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ATEL # 2791; K. Lo, B. M. Gaensler, T. Murphy, K. Bannister, R. W. Hunstead (Sydney Institute for Astronomy, School of Physics, The University of Sydney), Pat Slane (Harvard-Smithsonian Center for Astrophysics)
on 16 Aug 2010; 4:34 UT
Password Certification: Tara Murphy (firstname.lastname@example.org)
Subjects: Radio, Nova
We performed follow up radio observations of XMMU J115113.3-623730 (ATel #2746, #2777, #2771) with the Australia Telescope Compact Array at two separate epochs (2010 July 20, 22:16 UT and 2010 August 7, 01:00 UT), each for two hours in duration. The array was in the compact east-west configuration (EW352) in the July 20 epoch and the hybrid configuration (H168) in the August 7 epoch. We observed at 5.5 GHz and 9 GHz simultaneously, each with a bandwidth of 2 GHz, using the new Compact Array Broadband Backend.
We detect a source at 5.5 GHz and 9 GHz in both epochs. The measured positions are RA(2000) = 11:51:13.4 (+-3.7’’), Dec(2000) = -62:37:30.7 (+-3.1’’) at 5.5 GHz and RA(2000) = 11:51:12.9 (+-0.8’’), Dec(2000) = -62:37:28.3 (+-0.7’’) at 9 GHz in the August 7 epoch. The source is within the error circle of XMMU J115113.3-623730 and its likely optical/NIR counterpart (ATel #2746), the coordinates of the latter is RA(2000) = 11:51:13.00, Dec(2000) = -62:37:28.8 (+-0.3" in both coordinates). In the July 20 epoch, we measure the flux density at 5.5 GHz to be 1.2 +- 0.2 mJy and at 9 GHz to be 1.1 +- 0.1 mJy; and at the August 7 epoch, we measure the flux density at 5.5 GHz to be 1.6 +- 0.3 mJy and at 9 GHz to be 1.0 +- 0.1 mJy. The signal-to-noise ratio of the images, at 5.5 GHz and 9 GHz, are 8 and 18 for the July 20 epoch, and 6 and 18 for the August 7 epoch respectively.
A search in the Molonglo archive yielded a 5 sigma upper flux limit of 5 mJy at 843 MHz on 2003 May 18.
Based on its optical spectrum and the long term optical behaviour, XMMU J115113.3-623730 has been classified as a nova. The optical light curve from the All Sky Automated Survey indicates this source erupted sometime before November 2008. We calculate the spectral index of the radio emission to be alpha = -0.2 +- 0.9 in the 20 July epoch and alpha = -1 +- 1 in the 7 August epoch (flux density goes as nu^alpha). The uncertainty on alpha makes it hard to infer whether the spectrum has varied between epochs. The radio emission from most novae is dominated by thermal bremsstralung (Bode and Evans (ed.) 1989, Classical Novae, Wiley, Oxford). At early times, when the emitting region is optically thick, spectral index alpha = +2 and subsequently drops to alpha = -0.1 when the emitting region becomes optically thin. Novae radio light curves evolve slowly over hundreds of days (Bode 2010, Astron. Nachr., 331, 160). Hence the lack of any obvious change in flux density between our two observed epochs is expected.
Our radio observations corroborate the nova classification of this object. Its spectral index suggests it has transitioned to the optically thin regime and we are sampling the radio light curve in decay. The radio behaviour of this source is comparable to other classical novae such as HR Del, FH Ser and V1500 Cyg (Hjellming et al. 1979, AJ, 84, 1619-1631).