Low-frequency radio absorption in Cassiopeia A

Abstract

Context. Cassiopeia A is one of the best-studied supernova remnants. Its bright radio and X-ray emission is due to shocked ejecta. Cas A is rather unique in that the unshocked ejecta can also be studied: through emission in the infrared, the radio-active decay of 44 Ti, and the low-frequency free-free absorption caused by cold ionised gas, which is the topic of this paper. Aims. Free-free absorption processes are affected by the mass, geometry, temperature, and ionisation conditions in the absorbing gas. Observations at the lowest radio frequencies can constrain a combination of these properties. Methods. We used Low Frequency Array (LOFAR) Low Band Antenna observations at 30-77 MHz and Very Large Array (VLA) L-band observations at 1-2 GHz to fit for internal absorption as parametrised by the emission measure. We simultaneously fit multiple UV-matched images with a common resolution of 17 (this corresponds to 0.25 pc for a source at the distance of Cas A). The ample frequency coverage allows us separate the relative contributions from the absorbing gas, the unabsorbed front of the shell, and the absorbed back of the shell to the emission spectrum. We explored the effects that a temperature lower than the ∼100-500 K proposed from infrared observations and a high degree of clumping can have on the derived physical properties of the unshocked material, such as its mass and density. We also compiled integrated radio flux density measurements, fit for the absorption processes that occur in the radio band, and considered their effect on the secular decline of the source. Results. We find a mass in the unshocked ejecta of M = 2.95 ± 0.48 M for an assumed gas temperature of T = 100 K. This estimate is reduced for colder gas temperatures and, most significantly, if the ejecta are clumped. We measure the reverse shock to have a radius of 114 ±6 and be centred at 23:23:26, +58:48:54 (J2000). We also find that a decrease in the amount of mass in the unshocked ejecta (as more and more material meets the reverse shock and heats up) cannot account for the observed low-frequency behaviour of the secular decline rate. Conclusions. To reconcile our low-frequency absorption measurements with models that reproduce much of the observed behaviour in Cas A and predict little mass in the unshocked ejecta, the ejecta need to be very clumped or the temperature in the cold gas needs to be low (∼ 10 K). Both of these options are plausible and can together contribute to the high absorption value that we find.Article number, page 16 of 16

The mass, location, and heating of the dust in the Cassiopeia A supernova remnant

“…The SNR also contains ejecta which has not yet encountered the reverse shock, and is consequently much cooler. Smith et al (2009) estimated a maximum electron density of ne 100 cm −3 for the unshocked ejecta based on forbidden line ratios, while observations of radio absorption by DeLaney et al (2014) and Arias et al (2018) give ne ∼ 10 cm −3 and T ∼ 100 K. Raymond et al (2018) inferred a preshock temperature of ∼ 100 K from [Si I] IR emission lines. Krause et al (2008) determined that the Cas A SN was of type IIb from a spectrum of its light echo, meaning that the progenitor star must have lost most of its hydrogen envelope pre-explosion.…”

Low-frequency Radio Absorption in Tycho’s Supernova Remnant

“…The amount of mass in ionised material internal to the SNR reverse shock is given by (see Arias et al 2018):…”

Cassiopeia A, Cygnus A, Taurus A, and Virgo A at ultra-low radio frequencies

“…Cassiopeia A: LOFAR LBA data for Cassiopeia A, a ∼ 330 yr old supernova remnant, has been analysed recently by Arias et al (2018). However, we note that the data and reduction methods presented in this work are new.…”

“…However, we note that the data and reduction methods presented in this work are new. The most striking feature of Cassiopeia A at low radio frequencies is the effect of internal free-free absorption from cold (∼ 100 K; Arias et al 2018;Oonk et al 2017), unshocked supernova ejecta material. As a result, the central region of Cassiopeia A (interior to the bright shell) is less bright than in the gigahertz band.…”

“…The overall flux density within a beam centred on Cassiopeia A is always affected by free-free absorption by relatively cool free electrons between us and the source, as well as internal absorption in the central region. As a consequence, for a given beam, the flux density can be described as (Arias et al 2018)…”