Constraining properties of neutron star merger outflows with radio observations

The jet opening angle and inclination of GW170817 -- the first detected binary neutron star merger -- were vital to understand its energetics, relation to short gamma-ray bursts, and refinement of the standard siren-based determination of the Hubble constant, $H_0$. These basic quantities were determined through a combination of the radio lightcurve and Very Long Baseline Interferometry (VLBI) measurements of proper motion. In this paper we discuss and quantify the prospects for the use of radio VLBI observations and observations of scintillation-induced variability to measure the source size and proper motion of merger afterglows, and thereby infer properties of the merger including inclination angle, opening angle and energetics. We show that these techniques are complementary as they probe different parts of the circum-merger density/inclination angle parameter space and different periods of the temporal evolution of the afterglow. We also find that while VLBI observations will be limited to the very closest events it will be possible to detect scintillation for a large fraction of events beyond the range of current gravitational wave detectors. Scintillation will also be detectable with next generation telescopes such as the Square Kilometre Array, 2000 antenna Deep Synoptic Array and the next generation Very Large Array, for a large fraction of events detected with third generation gravitational wave detectors. Finally, we discuss prospects for the measurement of the $H_0$ with VLBI observations of neutron star mergers and compare this technique to other standard siren methods

The period-luminosity relation of red supergiants with Gaia DR2

We revisit the K -band period-luminosity (P-L) relations of Galactic red supergiants using Gaia Data Release 2 parallaxes and up to 70 yr of photometry from AAVSO and ASAS campaigns. In addition, we examine 206 LMC red supergiants using 50 yr of photometric data from the Digitised Harvard Astronomical Plate Collection. We identified periods by computing power spectra and calculated the period-luminosity relations of our samples and compared them with the literature. Newly available data tighten the P-L relations substantially. Identified periods form two groups: one with periods of 300-1000 days, corresponding to pulsations, and another with Long Secondary Periods between 1000 and 8000 days. Among the 48 Galactic objects we find shorter periods in 25 stars and long secondary periods in 23 stars. In the LMC sample we identify 85 and 94 red supergiants with shorter and long secondary periods, respectively. The P-L relation of the Galactic red supergiants is in agreement with the red supergiants in both, the Large Magellanic Cloud and the Andromeda Galaxy. We and no clear continuity between the known red giant period-luminosity sequences, and the red supergiant sequences investigated here.Comment: Resubmitted with the supplementary material. Acknowledgements section and affiliations have been update

Periodic Radio Emission from the T8 Dwarf WISE J062309.94-045624.6

We present the detection of rotationally modulated, circularly polarized radio emission from the T8 brown dwarf WISE J062309.94-045624.6 between 0.9 and 2.0 GHz. We detected this high proper motion ultracool dwarf with the Australian SKA Pathfinder in $1.36$ GHz imaging data from the Rapid ASKAP Continuum Survey. We observed WISE J062309.94-045624.6 to have a time and frequency averaged Stokes I flux density of $4.17\pm0.41$ mJy beam$^{-1}$, with an absolute circular polarization fraction of $66.3\pm9.0\%$, and calculated a specific radio luminosity of $L_{\nu}\sim10^{14.8}$ erg s$^{-1}$ Hz$^{-1}$. In follow-up observations with the Australian Telescope Compact Array and MeerKAT we identified a multi-peaked pulse structure, used dynamic spectra to place a lower limit of $B>0.71$ kG on the dwarf's magnetic field, and measured a $P=1.912\pm0.005$ h periodicity which we concluded to be due to rotational modulation. The luminosity and period we measured are comparable to those of other ultracool dwarfs observed at radio wavelengths. This implies that future megahertz to gigahertz surveys, with increased cadence and improved sensitivity, are likely to detect similar or later-type dwarfs. Our detection of WISE J062309.94-045624.6 makes this dwarf the coolest and latest-type star observed to produce radio emission.Comment: Accepted for publication in ApJ Letters; 11 pages, 3 figures and 2 table

Serendipitous Discovery of PSR J1431-6328 as a Highly-Polarized Point Source with the Australian SKA Pathfinder

