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Detection of compact radio emission from Circinus X-1 with the first Southern hemisphere e-VLBI experiment


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Phillips, CJ, Dickey, JM, Fender, RP, Tudose, V, Nicolson, GD, Deller, A, Amy, SW, Tingay, SJ, Tzioumis, AK, Reynolds, JE, Jauncey, DL, Stevens, JB and Ellingsen, SP 2007 , 'Detection of compact radio emission from Circinus X-1 with the first Southern hemisphere e-VLBI experiment' , Monthly Notices of the Royal Astronomical Society, vol. 380, no. 1 , L11-L14 , doi: 10.1111/j.1745-3933.2007.00339.x.

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Circinus X-1 has recently returned to a state of strong radio flaring. Here we report on the first
VLBI observations, and detection, undertaken in the 25 years since the 1975–1985 period of
strong recurrent flaring activity.We detected Circinus X-1 with the first observations conducted
by a recently developed Southern hemisphere e-VLBI array, at both 1.6 and 8.4 GHz, over a
three-day period. At 1.6 GHz, the compact source has a total flux density of 11 mJy and a size of
60 ± 15 mas (Gaussian model full width at half maximum). At 8.4 GHz, the compact source is
less than 60 mas. The size variation with frequency is consistent with a broadened image
due to scattering in the turbulent, ionized interstellar medium of our Galaxy. However, these
size measurements appear inconsistent with the λ2.2 variation expected for strong interstellar
scattering and previous VLBI observations made at 2.3 GHz in the early 1980s. To explain this
apparent inconsistency, we suggest that Circinus X-1 supports a weak, non-varying component
of 35 mas extent (175 au at 5 kpc distance), corresponding to compact structure in the extended
radio nebula. No significant variation in the flux density at 1.6 GHz is evident between two
observations 24 h apart. No jet-like structures are evident on scales of tens of mas, simply a
scatter broadened source, presumably coincident with the suggested neutron star in the binary

Item Type: Article
Authors/Creators:Phillips, CJ and Dickey, JM and Fender, RP and Tudose, V and Nicolson, GD and Deller, A and Amy, SW and Tingay, SJ and Tzioumis, AK and Reynolds, JE and Jauncey, DL and Stevens, JB and Ellingsen, SP
Keywords: instrumentation: interferometers • techniques: interferometric • radio continuum: stars • X-rays: stars • X-rays: individual: Circinus X
Journal or Publication Title: Monthly Notices of the Royal Astronomical Society
Publisher: Blackwell Publishing
ISSN: 0035-8711
DOI / ID Number: 10.1111/j.1745-3933.2007.00339.x
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