Odd Radio Circles as supernovae remnants in the intragroup medium

Some five Odd Radio Circles (ORCs) were recently discovered as a new population of extremely faint (surface brightness a few tens of $\mu$Jy per beam) diffuse radio sources in the celestial sky, all having circular ring-like morphology with nearly identical angular diameters of $\sim 1$ arcmin (Norris et al., 2021a; Norris, Crawford & Macgregor, 2021; Koribalski et al., 2021). These sources were identified either in a deep pilot radio survey named as Evolutionary Map of the Universe (EMU; Norris et al. 2021b), made using the Australian Square Kilometre Array Pathfinder (ASKAP) or in the Giant Meter-wave Radio Telescope (GMRT) archival data. The EMU survey attained surface brightness sensitivity of $\sim 30$ $\mu$Jy per beam rms at an angular resolution of nearly 12 arcsec near 1 GHz (0.944 GHz) frequency (Norris et al., 2021b). Three of these ORCs were found in a search area of 270 deg² of the ASKAP data, one in 40 deg² area around the galaxy NGC 253 in the ASKAP data, and one in the archival observation data at 325 MHz in the cluster field Abell 2142 from the GMRT. The EMU data cover fields around the Galactic latitude of $-40$ degree. These radio objects are named as ORC-1 to ORC-5 with their Galactic $(l,b)$ coordinates at (333.42, -39.0), (339.09, -39.5), (339.08, -39.6), (44.36, 49.4) and (170.78, -86.6) degree respectively.

The morphological appearances of the five ORCs are strikingly similar to a typical Galactic supernovae remnant (SNR). However, Norris et al. (2021a) ruled out these as Galactic SNRs based on the high latitude positions of the ORCs coming at odd with the statistical distribution of the known SNRs as a function of the Galactic latitude. Although, a possibility for ORCs as a previously unknown population of SNRs was not ruled out. All the five ORCs have a similar angular diameter of nearly 1 arcmin in the sky. Optical counterparts have been associated near the centers of ORC-1, ORC-4 and ORC-5 at photometric redshifts of 0.551, 0.457 and 0.270 respectively. ORC-2 and ORC-3 could not be associated with any optical counterparts. The ORC-2 and ORC-3 also form a close pair with an angular separation of nearly $1.5^{\prime}$ in the sky. The flux densities near 1 GHz for four of these ORCs are almost similar at $\sim 7$ mJy. The ORC-3 is the faintest with flux density at $\sim 2$ mJy. The mean radio surface brightness of 4 bright ORCs is nearly $10^{-21}$ W m^-2 Hz^-1 sr^-1 and that of the faintest ORC is nearly $3\times 10^{-22}$ W m^-2 Hz^-1 sr^-1. Their radio spectral indices are negative, indicating that the radio emission is of non-thermal origin.

The astrophysical origins of the ORCs are presently debated with a number of possibilities. Norris et al. (2021a) discussed various possibilities to associate ORCs with different types of known astrophysical sources after excluding a possibility of Galactic SNRs. However, majority of other possibilities are also ruled out on the basis of ORC’s distinct morphological features such as edge-brightening, symmetry and a high degree of circularity, which are not seen in other known similar diffuse radio sources such as lobes of radio galaxies, face-on star-forming galaxies and cluster halos. Moreover, ORCs are not physically associated with large face-on galaxies or rich galaxy clusters. Norris et al. (2021b) and Koribalski et al. (2021) mentioned three promising theoretical possibilities for the origins of some of the ORCs, viz., spherical shock resulting from a merger of two supermassive black-holes, vortex rings in radio lobes viewed end-on in radio galaxies, and termination shock of starburst winds from the galaxy. Presently, any single or all five ORCs can not be firmly associated with a definite type of astrophysical object or phenomena.

