ORCID Profile
0000-0002-0592-4152
Current Organisations
York University
,
Hartebeestheok Radio Astronomy Observatory
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Publisher: Oxford University Press (OUP)
Date: 21-11-2019
Abstract: We report on Very Long Baseline Interferometry (VLBI) observations of the fast and blue optical transient (FBOT), AT 2018cow. At ∼62 Mpc, AT 2018cow is the first relatively nearby FBOT. The nature of AT 2018cow is not clear, although various hypotheses from a tidal disruption event to different kinds of supernovae have been suggested. It had a very fast rise time (3.5 d) and an almost featureless blue spectrum, although high photospheric velocities (40 000 km s−1) were suggested early on. The X-ray luminosity was very high, ∼1.4 × 1043 erg s−1, larger than those of ordinary supernovae (SNe), and more consistent with those of SNe associated with gamma-ray bursts. Variable hard X-ray emission hints at a long-lived ‘central engine.’ It was also fairly radio luminous, with a peak 8.4-GHz spectral luminosity of ∼4 × 1028 erg s−1 Hz−1, allowing us to make VLBI observations at ages between 22 and 287 d. We do not resolve AT 2018cow. Assuming a circularly symmetric source, our observations constrain the average apparent expansion velocity to be ${\\lt}0.49\\, c$ by t = 98 d (3σ limit). We also constrain the proper motion of AT 2018cow to be ${\\lt}0.51\\, c$. Since the radio emission generally traces the fastest ejecta, our observations make the presence of a long-lived relativistic jet with a lifetime of more than 1 month very unlikely.
Publisher: American Astronomical Society
Date: 02-2022
Abstract: We present deep X-ray and radio observations of the fast blue optical transient (FBOT) AT 2020xnd/ZTF 20acigmel at z = 0.2433 from 13 days to 269 days after explosion. AT 2020xnd belongs to the category of optically luminous FBOTs with similarities to the archetypal event AT 2018cow. AT 2020xnd shows luminous radio emission reaching L ν ≈ 8 × 10 29 erg s −1 Hz −1 at 20 GHz and 75 days post-explosion, accompanied by luminous and rapidly fading soft X-ray emission peaking at L X ≈ 6 × 10 42 erg s −1 . Interpreting the radio emission in the context of synchrotron radiation from the explosion’s shock interaction with the environment, we find that AT 2020xnd launched a high-velocity outflow ( v ∼ 0.1 c –0.2 c ) propagating into a dense circumstellar medium (effective M ̇ ≈ 10 − 3 M ⊙ yr −1 for an assumed wind velocity of v w = 1000 km s −1 ). Similar to AT 2018cow, the detected X-ray emission is in excess compared to the extrapolated synchrotron spectrum and constitutes a different emission component, possibly powered by accretion onto a newly formed black hole or neutron star. These properties make AT 2020xnd a high-redshift analog to AT 2018cow, and establish AT 2020xnd as the fourth member of the class of optically luminous FBOTs with luminous multiwavelength counterparts.
Publisher: Oxford University Press (OUP)
Date: 17-06-2020
Abstract: We present high-cadence multifrequency radio observations of the long gamma-ray burst (GRB) 190829A, which was detected at photon energies above 100 GeV by the High Energy Stereoscopic System (H.E.S.S.). Observations with the Meer Karoo Array Telescope (MeerKAT, 1.3 GHz) and Arcminute Microkelvin Imager – Large Array (AMI-LA, 15.5 GHz) began one day post-burst and lasted nearly 200 d. We used complementary data from Swift X-Ray Telescope (XRT), which ran to 100 d post-burst. We detected a likely forward shock component with both MeerKAT and XRT up to over 100 d post-burst. Conversely, the AMI-LA light curve appears to be dominated by reverse shock emission until around 70 d post-burst when the afterglow flux drops below the level of the host galaxy. We also present previously unpublished observations of the other H.E.S.S.-detected GRB, GRB 180720B from AMI-LA, which shows likely forward shock emission that fades in less than 10 d. We present a comparison between the radio emission from the three GRBs with detected very high energy (VHE) gamma-ray emission and a sensitivity-limited radio afterglow s le. GRB 190829A has the lowest isotropic radio luminosity of any GRB in our s le, but the distribution of luminosities is otherwise consistent, as expected, with the VHE GRBs being drawn from the same parent distribution as the other radio-detected long GRBs.
