Gloria M. Dubner

Geometry of the non-thermal emission in SN 1006 - Azimuthal variations of cosmic-ray acceleration

SN 1006 is the prototype of shell supernova remnants, in which non-thermal synchrotron emission dominates the X-ray spectrum. The non-thermal emission is due to the cosmic-ray electrons accelerated behind the blast wave. The X-ray synchrotron emission is due to the highest energy electrons, and is thus a tracer of the maximum energy electrons may reach behind a shock. We have put together all XMM-Newton observations to build a full map of SN 1006. The very low brightness above 2 keV in the interior indicates that the bright non-thermal limbs are polar caps rather than an equator. This implies that the ambient magnetic field runs southwest to northeast, along the Galactic plane. We used a combined VLA/Parkes radio map to anchor the spectrum at low energy, and model the spectra with synchrotron emission from a cut-off power-law electron distribution, plus a thermal component. We present radial and azimuthal profiles of the cut-off frequency. The cut-off frequency decreases steeply with radius towards the center and with position angle away from the maximum emission. The maximum energy reached by accelerated particles, as well as their number, must be higher at the bright limbs than elsewhere. This implies interesting constraints for acceleration at perpendicular shocks. Overall the XMM-Newton data is consistent with the model in which the magnetic field is amplified where acceleration is efficient.

Thermal emission, shock modification, and X-ray emitting ejecta in SN 1006

Context. Efficient particle acceleration can modify the structure of supernova remnants. We present the results of a combined analysis of the XMM-Newton EPIC archive observations of SN 1006. Aims. We attempt to describe the spatial distribution of the physical and chemical properties of the X-ray emitting plasma at the shock front. We investigate the contribution of thermal and non-thermal emission to the X-ray spectrum at the rim of the remnant to study how the acceleration processes affect the X-ray emitting plasma. Methods. We perform a spatially resolved spectral analysis of a set of regions covering the entire rim of the shell and we apply our results in producing a count-rate image of the “pure” thermal emission of SN 1006 in the 0.5−0.8 keV energy band (subtracting the non-thermal contribution). This image differs significantly from the total image in the same band, especially close to the bright limbs. Results. We find that thermal X-ray emission can be associated with the ejecta and study the azimuthal variation in the physical and chemical properties of the ejecta by identifying anisotropies in the temperature and chemical composition. By employing our thermal image, we trace the position of the contact discontinuity over the entire shell and compare it with that expected from 3D MHD models of SNRs with an unmodified shock. Conclusions. We conclude that the shock is modified everywhere in the rim and that the aspect angle between the interstellar magnetic field and the line of sight is significantly lower than 90 ◦ .

Aspect angle for interstellar magnetic field in SN 1006

A number of important processes taking place around strong shocks in supernova remnants (SNRs) depend on the shock obliquity. The measured synchrotron flux is a function of the aspect angle between interstellar magnetic field (ISMF) and the line of sight. Thus, a model of non-thermal emission from SNRs should account for the orientation of the ambient magnetic field. We develop a new method for the estimation of the aspect angle, based on the comparison between observed and synthesized radio maps of SNRs, making different assumptions about the dependence of electron injection efficiency on the shock obliquity. The method uses the azimuthal profile of radio surface brightness as a probe for orientation of ambient magnetic field because it is almost insensitive to the downstream distribution of magnetic field and emitting electrons. We apply our method to a new radio image of SN 1006 produced on the basis of archival Very Large Array and Parkes data. The image recovers emission from all spatial structures with angular scales from a few arcsec to 15 arcmin. We explore different models of injection efficiency and find the following best-fitting values for the aspect angle of SN 1006: Φ o = 70° ± 4.2° if the injection is isotropic, Φ o = 64° ± 2.8° for quasi-perpendicular injection (SNR has an equatorial belt in both cases) and Φ o = 11° ± 0.8° for quasi-parallel injection (polar-cap model of SNR). In the last case, SN 1006 is expected to have a centrally peaked morphology contrary to what is observed. Therefore, our analysis provides some indication against the quasi-parallel injection model.