In this paper, we discuss a possibility that some ORCs may trace their origins in SNRs in low density intragroup medium (IGrM). We will use single abbreviation IGrM to denote both intragroup medium commonly associated with the local group, and intergalactic medium or IGM commonly used for other groups of galaxies. The motivation for exploring this possibility comes from detections of a subtle population of stars in the IGrM in the form of various known stellar streams in our local group and in other nearby groups of galaxies, diffuse intracluster light in some clusters of galaxies, and detections of a significant number of host-less supernovae.

Based on a similar appearance as those of the Galactic SNRs, the ORCs are very likely dynamically evolving objects, created via a catastrophic event in some astrophysical objects. As the interest here is to explore a possibility of some ORCs being SNR, and considering that the ORCs do not appear to be residing in the Milky-Way or in any other large galaxy, a discussion of evolution of SNRs in low-density hot medium, typical in IGrM environments will be the most relevant here.

The radio brightness and morphology of a SNR depends on various factor such as age, type (I or II) based on the properties of the progenitor star, and density and temperature of the ambient medium in which it is evolving. Early numerical studies related to surface brightness and size evolution of SNRs as a function of ambient temperature and gas density were carried out by Tomisaka, Habe & Ikeuchi (1980) and Higdon & Lingenfelter (1980). In their minimalistic theoretical framework, the sizes of SNRs are expected to become larger and surface brightness to become fainter in low-density warm medium (e.g., $n\sim 10^{-4}$ cm^-3, $T\sim 10^{5}$ K), compared to their counterparts in dense and cold (e.g., $n\sim 10^{-1}$ cm^-3, $T\sim 10^{2}$ K) interstellar medium (ISM) in the Galactic plane. Evolutions of SNRs in hot medium with a temperature up to $5\times 10^{7}$ K are described in Tang & Wang (2005). Based on detailed hydrodynamical simulations, estimates for size and radio surface brightness variations of SNRs with ambient density and time are provided in more recent work of Pavlović et al. (2018). The main results from all these studies can be summarized as follows – (i) Expansion deviates quickly ($<10^{4}$ years) from the standard Sedov-Taylor solutions at ambient temperatures above $10^{6}$ K. (ii) The sizes of SNRs near the end of the Sedov-Taylor phase are expected to scale inversely with ambient density as a power-law with an exponent of $\sim 1/3$, (iii) The SNRs expanding in a hot medium are expected to have a lower radio surface brightness than those in a warm medium. This behaviour is due to lower Mach number of the shock in a hotter medium. (iv) The radio surface brightness decreases very fast in late phases (i.e., Sedov-Taylor regime) of evolution with surface brightness to diameter slopes in between $-4$ and $-6$. It means that while the diameter will not increase very fast (i.e. expansion is slowed down), the surface brightness decreases very fast in the late stages. These general results will also be used in the next section to obtain a range of possible distances to ORCs.

It is also worth to mention here that the shell component of SNRs in a low ambient density will brighten at late stages and are likely to be detected in radio surveys, compared to the plerionic component which will be bright in the beginning for only a very short duration and will become extremely faint after a few 100 years from the explosion (e.g., Bhattacharya 1990). The dynamical lifetime of SNRs are expected to be $10^{4}-10^{5}$ years, after which SNRs mix with the general ISM and cease to exist as a morphologically distinct structure. It may be noted that the synchrotron lifetime of relativistic electrons generated in a SNR are expected to be much higher (up to $\sim 100$ Myr) in low (a few $\mu$G) magnetic field strengths, however, these electrons will diffuse out to much larger extents during this period and will contribute to large-scale diffuse radio emission. This simple dynamical behaviour of SNRs will have two consequences - firstly, small size SNRs representative of younger age will be difficult to detect in a survey covering low volume in the space and secondly, beyond a certain stage of expansion at later times, SNRs will become too faint and large to be detected in a survey with a limited sensitivity. Hence, statistically, if some ORCs are SNRs, then these are detected at a particular stage of their evolution, when both surface brightness and angular diameter become favourable for their detections in the radio survey with a limited sensitivity. This stage will be neither too early nor too late.

