* Astronomy

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RE: Circinus X-1

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Title: Lord of the Rings: A Kinematic Distance to Circinus X-1 from a Giant X-Ray Light Echo
Author: Sebastian Heinz, Michael G. Burton, Catherine Braiding, William N. Brandt, Peter G. Jonker, Paul Sell, Robert P. Fender, Michael A. Nowak, Norbert S. Schulz

Circinus X-1 exhibited a bright X-ray flare in late 2013. Follow-up observations with Chandra and XMM-Newton from 40 to 80 days after the flare reveal a bright X-ray light echo in the form of four well-defined rings with radii from 5 to 13 arcminutes, growing in radius with time. The large fluence of the flare and the large column density of interstellar dust towards Circinus X-1 make this the largest and brightest set of rings from an X-ray light echo observed to date. By deconvolving the radial intensity profile of the echo with the MAXI X-ray lightcurve of the flare we reconstruct the dust distribution towards Circinus X-1 into four distinct dust concentrations. By comparing the peak in scattering intensity with the peak intensity in CO maps of molecular clouds from the Mopra Southern Galactic Plane CO Survey we identify the two innermost rings with clouds at radial velocity ~ -74 km/s and ~ -81 km/s, respectively. We identify a prominent band of foreground photoelectric absorption with a lane of CO gas at ~ -32 km/s. From the association of the rings with individual CO clouds we determine the kinematic distance to Circinus X-1 to be D_{Cir X-1} = 9.4^{+0.8}_{-1.0} kpc. This distance rules out earlier claims of a distance around 4 kpc, implies that Circinus X-1 is a frequent super-Eddington source, and places a lower limit of Gamma \gtrsim 22 on the Lorentz factor and an upper limit of theta_{jet} \lesssim 3° on the jet viewing angle.

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ATel 7311: A new outburst of Cir X-1

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Title: A return to strong radio flaring by Circinus X-1 observed with the Karoo Array Telescope test array KAT-7
Authors: R. P. Armstrong, R.P. Fender, G.D. Nicolson, S. Ratcliffe, M. Linares, J.Horrell, L. Richter, M. P. E. Schurch, M. Coriat, P. Woudt, J. Jonas, R. Booth, B. Fanaroff

Circinus X-1 is a bright and highly variable X-ray binary which displays strong and rapid evolution in all wavebands. Radio flaring, associated with the production of a relativistic jet, occurs periodically on a ~17-day timescale. A longer-term envelope modulates the peak radio fluxes in flares, ranging from peaks in excess of a Jansky in the 1970s to an historic low of milliJanskys during the years 1994 to 2007. Here we report first observations of this source with the MeerKAT test array, KAT-7, part of the pathfinder development for the African dish component of the Square Kilometre Array (SKA), demonstrating successful scientific operation for variable and transient sources with the test array. The KAT-7 observations at 1.9 GHz during the period 13 December 2011 to 16 January 2012 reveal in temporal detail the return to the Jansky-level events observed in the 1970s. We compare these data to contemporaneous single-dish measurements at 4.8 and 8.5 GHz with the HartRAO 26-m telescope and X-ray monitoring from MAXI. We discuss whether the overall modulation and recent dramatic brightening is likely to be due to an increase in the power of the jet due to changes in accretion rate or changing Doppler boosting associated with a varying angle to the line of sight.

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Title: The evolution of inner disk radius with orbital phase in Circinus X-1
Authors: Yanan Wang, Guoqiang Ding, Chunping Huang

Using RXTE observations, we investigate the evolution of inner disk radius R_in of Cir X-1 during two cycles and find obvious orbital modulation. We argue that the modulation is attributed to its high orbital eccentricity. The disk luminosity is inversely with the inner disk temperature kT_in, which is ascribed to the slow increase of kT_in and, however, the rapid decrease of R_in during the passage for the neutron star to depart from the companion star.

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Title: Millimetre observations of a sub-arcsecond jet from Circinus X-1
Authors: D.E. Calvelo (Southampton), R.P. Fender (Southampton), A.K. Tzioumis (ATNF), J.W. Broderick (Southampton)

We present results from the first successful millimetre (combined 33 GHz and 35 GHz) observations of the neutron star X-ray binary Circinus X-1, using the Australia Telescope Compact Array. The source was clearly detected in all three observing epochs. We see strong evidence for a periastron flare beginning at MJD 55519.9 ±0.04 with estimated peak flux densities of up to 50 mJy and which proceeds to decline over the following four days. We directly resolve jet structures on sub-arcsecond scales. Flux density variability and distance from the core of nearby components suggests recent shock re-energisation, though we are unable to directly connect this with the observed flare. We suggest that, if the emission is powered by an unseen outflow, then a phase delay exists between flare onset and subsequent brightening of nearby components, with flows reaching mildly relativistic velocities. Given resolved structure positions, in comparison to past observations of Cir X-1, we find evidence that jet direction may vary with distance from the core, or the source's precession parameters have changed.