Neutral Hydrogen in the Direction of the Vela Supernova Remnant

?????We have carried out a study of the distribution and kinematics of the neutral hydrogen in the direction of the Vela supernova remnant (SNR). A field of 68 ? 54 centered at l = 2641, b = -16 was surveyed using the Parkes 64 m radio telescope (half-power beamwidth 147 at 21 cm). Nearly 2300 H I profiles were obtained with a grid spacing of 75. The presence of a thin, almost circular H I shell, centered at v = 1.6 ? 0.8 km s-1, is revealed. This shell delineates the outer border of the X-ray emission as shown in the ROSAT observations of Aschenbach, Egger, & Tr?mper and wraps around the receding part of the remnant. In addition, two higher velocity features possibly associated with Vela are observed at about -30 and 30 km s-1. These features are interpreted as gas accelerated by the expansion of the supernova shock. The low systemic velocity observed suggests a distance shorter than 500 pc for the Vela SNR. The H I shell is ~7? in diameter and expands at v ~ 30 km s-1. By assuming a distance of 350 pc, we calculate for this shell a linear radius of 22 pc, a swept-up mass of ~1200?2300 M?, and an atomic preshock density of ~1?2 cm-3. The kinetic energy transferred by the supernova shock into the interstellar medium is ~(1?2) ? 1049 ergs, while the initial energy of the explosion is estimated to be ~(1?2.5) ? 1051 ergs. We present the distribution of the column density of the neutral material absorbing the X-radiation, an essential parameter in the analysis of X-ray data. A comparison between the H I and H? emission suggests that the H I shell contains embedded dust that might be responsible for increased optical absorption in this region. On the other hand, the brightest arc-shaped optical filaments associated with the western side of Vela show good correspondence with the H I features. From a comparison between the H I and Molonglo Observatory Synthesis Telescope 843 MHz radio continuum emission, we find that the outermost arched radio filaments correlate well with the main ridge of the H I shell. No strong inhomogeneities were found in the ambient H I medium in the direction of Vela X (the central nebula, powered by the pulsar PSR B0833-45).

The supernova remnant W44: Confirmations and challenges for cosmic-ray acceleration

The middle-aged supernova remnant (SNR) W44 has recently attracted attention because of its relevance regarding the origin of Galactic cosmic-rays. For the first time for a SNR, the gamma-ray missions AGILE and Fermi have established the spectral continuum below 200 MeV, which can be attributed to a neutral pion emission. Confirming the hadronic origin of the gamma-ray emission near 100 MeV is then of the greatest importance. Our paper is focused on a global re-assessment of all available data and models of particle acceleration in W44 with the goal of determining the hadronic and leptonic contributions to the overall spectrum on a firm ground. We also present new gamma-ray and CO NANTEN2 data on W44 and compare them to recently published AGILE and Fermi data. Our analysis strengthens previous studies and observations of the W44 complex environment and provides new information for more detailed modeling. In particular, we determine that the average gas density of the regions emitting 100 MeV‐10 GeV gammarays is relatively high (n 250‐300 cm 3 ). The hadronic interpretation of the gamma-ray spectrum of W44 is viable and supported by strong evidence. It implies a relatively large value for the average magnetic field (B 10 2 G) in the SNR surroundings,which is a sign of field amplification by shock-driven turbulence. Our new analysis establishes that the spectral index of the proton energy distribution function is p1 = 2:2 0:1 at low energies and p2 = 3:2 0:1 at high energies. We critically discuss hadronic versus leptonic-only models of emission taking radio and gamma-ray data into account simultaneously. We find that the leptonic models are disfavored by the combination of radio and gamma-ray data. Having determined the hadronic nature of the gamma-ray emission on firm ground, a number of theoretical challenges remain to be addressed.

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 regi on 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 ′′ × 7 ′′ size is prominent in the 24 � m, 70 � m, and 2.2 � m 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 drive n 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 a lso discussed. Subject headings:ISM: individual (IC 443) — supernova remnants — X-rays: ISM

A new young galactic supernova remnant containing a compact object : G15.9+0.2

We identify the radio-emitting shell-type supernova remnant G15.9+0.2 as a relatively young remnant containing an X-ray point source that may be its associated neutron star. The integrated spectrum of the remnant shell obtained from our 30 ks exploratory Chandra observation shows very strong lines that require elevated element abundances from ejecta, in particular of sulfur. A plane-shock model fit gives a temperature kT = 0.9 (0.8, 1.0) keV, an ionization timescale net = 6 (4, 9) × 1010 cm-3 s, and a sulfur abundance of 2.1 (1.7, 2.7) times solar (90% confidence limits). Two-component models with one solar and one enriched component are also plausible, but they are not well constrained by the data. Various estimates give a remnant age of order 103 yr, which would make G15.9+0.2 among the dozen or so youngest remnants in the Galaxy. The sparse point-source spectrum is consistent with either a steep Γ ~ 4 power law or a kT ~ 0.4 keV blackbody. The spectrum is absorbed by a H column density NH ~ 4 × 1022 cm-2 similar to that required for the remnant shell. The implied 2-9.5 keV source luminosity is about 1033 ergs s-1 for an assumed distance of 8.5 kpc consistent with the high absorption column. We suggest that the point source is either a rotation-powered pulsar or a compact central object.