We identified a highly-polarized, steep-spectrum radio source in a deep image with the Australian Square Kilometre Array Pathfinder (ASKAP) telescope at 888 MHz. After considering and rejecting a stellar origin for this source, we discovered a new millisecond pulsar (MSP) using observations from the Parkes radio telescope. This pulsar has period 2.77 ms and dispersion measure 228.27 pc/cm**3. Although this pulsar does not yet appear to be particularly remarkable, the short spin period, wide profile and high dispersion measure do make it relatively hard to discover through traditional blind periodicity searches. Over the course of several weeks we see changes in the barycentric period of this pulsar that are consistent with orbital motion in a binary system, but the properties of any binary need to be confirmed by further observations. While even a deep ASKAP survey may not identify large numbers of new MSPs compared to the existing population, it would be competitive with existing all-sky surveys and could discover interesting new MSPs at high Galactic latitude without the need for computationally-expensive all-sky periodicity searches.Comment: ApJ, in pres

A search for radio afterglows from gamma-ray bursts with the Australian Square Kilometre Array Pathfinder

We present a search for radio afterglows from long gamma-ray bursts using the Australian Square Kilometre Array Pathfinder (ASKAP). Our search used the Rapid ASKAP Continuum Survey, covering the entire celestial sphere south of declination $+41^\circ$, and three epochs of the Variables and Slow Transients Pilot Survey (Phase 1), covering $\sim 5,000$ square degrees per epoch. The observations we used from these surveys spanned a nine-month period from 2019 April 21 to 2020 January 11. We crossmatched radio sources found in these surveys with 779 well-localised (to $\leq 15''$) long gamma-ray bursts occurring after 2004 and determined whether the associations were more likely afterglow- or host-related through the analysis of optical images. In our search, we detected one radio afterglow candidate associated with GRB 171205A, a local low-luminosity gamma-ray burst with a supernova counterpart SN 2017iuk, in an ASKAP observation 511 days post-burst. We confirmed this detection with further observations of the radio afterglow using the Australia Telescope Compact Array at 859 days and 884 days post-burst. Combining this data with archival data from early-time radio observations, we showed the evolution of the radio spectral energy distribution alone could reveal clear signatures of a wind-like circumburst medium for the burst. Finally, we derived semi-analytical estimates for the microphysical shock parameters of the burst: electron power-law index $p = 2.84$, normalised wind-density parameter $A_* = 3$, fractional energy in electrons $\epsilon_{e} = 0.3$, and fractional energy in magnetic fields $\epsilon_{B} = 0.0002$.Comment: 18 pages, 7 figures; accepted for publication in MNRA

Gaia GraL: Gaia DR2 Gravitational Lens Systems. VII. XMM-Newton Observations of Lensed Quasars

© 2022. The Author(s). Published by the American Astronomical Society. This is the accepted manuscript version of an article which has been published in final form at https://doi.org/10.3847/1538-4357/ac4476We present XMM-Newton X-ray observations of nine confirmed lensed quasars at 1 ≲ z ≲ 3 identified by the Gaia Gravitational Lens program. Eight systems are strongly detected, with 0.3-8.0 keV fluxes F 0.3-8.0 ≳ 5 ×10-14 erg cm-2 s-1. Modeling the X-ray spectra with an absorbed power law, we derive power-law photon indices and 2-10 keV luminosities for the eight detected quasars. In addition to presenting sample properties for larger quasar population studies and for use in planning for future caustic-crossing events, we also identify three quasars of interest: a quasar that shows evidence of flux variability from previous ROSAT observations, the most closely separated individual lensed sources resolved by XMM-Newton, and one of the X-ray brightest quasars known at z > 3. These sources represent the tip of the discoveries that will be enabled by SRG/eROSITA.Peer reviewe

An ASKAP search for a radio counterpart to the first high-significance neutron star-black hole merger LIGO/Virgo S190814bv

We present results from a search for a radio transient associated with the LIGO/Virgo source S190814bv, a likely neutron star–black hole (NSBH) merger, with the Australian Square Kilometre Array Pathfinder. We imaged a 30 deg² field at ΔT = 2, 9, and 33 days post-merger at a frequency of 944 MHz, comparing them to reference images from the Rapid ASKAP Continuum Survey observed 110 days prior to the event. Each epoch of our observations covers 89% of the LIGO/Virgo localization region. We conducted an untargeted search for radio transients in this field, resulting in 21 candidates. For one of these, AT2019osy, we performed multiwavelength follow-up and ultimately ruled out the association with S190814bv. All other candidates are likely unrelated variables, but we cannot conclusively rule them out. We discuss our results in the context of model predictions for radio emission from NSBH mergers and place constrains on the circum-merger density and inclination angle of the merger. This survey is simultaneously the first large-scale radio follow-up of an NSBH merger, and the most sensitive widefield radio transients search to-date