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Title: Radio observations of Circinus X-1 over a complete orbit at an historically faint epoch
Authors: D.E.Calvelo (Southampton), R.P.Fender (Southampton), A.K.Tzioumis (ATNF), N.Kawai (Tokyo Tech), J.W.Broderick (Southampton), M.E.Bell (Southampton)

We present results from the first radio observations of a complete orbit (~ 17 days) of the neutron star X-ray binary Circinus X-1 using the Australia Telescope Compact Array Broadband Backend, taken while the system was in an historically faint state. We have captured the rapid rise and decline of a periastron passage flare, with flux densities for 9 days prior to the event stable at ~ 1 mJy at 5.5 GHz and ~ 0.5 mJy at 9 GHz. The highest flux densities of 43.0 ±0.5 mJy at 5.5 GHz and 29.9 ±0.6 mJy at 9 GHz were measured during the flare's decline (MJD 55206.69) which continues towards pre-flare flux densities over the following 6 days. Imaging of pre-flare data reveals steady structure including two stable components within 15 arc-seconds of the core which we believe may be persistent emission regions within the system's outflows, one of which is likely associated with the system's counter-jet. Unlike past observations carried out in the system's brighter epochs, we observe no significant structural variations within ~ 3 arc-seconds of the core's position. Model subtraction and difference mapping provide evidence for variations slightly further from the core: up to 5" away. If related to the observed core flare, then these variations suggest very high outflow velocities with {\Gamma} > 35, though this can be reduced significantly if we invoke phase delays of at least one orbital period. Interestingly, the strongest structural variations appear to the north west of the core, opposite to the strongest arcsec-scale emission historically. We discuss the implications of this behaviour, including the possibility of precession or a kinked approaching jet.

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Title: The return of the bursts: Thermonuclear flashes from Circinus X-1
Authors: M. Linares (1,2), A. Watts (3), D. Altamirano (3), P. Soleri (4), N. Degenaar (3), Y. Yang (3), R. Wijnands (3), P. Casella (5), J. Homan (1), D. Chakrabarty (1), N. Rea (6), M. Armas-Padilla (3), Y. Cavecchi (3,7), M. Kalamkar (3), R. Kaur (3), A. Patruno (3), M. van der Klis (3), ((1) MIT, (2) Rubicon Fellow, (3) Amsterdam, (4) Groningen, (5) Southampton, (6) CSIC-IEEC, (7) Leiden)
(Version v2)

We report the detection of 15 X-ray bursts with RXTE and Swift observations of the peculiar X-ray binary Circinus X-1 during its May 2010 X-ray re-brightening. These are the first X-ray bursts observed from the source after the initial discovery by Tennant and collaborators, twenty-five years ago. By studying their spectral evolution, we firmly identify nine of the bursts as type I (thermonuclear) X-ray bursts. We obtain an arcsecond location of the bursts that confirms once and for all the identification of Cir X-1 as a type I X-ray burst source, and therefore as a low magnetic field accreting neutron star. The first five bursts observed by RXTE are weak and show approximately symmetric light curves, without detectable signs of cooling along the burst decay. We discuss their possible nature. Finally, we explore a scenario to explain why Cir X-1 shows thermonuclear bursts now but not in the past, when it was extensively observed and accreting at a similar rate.

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A team of astronomers has discovered a neutron star emitting an extended stream of powerful X rays, marking the first time such an extended X-ray jet has been detected originating from any class of object other than black holes.

"This discovery shows that the unusual properties of black holes, such as the lack of an actual surface,- may not be required to form powerful X-ray jets, as was previously thought" - Niel Brandt, professor of astronomy and astrophysics, and one of the scientists on the team that made the discovery with NASA's Chandra X-ray Observatory.

The discovery, which is described in a paper recently published in The Astrophysical Journal Letters, may help astronomers to understand how X-ray jets can erupt from a neutron star -- the extremely dense ball of tightly packed neutrons that remains after a more "normal" star has exploded at the end of its lifetime.

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Circinus X-1: Neutron Stars Join The Black Hole Jet Set