Publisher: American Astronomical Society
Date: 2022
Abstract: We present new radio and optical data, including very-long-baseline interferometry, as well as archival data analysis, for the luminous, decades-long radio transient FIRST J141918.9+394036. The radio data reveal a synchrotron self-absorption peak around 0.3 GHz and a radius of around 1.3 mas (0.5 pc) 26 yr post-discovery, indicating a blastwave energy ∼5 × 10 50 erg. The optical spectrum shows a broad [O iii ] λ 4959,5007 emission line that may indicate collisional excitation in the host galaxy, but its association with the transient cannot be ruled out. The properties of the host galaxy are suggestive of a massive stellar progenitor that formed at low metallicity. Based on the radio light curve, blastwave velocity, energetics, nature of the host galaxy and transient rates, we find that the properties of J1419+3940 are most consistent with long gamma-ray burst (LGRB) afterglows. Other classes of (optically discovered) stellar explosions as well as neutron star mergers are disfavored, and invoking any exotic scenario may not be necessary. It is therefore likely that J1419+3940 is an off-axis LGRB afterglow (as suggested by Law et al. and Marcote et al.), and under this premise the inverse beaming fraction is found to be f b − 1 ≃ 280 − 200 + 700 , corresponding to an average jet half-opening angle θ j ≃ 5 − 2 + 4 degrees (68% confidence), consistent with previous estimates. From the volumetric rate we predict that surveys with the Very Large Array, Australian Square Kilometre Array Pathfinder, and MeerKAT will find a handful of J1419+3940-like events over the coming years.
Publisher: Elsevier BV
Date: 08-2018
Publisher: IOP Publishing
Date: 27-07-2023
Abstract: The Einstein Equivalence Principle (EEP) is a cornerstone of general relativity and predicts the existence of gravitational redshift. We report on new results of measuring this shift with RadioAstron (RA), a space very long baseline interferometry (VLBI) spacecraft launched into an evolving high eccentricity orbit around Earth with geocentric distances reaching 353 000 km. The spacecraft and ground tracking stations at Pushchino, Russia, and Green Bank, USA, were each equipped with a hydrogen maser frequency standard allowing a possible violation of the predicted gravitational redshift, in the form of a violation parameter ɛ , to be measured. By alternating between RA’s frequency referencing modes during dedicated sessions between 2015 and 2017, the recorded downlink frequencies can essentially be corrected for the non-relativistic Doppler shift. We report on an analysis using the Doppler-tracking frequency measurements made during these sessions and find ε = ( 2.1 ± 3.3 ) × 10 − 4 . We also discuss prospects for measuring ɛ with a significantly smaller uncertainty using instead the time-domain recordings of the spacecraft signals and envision how 10 −7 might be possible for a future space VLBI mission.
Publisher: Oxford University Press (OUP)
Date: 12-02-2022
Abstract: Tidal disruption events (TDEs) occur when a star is destroyed by a supermassive black hole at the centre of a galaxy, temporarily increasing the accretion rate on to the black hole and producing a bright flare across the electromagnetic spectrum. Radio observations of TDEs trace outflows and jets that may be produced. Radio detections of the outflows from TDEs are uncommon, with only about one-third of TDEs discovered to date having published radio detections. Here, we present over 2 yr of comprehensive, multiradio frequency monitoring observations of the TDE AT2019azh taken with the Very Large Array and MeerKAT radio telescopes from approximately 10 d pre-optical peak to 810 d post-optical peak. AT2019azh shows unusual radio emission for a thermal TDE, as it brightened very slowly over 2 yr, and showed fluctuations in the synchrotron energy index of the optically thin synchrotron emission from 450 d post-disruption. Based on the radio properties, we deduce that the outflow in this event is likely non-relativistic and could be explained by a spherical outflow arising from self-stream intersections or a mildly collimated outflow from accretion on to the supermassive black hole. This data set provides a significant contribution to the observational data base of outflows from TDEs, including the earliest radio detection of a non-relativistic TDE to date, relative to the optical discovery.
