Investigation of Dense Gas Towards Relativistic Outflow Sources

We probe the interstellar medium towards the objects Circinus X-1, a low-mass X-ray binary with relativistic jets; and the highly energetic Westerlund 2 stellar cluster, which is located towards TeV gamma-ray emission and interesting arc- and jet-like features seen in Nanten 12CO data. We have mapped both regions with the Mopra radio telescope, in 7 mm and 12 mm wavebands, looking for evidence of disrupted/dense gas caused by the interaction between high energy outflows and the ISM. Towards Westerlund 2, peaks in CS(J=1-0) emission indicate high density gas towards the middle of the arc and the endpoint of the jet; and radio recombination line emission is seen overlapping the coincident HII region RCW49. Towards Circinus X-1, 12CO(J=1-0) Nanten data reveals three molecular clouds that lie in the region of Cir X-1. Gas parameters for each cloud are presented here.


Introduction
Circinus X-1 (Cir X-1) is a low-mass X-ray binary and microquasar, containing a young neutron star with an orbital period about its companion of ∼16.6 days 1 . Its highly relativistic jets (apparent velocity β app > 12) 1 are resolved in both radio and X-ray wavelengths 2,3 and interact with the interstellar medium (ISM) to create arcminute-scale synchrotron X-ray lobes 4 and a 5 ′ diameter radio nebula surrounding Cir X-1 1 . Given the evidence of multi-TeV particle acceleration in the region, Cir X-1 is a good candidate for TeV gamma-ray (γ-ray) emission, however no TeV γ-ray emission has been seen with H.E.S.S. (28 hrs) 5 . Our observations of the molecular gas towards Cir X-1 aim to provide insights into the physics of jet/ISM interactions on parsec (pc) scales, assess the potential of Cir X-1 as a cosmic-ray accelerator and provide additional constraints on its distance, which is ambiguous at 4-12 kpc.
Westerlund 2 is a young (1-3 Myr) and rich stellar cluster containing ongoing star formation and over two dozen high mass stars, including two Wolf-Rayet stars; and ionizes the nearby giant HII region RCW 49. Interesting arc-and jet-like features seen in Nanten 12 CO data 6 towards the cluster may be an indication of relativistic particle acceleration in the region and may have a common origin with coincident TeV γ-ray source HESS J1023-575 7 . Given the uncertainty in the origin of the TeV source and its link to the cluster, arc-jet feature and nearby pulsar PSR J1022-5746, a detailed picture of the dense and disrupted gas in this region is needed.

Observations
The area towards Cir X-1 was mapped in the 12 CO(J=1-0) line as part of the Nanten Galactic Plane Survey 8 . Follow-up observations with the Mopra radio telescope targeted tracers of denser and disrupted gas and were conducted in February 2012 in the 7 mm band and January 2010, February 2010 and April 2011 in the 12 mm band with a resultant T RMS of 0.1 K and 0.07 K respectively. Towards Westerlund 2, observations in the 12 mm band were conducted in January 2012 and were comprised of three 30 ′ ×30 ′ and one 10 ′ ×10 ′ mapping region to result in full coverage of the HII region, TeV emission and molecular arc-jet feature. The 7 mm observations were conducted in April 2012 and had coverage as seen in Fig. 2. The T RMS was ∼0.08 K per channel in all cases. The observations with the Mopra radio telescope in the 7 and 12 mm bands and data reduction were conducted as in Ref. 9.
Two narrow, ∼3 km/s FWHM, peaks of CS(J=1-0) emission were seen towards Westerlund 2, indicating gas densities in those regions of >10 4 cm −3 . A peak of 0.31 K (5.8σ) at ∼3 km/s is coincident with a dense component of giant molecular cloud (GMC) located at 1.2-8.7 km/s and believed to be one of the two parent GMCs of Westerlund 2 (Fig. 2a). It is 2.7 pc in radius (d/5.4 kpc) and has a virial mass between 1.5x10 3 and 5.5x10 3 M ⊙ . The other peak of 0.26 K (7.5 σ) is seen in the arc, which coincides precisely with the peak of the CO emission and is at the same Fig. 1. Cir X-1 region, showing velocity integrated Nanten 12 CO(J=1-0) data in units of K km/s, overlaid with MOST 843 MHz radio continuum contours (black) that show the radio nebula and nearby SNR G312.9-0.3, which is not thought to be associated with Cir X-1 11 . a)−85 to −64 km/s. The cloud has kinematic distance 4 or 11 kpc and the molecular mass inside an ellipse of 109 arcsec 2 encompassing the SNR and flanking CO emission is ∼2x10 5 M ⊙ (d/5.8kpc) 2 . b)−34 to −30 km/s with additional CS(J=1-0) contours (thin white lines) in steps of 1σ from 2σ and NH 3 (1,1) contours (thick white lines) in steps of 3σ from 5σ. The cloud has kinematic distance 2 or 13 kpc and the total molecular mass in the cloud is ∼4x10 5 M ⊙ (d/10kpc) 2 . c)6 to 14 km/s. The northern cloud has kinematic distance < 16 kpc and has mass of a few x10 4 M ⊙ (d/10kpc) 2 and density ∼70 cm −3 (d/10kpc) −1 . d) 12 CO(J=1-0) and NH 3 (1,1) spectra towards Cir X-1 (l,b=322.116 • ,0.033 • ) and EGO G321.94-0.01, (l,b=321.923 • ,0.013 • ).
velocity range (∼24-28 km/s) (see Fig. 2b). It is 2.8 pc in radius (d/7 kpc) and has a virial mass between 8.8x10 2 and 3.1x10 3 M ⊙ . Virial masses are bounded by r −2 and Gaussian density profile assumptions. Emission from radio recombination lines (RRLs) H62α (Fig. 2c), H65α and H69α was observed towards the HII region. It shows a morphological correspondence to the radio continuum emission and appears to be centred between the two Wolf-Rayet stars of the Westerlund 2 cluster. The emission in all three lines is broad (∼45 km/s FWHM) and peaks at ∼0 km/s.

Conclusions
The ambiguity in associating peaks in the 12 CO emission to Cir X-1 as well as the non-detection of any other molecular line emission has not enabled us to gain insights into the interaction between the relativistic jets and the ISM, nor to constrain the distance to Cir X-1. We tentatively associate the molecular gas located at 10 km/s with Cir X-1, and that at −75 km/s with SNR G321.9-0.3. EGO G321.94-0.01 has been associated with 12 CO(1-0), CS(1-0), H 2 O and NH 3 (1,1) emission at −30 km/s. Molecular cloud masses of the order of ∼10 4 M ⊙ towards Cir X-1 make γ-ray production likely given the jet energetics, and warrants deeper H.E.S.S observations. Two peaks of CS are seen towards Westerlund 2, providing evidence of dense gas towards both the jet feature and a parent molecular cloud of the cluster, either of which may be associated with the high energy emission in the region. RRL emission highlights the extent of the partially ionised gas in the HII region RCW 49.