Study of the γ-ray source 1AGL J2022+4032 in the Cygnus region

Context. Identification of -ray-emitting Galactic sources is a long-standing problem in astrophysics. One such source, 1AGL J2022+4032, coincident with the interior of the radio shell of the supernova remnant Gamma Cygni (SNR G78.2+2.1) in the Cygnus Region, has recently been identified by Fermi as a -ray pulsar, LAT PSR J2021+4026. Aims. We present long-term observations of 1AGL J2022+4032 with the AGILE -ray telescope, measuring its flux and light curve. Methods. We compare the light curve of 1AGL J2022+4032 with that of 1AGL J2021+3652 (PSR J2021+3651), showing that the flux variability of 1AGL J2022+4032 appears to be greater than the level predicted from statistical and systematic e ects and producing detailed simulations to estimate the probability of the apparent observed variability. Results. We evaluate the possibility that the -ray emission may be due to the superposition of two or more point sources, some of which may be variable, considering a number of possible counterparts. Conclusions. We consider the possibility of a nearby X-ray quiet microquasar contributing to the flux of 1AGL J2022+4032 to be more likely than the hypotheses of a background blazar or intrinsic -ray variabilty of LAT PSR J2021+4026.

High resolution radio study of the pulsar wind nebula within the supernova remnant G0.9+0.1

Aims. We have conducted a study in radio wavelengths and in X-rays of the pulsar wind nebula (PWN) in the supernova remnant (SNR) G0.9+0.1 with the goal of investigating in detail its morphology and to accurately determine its characteristic parameters. Methods. To carry out this research we have observed the PWN at λ3.6 and 6 cm using the Australia Telescope Compact Array (ATCA) and combined these data with existing multiconfiguration VLA data and single dish observations in order to recover information at all spatial scales. We have also reprocessed VLA archival data at λ20 cm. From all these observational data we have produced high-fidelity images at the three radio frequencies with angular resolution better than 3 �� . The radio data were compared to X-ray data obtained with Chandra and in two different observing runs with XMM-Newton. Results. The new observations revealed that the morphology and symmetry suggested by Chandra observations (torus and jet-like features) are basically preserved in the radio range in spite of the rich structure observed in the radio emission of this PWN, including several arcs, bright knots, extensions and filaments. The reprocessed X-ray images show for the first time that the X-ray plasma fills almost the same volume as the radio PWN. Notably the X-ray maximum does not coincide with the radio maximum and the neutron star candidate CXOU J174722.8-280915 lies within a small depression in the radio emission. From the new radio data we have refined the flux density estimates, obtaining S PWN ∼ 1.57 Jy, almost constant between λ3.6 and λ20 cm. For the whole SNR (compact core and shell), a flux density S 20 cm = 11.5 Jy was estimated. Based on the new and the existing λ90 cm flux density estimates, we derived a spectral index αPWN = −0.18 ± 0.04 and αshell = −0.68 ± 0.07. From the combination of the radio data with X-ray data, a spectral break is found near ν ∼ 2.4 × 10 12 Hz. The total radio PWN luminosity is Lradio = 1.2 × 10 35 erg s −1 when a distance of 8.5 kpc is adopted. By assuming equipartition between particle and magnetic energies, we estimate a nebular magnetic field B = 56 μG. The associated particle energy turns out to be Upart = 5 × 10 47 erg and the magnetic energy Umag = 2 × 10 47 erg. The high ratio between magnetic and particles flux energy density suggests that the pulsar wind just started to become particle dominated. Based on an empirical relation between X-ray luminosity and pulsar energy loss rate, and the comparison with the calculated total energy, a lower limit of 1100 yr is derived for the age of this PWN.

A molecular outflow revealing star formation activity in the vicinity of the HII region G034.8-0.7 and the SNR W44

Aims. This work aims at investigating the molecular gas component in the vicinity of two young stellar object (YSO) candidates identified at the border of the HII region G034.8-0.7 that is evolving within a molecular cloud shocked by the SNR W44. The purpose is to explore signatures of star forming activity in this complex region. Methods. We performed a near and mid infrared study towards the border of the HII region G034.8-0.7 and observed a 90 �� ×90 �� region near 18 h 56 m 48 s , +01 ◦ 18 � 45 �� (J2000) using the Atacama Submillimeter Telescope Experiment (ASTE) in the 12 CO J = 3−2, 13 CO J = 3−2, HCO + J = 4− 3a nd CSJ = 7−6 lines with an angular resolution of 22 �� . Results. Based on the infrared study we propose that the source 2MASS 18564827+0118471 (IR1 in this work) is a YSO candidate. We discovered a bipolar 12 CO outflow in the direction of the line of sight and a HCO + clump towards IR1, confirming that it is a YSO. From the detection of the CS J = 7−6 line we infer the presence of high density (>10 7 cm −3 )a nd warm (>60 K) gas towards IR1, probably belonging to the protostellar envelope where the YSO is forming. We investigated the possible connection of IR1 with the SNR and the HII region. By comparing the dynamical time of the outflows and the age of the SNR W44, we conclude that the possibility that the SNR triggered the formation of IR1 is unlikely. On the other hand, we suggest that the expansion of the HII region G034.8-0.7 is responsible for the formation of IR1 through the “collect and collapse” process.