Publisher: Oxford University Press (OUP)
Date: 31-10-2022
Abstract: Tidal disruption events (TDEs) occur when a star passes too close to a supermassive black hole and is destroyed by tidal gravitational forces. Radio observations of TDEs trace synchrotron emission from outflowing material that may be ejected from the inner regions of the accretion flow around the supermassive black hole or by the tidal debris stream. Radio detections of TDEs are rare, but provide crucial information about the launching of jets and outflows from supermassive black holes and the circumnuclear environment in galaxies. Here, we present the radio detection of the TDE AT2020opy, including three epochs of radio observations taken with the Karl G. Jansky Very Large Array, MeerKAT, and upgraded Giant Metrewave Radio telescope. AT2020opy is the most distant thermal TDE with radio emission reported to date, and from modelling the evolving synchrotron spectra we deduce that the host galaxy has a more dense circumnuclear medium than other thermal TDEs detected in the radio band. Based on an equipartition analysis of the synchrotron spectral properties of the event, we conclude that the radio-emitting outflow was likely launched approximately at the time of, or just after, the initial optical flare. We find no evidence for relativistic motion of the outflow. The high luminosity of this event supports that a dense circumnuclear medium of the host galaxy produces brighter radio emission that rises to a peak more quickly than in galaxies with lower central densities.
Publisher: Oxford University Press (OUP)
Date: 08-02-2023
Abstract: We present broad-band radio flux-density measurements supernova (SN) 1996cr, made with MeerKAT, ATCA, and ALMA, and images made from very long baseline interferometry (VLBI) observations with the Australian Long Baseline Array. The spectral energy distribution of SN 1996cr in 2020, at age t ∼8700 d, is a power-law, with flux density, S ∝ ν−0.588 ± 0.011 between 1 and 34 GHz, but may steepen at & GHz. The spectrum has flattened since t = 5370 d (2010). Also since t = 5370 d, the flux density has declined rapidly, with $S_{\\rm 9 \\, GHz} \\propto t^{-2.9}$. The VLBI image at t = 8859 d shows an approximately circular structure with a central minimum reminiscent of an optically-thin spherical shell of emission. For a distance of 3.7 Mpc, the average outer radius of the radio emission at t = 8859 d was (5.1 ± 0.3) × 1017 cm, and SN 1996cr has been expanding with a velocity of 4650 ± 1060 km s−1 between t = 4307 and 8859 d. It must have undergone considerable deceleration before t = 4307 d. Deviations from a circular shell structure in the image suggest a range of velocities up to ∼7000 km s−1, and hint at the presence of a ring- or equatorial-belt-like structure rather than a complete spherical shell.
Publisher: Oxford University Press (OUP)
Date: 27-04-2023
Abstract: A tidal disruption event (TDE) occurs when a star is destroyed by a supermassive black hole. Broad-band radio spectral observations of TDEs trace the emission from any outflows or jets that are ejected from the vicinity of the supermassive black hole. However, radio detections of TDEs are rare, with & published to date, and only 11 with multi-epoch broad-band coverage. Here we present the radio detection of the TDE AT2020vwl and our subsequent radio monitoring c aign of the outflow that was produced, spanning 1.5 yr post-optical flare. We tracked the outflow evolution as it expanded between 1016 and 1017 cm from the supermassive black hole, deducing it was non-relativistic and launched quasi-simultaneously with the initial optical detection through modelling the evolving synchrotron spectra of the event. We deduce that the outflow is likely to have been launched by material ejected from stream-stream collisions (more likely), the unbound debris stream, or an accretion-induced wind or jet from the supermassive black hole (less likely). AT2020vwl joins a growing number of TDEs with well-characterized prompt radio emission, with future timely radio observations of TDEs required to fully understand the mechanism that produces this type of radio emission in TDEs.
