A Radio Transient In M82: An SS433 Analogue?
Lying at a distance of 3.6Mpc (Freedman et al. 1994), M82 is observed to be one of the most tumultuous galaxies in the local group, with rapid periods of both star formation and evolution: a designated starburst galaxy (Kronberg et al. 1985; Rieke et al. 1993; Pedlar & Muxlow 1995). In addition to this intense star formation, radio observations have revealed approximately 60 compact radio sources in the central region of M82 (McDonald et al. 2002). A quarter of these sources are of unknown origin. As well as hosting a variety of compact sources, continual radio surveys have also found various transient sources of an undetermined nature (e.g Kronberg & Sramek 1985; Muxlow et al. 1994).
Recently, Joseph et al. (2011) have paid close attention to a peculiar radio transient that they believe is unlikely to be a “normal” microquasar, making way for a possible analogue to the exciting and infamous SS433 microquasar (see Margon 1984) in our own galaxy. Both SS433 and this new transient source have been observed to have low X-ray but high radio luminosities. However, SS433 possesses precessing, helical cone-like jets emanating from it’s poles due to accretion onto a compact object, most likely a black hole (Margon et al. 1979). Interestingly, if this extra-galactic transient is like SS433, then it will be only one of two such objects, after S26 in NGC7793, to be discovered (Pakull et al. 2010; Soria et al. 2010)
If the M82 transient is some form of microquasar, its luminosity suggests that it is quite probable that it can be associated with a massive black hole system of some type. This could range from an extreme form of X-ray binary to an intermediate-mass black hole system. Still, the high luminosity and temporal properties of the transient outburstimply that this type of accretion object is unusual and has not yet been seen within our Galaxy.
Muxlow et al. (2010) outlined that there are three plausible explanations for what this source might be: i.) an extragalactic microquasar, ii.) an AGN in the centre of M82, or iii.) a radio supernova. This provides the basis for Joseph et al. (2011) to home in on the exact nature of the transient source. A conclusion of their findings is such that: ii.) the source is unlikely to be an AGN since the supermassive black hole would have to be significantly displaced from the dynamical centre of the galaxy in order to match observations; iii.) the source is too faint to be either a type Ia/Ib or Ic supernova outburst due to its unchanging spectral index, which they were not able to fit with expectations for young radio supernovae. Therefore, they can test and either confirm or deny the theory that it is an extragalactic microquasar similar to SS433; yet, annoyingly, they still need to address two anomalous observations.
The first of which is that the transient source in M82 exhibits an X-ray to radio luminosity ratio that does not fit with exhisting empiral correlations for compact black hole binaries. Secondly, the source in M82 has been observed to flare which lasted much longer than the usual timescale of a few hours to a few days. What Joseph et al. (2011) can be sure of is that if this source is a microquasar, then it is nothing but conventional!
Joseph et al. (2011) address these concerns by suggesting that M82 may simply be a microquasar with unusual behavior analogous to the microquasar SS433 (Fabrika 2004). SS433 is is also relatively faint in X-rays compared to radio (Fiedler et al. 1987; Dubner et al. 1998), and it exhibits a similar spectral index and a similarly small amount of variability in its light curve (Margon et al. 1979; Kwitter et al. 1979; Crampton & Hutchings 1981; Margon 1984). And while the source in M82 has a significantly higher radio luminosity than SS433, this would make sense if it also had a more massive donor star than SS433’s 12 donor (Hillwig & Gies 2008).
After careful analysis, Joseph et al. (2011) conclude that M82’s peculair radio source is, like SS433, an unusual microquasar. An extragalactic analogue. However, they do issue caution; explaining that follow-up observations of the optical behaviour of M82’s source is needed, searching for the archetypical Doppler shifted spectral signatures strongly associated with SS433. An observation that would perhaps confirm M82’s transient radio source as SS433’s extragalactic sister.
- Crampton, D.; Hutchings, J. B. (1981) The SS 433 Binary System. The Astrophysical Journal, 251(1) pp. 604-610.
- Margon, B. (1984) Observations Of SS433. Annual Review: Astronomy & Astrophysics, 22 (1) pp.507-536.
- Rieke, G. H.; Loken, K.; Rieke, M. J.; Tamblyn, P. (1993) Starburst Modeling Of M82 – Test Case For A Biased Initial Mass Function. The Astrophysical Journal, 412 (1) pp.99-110.
- Muxlow, T.W.B. et al. (2010) Discovery Of An Unusual New Radio Source In The Star-Forming Galaxy M82: Faint Supernova, Supermassive Black Hole Or An Extragalactic Microquasar? Monthly Notices Royal Astronomical Society: Letters, 404 (1) pp. L109-L113.
- Joseph, T.; Maccarone, T.; Fender, R. (2011) The Unusual Radio Transient In M82: An SS433 Analogue? Monthly Notices Royal Astronomical Society: Letters, 415 (1) pp. L59-L63.