It is a well established fact that a large number of Galactic SNRs are missing in the present radio surveys due to multiple reasons (see e.g., Bhattacharya 1990; Green 2004). The main reason is believed to be that many SNRs evolve in low-density hot medium, created in the cavities blown by the progenitor massive star thereby making such SNRs large and faint. The biases created due to sensitivity limits in any radio surveys are also the reasons for missing some SNRs. For instance, sensitivities of the radio interferometric surveys such as ASKAP are limited in both flux and in detectable largest angular diameter, which is decided by the shortest distance in the pairs of the antennas in the radio array. Due to increase in sensitivity of the present generation radio telescopes and with the advent of new detection techniques, some very large and very faint SNRs have recently been detected. For instance, a large size ($\sim 4.4$ degree diameter) SNR named ’Hoinga (G249.7+24.7)’ with very low radio surface brightness ($4\times 10^{-23}$ W m^-2 Hz^-1 sr^-1 at 1.4 GHz) was detected at Galactic latitude of $\sim 25$ degree (Becker et al., 2021; Fesen et al., 2021). Another SNR ’G354-33’ with angular size of $11^{0}\times 14^{0}$ at Galactic latitude of $-33.5$ degree was detected in radio (Fesen et al., 2021). Detections of such large SNRs are obviously more favourable at higher Galactic latitudes due to reduced Galactic diffuse radio background at high latitudes.

Since ORCs have low radio surface brightness and have been detected due to a breakthrough in detection sensitivity with the ASKAP, it is worth to explore their origins in low-density hot medium SNRs, which are believed to be missing in previous generation radio surveys. The biggest challenge will be to constrain the distances as discussed in the next section.

It becomes worth at this point to compare properties of the ORCs with those of the Galactic SNRs and obtain some clues about their distances. Majority of the Galactic radio SNRs have their 1 GHz radio luminosity in the range of $10^{15}-10^{18}$ W Hz^-1 and radio surface brightness in the range of $10^{-22}-10^{-18}$ W m^-2 Hz^-1 sr^-1 (Green, 2004). The average radio surface brightness of five ORCs is in between $10^{-22}-10^{-21}$ W m^-2 Hz^-1 sr^-1, placing ORCs among the faintest bins of radio surface brightness of the Galactic SNRs.

Considering ORCs as SNRs at late stages where their expansion is slowed down, their angular sizes can provide another clue to their distances. The results provided in Pavlović et al. (2018) for SNRs evolving in ambient medium temperature of $10^{4}$ K and a range of densities between 2 to 0.005 cm^-3 for initial explosion energy in the range of 0.5 to 2.0$\times 10^{51}$ erg can be adjusted for lower densities and higher temperatures in the IGrM, using the general results described in the previous section. In ambient density $10^{-3}$ cm^-3, the 1 GHz radio surface brightness between $10^{-21}$ and $10^{-22}$ W m^-2 Hz^-1 sr^-1 will correspond to SNR diameter of $\sim 100$ pc. In lower ambient density, e.g., in $10^{-4}$ cm^-3 and $10^{-5}$ cm^-3, the sizes can be up to $\sim 200$ pc and $\sim 500$ pc respectively. It may be noted that the effect of increased ambient temperature will be on the radio surface brightness. The SNRs in hot (T$>10^{6}$ K) ambient medium (e.g., in galaxy clusters or massive groups with diffuse X-ray emission) are expected to become fainter than those in relatively warm (T$\sim 10^{5}-10^{6}$ K) ambient medium, typically seen in low mass groups of galaxies. This effect can reduce chances of detecting as well as identifying (from a background un-resolved radio source population) SNRs in the cores of galaxy clusters and in groups with diffuse X-ray emission.

