Radio emission from shell-type supernova remnants

Abstract

The evolution of the radio emission of shell-type Supernova remnants (SNRs) is modeled within the framework of the simple and commonly used assumptions that the mechanism of diffusive shock acceleration (DSA) is responsible for generating radio emitting electrons and that the magnetic field is the typical interstellar field compressed at the shock. It is considered that electrons are injected into the mechanism in test-particle regime directly from the high energy tail of the downstream Maxwellian distribution function. The model can be applied to most of the observed SNRs because the majority of detected SNRs are shell-types and have a more or less spherical shape and are sources of nonthermal radio emission. It is shown that the model successfully explains the many averaged observational properties of evolved shell-type SNRs. In particular, the radio surface brightness (Σ) evolves with diameter as ∼D −(0.3÷0.5) , while the bounding shock is strong (Mach number is M ≥ 10), followed by steep decrease (steeper than ∼D −4.5 ) for M < 10. Such evolution of the surface brightness with diameter and its strong dependence on the environmental parameters strongly reduce the usefulness of Σ − D relations as a tool for determining the distances to SNRs. The model predicts no radio emission from SNRs in the late radiative stage of evolution and the existence of radio-quiet but relatively active SNRs is possible. Our model easily explains very large-diameter radio sources such as the Galactic Loops and the candidates for Hypernova radio remnants. The model predicts that most of the observed SNRs with Σ 1 GHz < ∼ 10 −20 W m −2 sr −1 Hz −1 are located in a tenuous phase of the ISM. The model also predicts the existence of a population of 150−250 pc SNRs with Σ 1 GHz < ∼ 10 −22 W m −2 sr −1 Hz −1 if the kinetic energy of the explosion is ∼10 51 erg. From the comparison of the model results with the statistics of evolved shell-type SNRs, we were able to estimate the fraction of electrons accelerated from the thermal pool in the range (3 ÷ 11) × 10 −4 . If acceleration takes place directly from the high energy tail of the downstream Maxwellian distribution function, then the corresponding injection momentum is estimated as p inj ∼ (2.7−3) · p th .

The supergiant shell with triggered star formation in the dwarf irregular galaxy IC 2574: neutral and ionized gas kinematics

“…At least, with the ambient density of nHI ≃ 0.3 cm −3 the ring nebula fits well the empirical evolutionary sequence of SNRs on the log(vexp) versus log(Rs(nHI) 1/3 ) plot suggested by Lozinskaya (1980a,b). The expected radio surface brightness of an SNR 210 pc in diameter is about Σ1GHz = 10 −22 − 10 −21 Wm −2 Hz −1 sr −1 (see the Σ−D relation in Lozinskaya 1981; Berezhko & Volk 2004;Asvarov 2006). This surface brightness is less than the limiting brightness at λ = 6 cm in the observations by Walter et al (1998), which were the basis for the identification of the SNR in Region # 11.…”

Radio lobes and X-ray hotspots in the microquasar S26

“…The system was originally classified as a supernova remnant (Blair & Long 1997). In that scenario, Asvarov (2006) showed that an input energy ≈5 × 10 52 erg was required to explain its size and radio luminosity, well beyond the energy that can be supplied by an individual supernova. Based on the clear radio and X‐ray evidence for a collimated jet pair (lobes, hotspots), PSM10 showed that such a large amount of energy has been supplied by the BH over the lifetime of the bubble (characteristic age ≈2 × 10 5 yr).…”

The Radio Spectra of the Compact Sources in Arp 220: A Mixed Population of Supernovae and Supernova Remnants

“…The SNR models of Berezhko & Völk (2004) predict that in the early Sedov phase, spectral breaks that lie well above the radio-regime Adiabatic expansion caused by an SNR evolving into a low-density region (perhaps escaping its parent molecular cloud) could shift the break to lower frequencies, but only at the cost of dramatically dimming the source, which is clearly inconsistent with the fact that these sources are among the brightest observed at 18 cm. Asvarov (2006) note that it is difficult under the standard theory of diffusive shock acceleration in SNRs to get an injected electron energy distribution giving spectral indices < À0:6, although a suggested possibility is that such spectra could arise if a SN exploded into a preexisting cavity with increasing density versus radius.…”