Dramatic radio brightening of Circinus X-1

ATel #563; Rob Fender (Southampton), Tasso Tzioumis (ATNF), Valeriu Tudose (Amsterdam)
on 11 Jul 2005; 14:00 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Rob Fender (rpf@phys.soton.ac.uk)

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We report on observations of the neutron star X-ray binary system and relativistic jet source Circinus X-1 with the Australia Telescope Compact Array (ATCA). A 12hr observation on 2005 June 17 made simultaneously at 4.8 and 8.6 GHz revealed the source to be bright and highly variable, with flux densities at both frequencies varying between 20--50 mJy and flips from optically thick to optically thin spectra. These levels are equal to the largest radio flux densities measured with our (admittedly sporadic) ATCA monitoring program since 1996, and are contrary to the trend of declining radio flux density of both core and extended jets over the period 1998-2001 which was reported in Fender et al. (Nature, 427, 222, 2004). This trend may be traced back to the 1970s, when the radio flares were more than an order of magnitude brighter than they have been since the 1990s. The observations took place within five days of one of the X-ray outbursts from the source, which generally occur with a period of 16.6 days. Preliminary analysis of the radio lightcurves suggest a decline from even higher radio flux levels prior to our observation. It is not clear at present whether this event is related to the secular evolution of Circinus X-1, but if so it may be associated with the near-monotonic fading of the source in X-rays since about 1999, and/or the recent apparent recovery from this decline. The origin of this secular evolution may relate to a varying orbitally-averaged accretion rate and/or precession of the relativistic jet, which seems to be aligned very close to the line of sight. Whatever the origin of the radio rebrightening, it may well have observable consequences in other bands; observations at other wavelenths are therefore strongly encouraged.