maser in W44 is likely associated with a late-type star and not a shock in the supernova remnant. The nondetection of water masers toward shocked regions in these supernova remnants places constraints on the postshock density and is consistent with the density determined by molecular observations and the limits from 1720 MHz OH maser pumping.

Gamma‐Ray Production in Supernova Remnants

The bulk of the cosmic rays up to about 100 TeV are thought to be accelerated by the 1st order Fermi mechanism at supernova shocks, producing a power-law spectrum. Both electrons and protons should be accelerated, but their ratio on acceleration is not well known. Recently, the EGRET instrument on the Compton Gamma Ray Observatory has observed supernova remnants IC 443 and $\gamma$ Cygni at GeV energies. On the assumption that the observed gamma-rays are produced by accelerated particles in the remnants (rather than, for example, from a central compact object) we model the contributions due to pion production, bremsstrahlung, and inverse Compton scattering on the cosmic microwave, diffuse galactic radiation, and locally produced radiation fields. In the case of the same spectral index for both electrons and nuclei, and a cut-off at 80 TeV, we find that a spectral index of accelerated particles close to 2.4, and a ratio of electrons to protons in the range 0.2 to 0.3, gives a good fit to the observed spectra. For lower cut-off energies flatter spectra are possible. We also investigate the case where the electron spectrum is steeper than that of nuclei. We discuss the implications of our results for observations at air shower energies, and for the propagation of cosmic rays.

Polarization Observations of 1720 MHz OH Masers toward the Three Supernova Remnants W28, W44, and IC 443

We present arcsecond resolution observations from the VLA with full Stokes polarimetry of the ground-state satellite line of the hydroxyl molecule (OH) at 1720.53 MHz (2Π3/2, J = 3/2, F = 2 → 1) toward three Galactic supernova remnants: W28, W44, and IC 443. The total number of individual OH (1720 MHz) spots we detect in each of these three remnants is 41, 25, and 6, respectively. The OH (1720 MHz) features appear to lie along the edge of radio continuum emission from the supernova remnants, but they are displaced behind the leading edge of the shock as traced by the synchrotron emission. The brightness temperatures of the OH (1720 MHz) emission features range from 2 × 104 to 108 K, convincingly demonstrating the maser nature of the OH (1720 MHz) features. We argue that the partially resolved angular diameters that we measure for the masers are neither intrinsic sizes nor scattering disks, but result from a blend of several unresolved maser features near the same velocity. Thus, our computed brightness temperatures are lower limits to the true values. The characteristic antisymmetric S profile, indicative of Zeeman splitting in the weak-field case, is identified in the Stokes V spectrum of several of the brighter maser spots. The derived line-of-sight magnetic fields are of order 0.2 mG and are remarkably constant in both direction and magnitude over regions several parsecs apart. These are the first measurements of postshock magnetic fields in supernova remnants and demonstrate the importance of magnetic pressure in these molecular shocks. The velocity dispersion of the maser features is typically less than a few km s-1, and, except in the special case of W28, the mean maser velocity is equal to the systemic velocity of the remnant. We suggest that the maximum amplification of the maser transition will occur when the acceleration produced by the shock is transverse to the line of sight. Additional support for this point comes from the location of the masers in IC 443, and from molecular observations that allow the shock geometry to be determined. All of our observations are consistent with a model in which the OH (1720 MHz) is collisionally excited by H2 molecules in the postshock gas heated by a nondissociative shock. Finally, we end with a discussion of the importance of supernova remnants with OH (1720 MHz) maser emission as promising candidates to conduct high-energy searches for the sites of cosmic-ray acceleration.