e-MERLIN reveals the structure of a new 'superluminal transient' source in M82
Recent observations using the newly available e-MERLIN telescope have revealed the resolved structure of 'superluminal transient' radio source in M82. Results from these new observations, which have been obtained for the LeMMINGs e-MERLIN legacy project have recently been presented at a large international conference in Bordeaux, France.
This bizarre, faint radio source was initially detected during routine monitoring observations using the MERLIN array of radio telescopes in May 2009. This new source was seen to appear suddenly at a position where no source was visible in observations just 1 week earlier (see fig 1). Continued monitoring of this new point source tentatively showed that it was moving across the sky at with an apparent speed several times larger than the speed of light. This discovery and initial results were published by Muxlow et al in Monthly Notices of the Royal Astronomical Society (Muxlow et al., 2010 MNRAS, 404L, 109).
The initial detection of a new transient source in M82 using 'old'-MERLIN. This source’s appearance was detected within one week of its outburst (Muxlow et al 2010).
Since then the MERLIN has been completely upgraded to form e-MERLIN. This upgrade, involving the installation of a new dedicated fibre network, new receiver, data communication and control systems, along with a powerful new correlator has transformed the array of telescopes ultimately increasing its sensitivity by more than an order of magnitude. Scientifically this upgrade has revolutionised the capabilities of the array. Since early 2012 e-MERLIN has been in operations undertaking peer reviewed open-time observations and initial science observations for its large key science programme.
As part of the second largest legacy programme LeMMINGs [PI: R. Beswick (Manchester) and I. McHardy (Southampton)], initial exploratory observations have been undertaken of a series of nearby galaxies as a forerunner for this legacy project which will image at a large sample of galaxies in the nearby Universe with microJansky sensitivities and sub-arcsecond angular resolutions. One of these targets is the famous nearby star-forming galaxy M82. These initial short observations of M82 using a bandwidth of 512MHz centred at 5.5GHz have provided a stunning indication of future science with e-MERLIN, by providing high fidelity (the direct result of the increased bandwidth of e-MERLIN compared to the old narrow band MERLIN system) and high sensitivity images of numerous individual Supernova remnants and HII regions in M82 (see below).
An e-MERLIN C-band image of typical radio supernova remnant in M82. These observations were taken as part of the e-MERLIN LeMMINGs legacy programme.
Amongst this menagerie of star-formation products, the study of which are one of the primary science drivers e-MERLIN has detected and critically resolved the structure of this new, and as yet not understood, transient object. These new e-MERLIN observations show the source to have elongated resolved east-west structure with a size of ~40milliarcseconds. Assuming that this sources is situated in M82 itself this implies that that it has experienced a symmetric averaged expansion rate of 110,000km/s. This structure and expansion has been independently confirmed by global VLBI observations made by this group. With such an expansion speed and considering its radio characteristics, it is highly unlikely that this source is supernovae in origin, but more likely the radio emission from a flaring Intermediate Mass Blackhole. As such this time series of initially MERLIN and now more sensitive e-MERLIN observations have afforded a unique opportunity to study the evolution of this type of object. These results were presented at a large international conference in Bordeaux, France in October 2012.
The left image shows a high resolution Global VLBI image at 5GHz of the transient source in M82. The right-hand image show the same source observed with e-MERLIN in 2012. Whilst at lower resolution the sensitivity and imaging quality of e-MERLIN allows this source to be resolved in an east-west direction confirming the earlier MERLIN proper motion estimates of its expansion and the VLBI imaging. These e-MERLIN (and VLBI) observations are part of ongoing research under the e-MERLIN legacy project, LeMMINGs.
Over the next few years LeMMINGs e-MERLIN legacy programme will complete a survey of 300 nearby galaxies at unprecedented sensitivity and resolution using e-MERLIN, including M82. It will probe both the star-formation and accretion processes in the full range of galaxies in our locality. These initial observations give a glimpse the wide ranging science that this survey will produce, including the as yet unknown.