Supernova remnants

Supernova remnants are produced by the violent explosion of a star at the end of its lifetime. If the star is particularly massive, then its core will collapse and release a huge amount of energy. This will cause a blast wave that ejects the star's envelope into interstellar space. The result of the collapse may be, in some cases, a compact object such as a rapidly rotating neutron star that can be observed many years later as a radio pulsar, or a black hole.

Since 1992 the group have carried out multifrequency investigations of new SNRs and their possible relations with high-energy sources. The major problem in the effective identification of weak and extended sources at radio frequencies is the contamination of the radio images produced by the background galactic radiation. However, this diffuse component can be appropriately subtracted by means of a filtering technique. Continuum radio observations can be used to detect those candidates that are expected to accelerate electrons (and positrons) to high energies, producing non-thermal emission in the process. Some examples follow.

Left: from Radio Detection of the Supernova Remnant RX J0852.0-4622 by Combi, Romero, Benaglia in Ap. J. Lett. 519, 77 1999. Right: new SNR and superposed gamma-ray source from An inquiry into the nature of 3EG 1828+01 by Punsly, Romero, Torres and Combi [astro-ph/0007465], to appear in Astronomy and Astrophysics.

Several candidates for SNRs have been detected in the southern sky at 408 MHz, 1.42 GHz and 2.4 GHz. In particular, we have presented observational evidence supporting a picture where the gamma-ray source 3EG J1659-6251 is the result of the interaction between cosmic rays accelerated in a SNR and a nearby HI cloud. With the aim to obtain additional information about the structure of the ISM in the line of sight to the unidentified gamma-ray sources 3EG J1834-2138, 3EG J0724-4713 and 3EG J0725-5140, we have carried out radio continuum and HI-line observations and gathered information about the high-energy processes operating at them. These studies provide an unique insight into various aspects of the interstellar medium (ISM), in particular of the cosmic ray components. 

Some new SNR candidates have been detected at relatively high latitudes in the regions of Upper-Sco, Centaurus and Ara at radio wavelenghts. We have also detected the radio counterpart at 2.4 GHz of the X-Ray source RX J0852-4622, which has been recently proposed as a candidate of young nearby SNR on the basis of its X-ray (> 1.3 KeV) morphology. The radio images match the X-ray morphology very well and show a limb-brightened source with some elongated features protruding from the outer shell. These high resolution observations were also used to examine spectral index characteristics of its emission.

Recently, we have studied the surroundings of the gamma-ray source 3EG J1828+0142 using radio data from large-scale surveys and small-scale VLA observations from the NVSS Sky Survey. An image of the radio field around of this gamma-ray source is shown in the right panel of the Figure shown above. A large, shell-type structure can be clearly seen. It is a weak source with a low surface brightness that very much resembles a typical SNR. The probability location contours of the gamma-ray source are superposed to the 1.42 GHz radio image.

The SNR G347.3-0.5 (Butt, Torres, Romero, Dame & Combi 2002, Nature, 418, p.499)

Cosmic-rays particles are most likely accelerated in strong fronts of supernova remnants by diffusive shock acceleration. Although this conjecture has eluded direct observational confirmation, since it was first proposed in 1953 by Shklovskii, additional evidence can be gathered using multi -frequencies observations. 

We have associated the unidentified EGRET GeV gamma-ray source, 3EG J1714-3857, with a very massive (3*10^5 Mo ) and dense (~500 nucleons cm^-3) molecular cloud interacting with SNR RX J1713.7-3946 (see the Figure below). Since the cloud region is of low radio and X-ray brightnes, we dismiss an electronic origin of the bulk of the GeV emission there. Furthermore, the ambient cloud medium is directly measured to be highly excited indicating that it is indeed being overtaken by the SNR shock front. The ratio of the intensities of first two rotational transitions of the CO molecules, is ~2.4, more than 3.5 s above the average Galactic value. A picture thus emerges where both electrons and nuclei are being accelerated at the SNR blast wave of RX J1713.7-3946. Whereas the relativistic electrons dominate the local non-thermal radio, X-ray and TeVemission, the shock accelerated CR protons and ions (hadrons) are exposed through their interactions in the adjacent massive cloud, leading to the observed GeVemission via the amma-decay of neutral pions. Such a scenario had been anticipated by Aharonian et al. [A&A 285 (1994) 645].

Overlay map in Galactic coordinates showing SNR RX J1713.7-3946 (G347.3,-0.5) in gray (ROSAT PSPC X-ray) contours from Slane et al. (1999). The red depicts the TeV significance contours from Muraishi et al. (2000). In white are the location probability contours (successively, 50%, 68%, 95%, and 99%) of the GeV EGRET source 3EG J1714-3857 from Hartman et al. (1999). The color scale indicates the intensity of CO (j=1->0) emission, and consequently the column density of the ambient molecular cloud, in the LSR velocity interval -105 to -80 km/s associated with the SNR, corresponding to a kinematic distance of 6.3 +/- 0.4 kpc. The elongated CO emission feature near (l,b)~(348.5,+0.2) derives from the large velocity wings of a much more distant (11.3 kpc) and unrelated cloud centered at V=-68 km/s.

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