Considering an upper limit to physical diameter as 500 pc corresponding to the detected range of radio surface brightness of the ORCs, the maximum distance to the ORCs in a low density ($10^{-5}$ cm^-3) warm ($10^{5}$K) IGrM environment can be about 1.5 Mpc for an average angular diameter of 1 arcmin. If ORCs are evolving in higher density (e.g., $10^{-3}$ cm^-3) ambient medium, this distance can be revised down to 0.3 Mpc. The 1 GHz radio luminosity of the Galactic radio SNRs are generally in the range of $10^{15}-10^{18}$ W Hz^-1 (Green, 2004). Assuming that the radio luminosities of the ORCs are similar to that of the general population of the Galactic SNRs, the ORCs with their estimated flux density can be placed in a distance bracket within a range of $\sim 0.1$ Mpc to $\sim 3$ Mpc. Therefore, the range of estimated distances of the ORCs is nearly 0.1 - 3 Mpc and the two arguments, viz., using radio brightness and size, and radio luminosity fetch similar results within the uncertainties.

In this paper, radio surface brightness, luminosity, and angular size of the ORCs were discussed. We attempted here to trace origins of some ORCs in the SNRs in the IGrM. Our estimates made under a minimalistic framework, suggest that some ORCs can be SNRs in the IGrM of the local group and in its immediate neighbour groups of galaxies within a distance of about 3 Mpc. The estimated host-less (presumably in IGrM) optical supernovae rate in other nearby groups suggests that only about 1.3 supernovae per 1000 square degree may be detected, assuming a detectability in radio for about $10^{4}$ years. The detectability of old SNRs at an epoch may depend upon the temperature of the IGrM in the sense that in hot IGrM (T$>10^{6}$ K), radio bright phase of the SNRs will last for a shorter period than those in a relatively warmer IGrM. This effect may favour larger number of SNR detections in the galaxy groups than in the galaxy clusters. As the present detection rate of ORCs is much higher at about 1.6 every 100 square degree, the ORCs as a class of objects will be heterogeneous in nature and will very likely trace their origins in multiple astrophysical objects or events. A similarity in physical appearances of five ORCs indicates that although the ORCs may trace their origins in multiple types of objects or events, the underlying phenomena responsible for their morphologies is likely to be the expanding shock waves in IGrM. With the present detection rate estimated from just five ORCs, not more than about 8 per cent of ORCs may be associated as SNRs in the IGrM of the local group and its immediate neighbours. This value is likely to change, as more ORCs will be detected in future.

Out of the five known ORCs, three ORCs have been associated with an optical counterpart and two ORCs do not have optical counterparts. Presently, possible explanations of the origins of ORCs do not seamlessly fit into a single previously observed energetic phenomena. As some ORCs have an optical galaxy and some appear host-less, finding origins of ORCs in multiple types of physical objects and processes appears a clever choice. Given that the present sample of ORCs is statistically very small and so any extrapolation of one possible explanation to all such objects need not be carried out and at the same time any possible or viable scenario should not be left out. Presently, these important questions remain to be answered - (1) The nearly identical sizes and radio flux of ORCs associated with an optical galaxy pose a challenge to understand their origins in deep extra-galactic objects at widely different redshifts. (2) Two ORCs with a very close separation in the sky. (3) Missing smaller angular size ORCs in the ASKAP survey.

Finally, it is worth to mention here some important consequences of confirming detections of SNRs in the IGrM. Domainko et al. (2004) and Zaritsky et al. (2004) discussed importance of intracluster supernovae to match the observed Fe abundance excess in the cool-core clusters. Supernovae are also considered efficient sources of heating and turbulence over large volumes in low-density hot medium typical of that in the intracluster medium (Valdarnini, 2003; Domainko et al., 2004; Tang & Wang, 2005). Omar (2019) discussed a possibility that radio emission from mini halos, another low radio surface brightness structures on physical scales of a few hundred kpc, undoubtedly associated with the brightest cluster galaxies in cool-core galaxy clusters, can be explained in terms of relativistic electrons accelerated in the intracluster supernovae events. A direct confirmed detection of intergalactic SNRs will strengthen this possibility. The unprecedented sensitivity combined with the anticipated sub-arcsec resolution of the upcoming Square Kilometer Array telescope may enable us to identify SNRs in the IGrM of some nearby galaxy groups and clusters.