VLA & Swift Observations of Liller 1 Indicate CXOU J173324.6-332321 is Likely a Neutron Star X-Ray Binary

ATel #11646; Arash Bahramian (Curtin University), Alexandra Tetarenko, Gregory Sivakoff (University of Alberta), Jay Strader, Laura Chomiuk (Michigan State University), Craig Heinke (University of Alberta), James Miller-Jones (Curtin University), Evangelia Tremou (Michigan State University), Vlad Tudor (Curtin University)
on 16 May 2018; 14:15 UT
Credential Certification: Arash Bahramian (bahramian@pa.msu.edu)

Tweet

Following the detection of CXOU J173324.6-332321 as a new transient X-ray source in the globular cluster Liller 1 (ATel #11598), we acquired radio observations with the VLA to classify the source via its radio emission. The VLA was on source (in its most extended, A configuration), targeting near the Chandra X-ray position, between 06:22:49-07:07:36 UTC on 2018 May 14 (MJD~58252.28) with a total exposure of 26 min. We took data using two 2048-MHz basebands centred at 9 and 11 GHz. The source was observed at low elevations (~12-16 degrees), elongating the radio beam to 1.0x0.21 arcseconds (at a position angle of -31 deg) at 10 GHz.

We marginally detected an unresolved point source combining both basebands. Fitting the data to a point source model in the image plane, we derive a preliminary flux density of 19.3 +/- 5.3 μJy/beam across 8-12 GHz. The luminosity of the radio source (L_R = ν L_ν) is 7.8e27 (d/8.2 kpc)^2 at 5 GHz, where we assume a flat radio spectral index and d is the distance to the source (scaled to the Harris 1996, AJ, 112, 1487; 2010 edition distance of Liller 1). This source is preliminarily located at
RA=17h:33:24.5931 +/- 0.0039s
Dec=-33:23:20.65 +/- 0.12 "
, 0.8" away from the Chandra position (which had an absolute astrometric error of 0.6"). Based on the X-ray positions in Homan et al. (2018; ATel #11598) and the radio position of the Rapid Burster from Moore et al. (2000; ApJ 532, 1181), the X-ray position of CXOU J173324.6-332321 is RA=17h:33:24.61 +/- 0.007s
Dec=-33:23:21.2 +/- 0.1"
. Since the radio detection is <5-sigma and outside the corrected 3-sigma (0.3") confidence interval of CXOU J173324.6-332321, we are equivocal about whether the radio source we detect is real and, if real, whether the radio source is associated with CXOU J173324.6-332321.

As part of the MAVERIC project of deep radio continuum imaging of Galactic globular clusters, we serendipitously obtained (A configuration) VLA data in two subbands centered at 5.0 and 7.0 GHz preceding the outburst of CXOU J173324.6-332321 on UT April 9 (10:03-11:33). There is no source consistent with the position of the CXOU J173324.6-332321 or the marginally detected 10 GHz radio source discussed above in a stacked image at a mean frequency of 6.0 GHz (rms noise 3.0 µJy/beam). Hence, on 2018 April 9 we set a 3-sigma upper limit of 9.0 µJy on the flux density of a radio source associated with CXOU J173324.6-332321.

We obtained Swift/XRT observations on 2018 May 15, nearly contemporaneous with the VLA observation. The source is about two times brighter in X-rays than in the observation on 2018 May 06, suggesting either a re-brightening of CXOU J173324.6-332321 or contamination of the XRT light curve from a separate, new outburst from the Rapid Burster (or a new transient). Fitting the new observations with an absorbed power law, we find L_X = 4.7(-0.6/+1.2) e35 erg/s (1-10 keV), with N_H = 3.8(-1.2/+1.5)e22 cm^-2 and a photon index of 1.8(-/+0.5). These values are consistent with the (Chandra) values reported in ATel #11598.

To make a preliminary classification of the source, we place this object on the L_R-L_X plane (e.g., Tudor et al. 2017, MNRAS, 470, 324). We note that a 3-sigma upper limit to the flux density of 15 µJy yields a less conservative upper limit of 6.0e27 (d/8.2 kpc)^2 erg/s at 5 GHz (compared to the marginal 7.8e29 erg/s source noted above). Based on either of these values and the Swift X-ray luminosity, this transient is inconsistent with black hole X-ray binaries but is consistent with measurements or limits from hard state neutron star binaries (see the linked figure below). Hence, we conclude that CXOU J173324.6-332321 is likely to be a neutron star X-ray binary.

We thank NRAO for awarding Director's Discretionary Time and rapidly scheduling our observations, as well as Swift for its continued monitoring of this source's outburst.

Radio-X-ray luminosity plot