Publisher: American Astronomical Society
Date: 10-2022
Abstract: We present extensive multifrequency Karl G. Jansky Very Large Array (VLA) and Very Long Baseline Array (VLBA) observations of the radio-bright supernova (SN) IIb SN 2004C that span ∼40–2793 days post-explosion. We interpret the temporal evolution of the radio spectral energy distribution in the context of synchrotron self-absorbed emission from the explosion’s forward shock as it expands in the circumstellar medium (CSM) previously sculpted by the mass-loss history of the stellar progenitor. VLBA observations and modeling of the VLA data point to a blastwave with average velocity ∼0.06 c that carries an energy of ≈10 49 erg. Our modeling further reveals a flat CSM density profile ρ CSM ∝ R −0.03±0.22 up to a break radius R br ≈ (1.96 ± 0.10) × 10 16 cm, with a steep density gradient following ρ CSM ∝ R −2.3±0.5 at larger radii. We infer that the flat part of the density profile corresponds to a CSM shell with mass ∼0.021 M ☉ , and that the progenitor’s effective mass-loss rate varied with time over the range (50–500) × 10 −5 M ☉ yr −1 for an adopted wind velocity v w = 1000 km s −1 and shock microphysical parameters ϵ e = 0.1, ϵ B = 0.01. These results add to the mounting observational evidence for departures from the traditional single-wind mass-loss scenarios in evolved, massive stars in the centuries leading up to core collapse. Potentially viable scenarios include mass loss powered by gravity waves and/or interaction with a binary companion.
Publisher: American Astronomical Society
Date: 02-2021
Abstract: We assemble a large set of 2–10 GHz radio flux density measurements and upper limits of 294 different supernovae (SNe), from the literature and our own and archival data. Only 31% of SNe were detected. We characterize the SN radio lightcurves near the peak using a two-parameter model, with t pk being the time to rise to a peak and L pk the spectral luminosity at that peak. Over all SNe in our s le at D 100 Mpc, we find that t pk = 10 1.7±0.9 days and that L pk = 10 25.5±1.6 erg s −1 Hz −1 , and therefore that generally 50% of SNe will have L pk 10 25.5 erg s −1 Hz −1 . These L pk values are ∼30 times lower than those for only detected SNe. Types Ib/c and II (excluding IIn’s) have similar mean values of L pk but the former have a wider range, whereas Type IIn SNe have ∼10 times higher values with L pk = 10 26.5±1.1 erg s −1 Hz −1 . As for t pk , Type Ib/c have t pk of only 10 1.1±0.5 days while Type II have t pk = 10 1.6±1.0 and Type IIn the longest timescales with t pk = 10 3.1±0.7 days. We also estimate the distribution of progenitor mass-loss rates, , and find that the mean and standard deviation of are −5.4 ± 1.2 (assuming v wind = 1000 km s −1 ) for Type Ib/c SNe, and −6.9 ± 1.4 (assuming v wind = 10 km s −1 ) for Type II SNe excluding Type IIn.
Publisher: American Astronomical Society
Date: 10-2022
Abstract: We present late-time radio/millimeter (as well as optical/UV and X-ray) detections of tidal disruption event (TDE) AT2018hyz, spanning 970–1300 d after optical discovery. In conjunction with earlier deeper limits, including those at ≈700 days, our observations reveal rapidly rising emission at 0.8–240 GHz, steeper than F ν ∝ t 5 relative to the time of optical discovery. Such a steep rise cannot be explained in any reasonable scenario of an outflow launched at the time of disruption (e.g., off-axis jet, sudden increase in the ambient density), and instead points to a delayed launch. Our multifrequency data allow us to directly determine the radius and energy of the radio-emitting outflow, and we find from our modeling that the outflow was launched ≈750 days after optical discovery. The outflow velocity is mildly relativistic, with β ≈ 0.25 and ≈0.6 for a spherical geometry and a 10° jet geometry, respectively, and the minimum kinetic energy is E K ≈ 5.8 × 10 49 and ≈6.3 × 10 49 erg, respectively. This is the first definitive evidence for the production of a delayed mildly relativistic outflow in a TDE a comparison to the recently published radio light curve of ASASSN-15oi suggests that the final rebrightening observed in that event (at a single frequency and time) may be due to a similar outflow with a comparable velocity and energy. Finally, we note that the energy and velocity of the delayed outflow in AT2018hyz are intermediate between those of past nonrelativistic TDEs (e.g., ASASSN-14li, AT2019dsg) and the relativistic TDE Sw J1644+57. We suggest that such delayed outflows may be common in TDEs.
No related grants have been discovered for Michael Bietenholz.