arXiv subjects

Michael F. Bietenholz

Publications and source records attributed to Michael F. Bietenholz.

At least 19 recordsLinked to original sources

arXiv2025

Continued Rapid Radio Brightening of the Tidal Disruption Event AT2018hyz

We present ongoing radio observations of the tidal disruption event (TDE) AT2018hyz, which was first detected in the radio at 972 days after disruption, following multiple non-detections from earlier searches. The new observations presented here span approximately 1370-2160 days and 0.88-240 GHz. We find that the light curves continue to rise at all frequencies during this time period, following a power law of about F ~ t^3 (compared to F_nu ~ t^5.7 at 972-1400 days), and reaching a peak luminosity of L~ 10^40 erg/s, comparable to the luminosity of the relativistic TDE Swift 1644+57 on the same timescale. The multi-frequency data indicate that the peak frequency does not significantly evolve over the 1030-day span of our observations, while the peak flux density increases by an order of magnitude. The observed behavior is consistent with two possible scenarios: (i) a delayed spherical outflow launched about 620 days post-disruption with a velocity of ~0.3c and an energy of ~10^50 erg, and (ii) a highly off-axis (~80-90 deg) relativistic jet with a Lorentz factor of Gamma ~8 and E_K ~ 10^52 erg. Continued radio observations to capture the light curve peak, as well as VLBI observations, could distinguish between these scenarios.

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arXiv2024

Ubiquitous Late Radio Emission from Tidal Disruption Events

We present radio observations of 23 optically discovered tidal disruption events (TDEs) on timescales of 500-3200 days post discovery. We detect nine new TDEs that did not have detectable radio emission at earlier times, indicating a late-time brightening after several hundred (and up to 2300) days; an additional seven TDEs exhibit radio emission whose origin is ambiguous or may be attributed to the host galaxy or an active galactic nucleus. We also report a new rising component in one TDE previously detected in the radio at 10^3 days. While the radio emission in some of the detected TDEs peaked on a timescale 2-4 yr, over half of the sample still show rising emission. The range of luminosities for the sample is 10^37-10^39 erg/s, about 2 orders of magnitude below the radio luminosity of the relativistic TDE Sw J1644+57. Our data set indicates 40% of all optical TDEs are detected in radio hundreds to thousands of days after discovery, and that this is probably more common than early radio emission peaking at 10^2 days. Using an equipartition analysis, we find evidence for a delayed launch of the radio-emitting outflows, with delay timescales of 500-2000 days, inferred velocities of 0.02-0.15c, and kinetic energies of 10^47-10^49 erg. We rule out off axis relativistic jets as a viable explanation for this population, and conclude delayed outflows are a more likely explanation, possibly from delayed disk formation. We conclude late radio emission marks a fairly ubiquitous but heretofore overlooked phase of TDE evolution.

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arXiv2023

The Bright Supernova 1996cr in the Circinus Galaxy Imaged with VLBI: Shell Structure with Complex Evolution

We present broadband 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 \sim$8700 d, is a power-law, with flux density, $S \propto ν^{-0.588 \pm 0.011}$ between 1 and 34 GHz, but may steepen at $>35$ 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 $(5.1 \pm 0.3) \times 10^{17}$ cm, and SN 1996cr has been expanding with a velocity of $4650 \pm 1060$ 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 $\sim$7000 km s$^{-1}$, and hint at the presence of a ring- or equatorial-belt-like structure rather than a complete spherical shell.

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arXiv2022

Seven years of coordinated Chandra-NuSTAR observations of SN2014C unfold the extreme mass-loss history of its stellar progenitor

We present the results from our seven-year long broad-band X-ray observing campaign of SN\,2014C with \emph{Chandra} and \emph{NuSTAR}. These coordinated observations represent the first look at the evolution of a young extragalactic SN in the 0.3-80 keV energy range in the years after core collapse. We find that the spectroscopic metamorphosis of SN\,2014C from an ordinary type Ib SN into an interacting SN with copious hydrogen emission is accompanied by luminous X-rays reaching $L_x\approx 5.6\times10^{40}\, \rm{erg\,s^{-1}}$ (0.3--100 keV) at $\sim 1000$ days post explosion and declining as $L_x\propto t^{-1}$ afterwards. The broad-band X-ray spectrum is of thermal origin and shows clear evidence for cooling after peak, with $T(t)\approx 20 \,{\rm keV}(t/t_{\rm pk})^{-0.5}$. Soft X-rays of sub-keV energy suffer from large photoelectric absorption originating from the local SN environment with $NH_{\rm int}(t)\approx3\times 10^{22}(t/400 \,\rm{days})^{-1.4}\,\rm{cm^{-2}}$. We interpret these findings as the result of the interaction of the SN shock with a dense ($n\approx 10^{5}-10^{6}\,\rm{cm^{-3}}$), H-rich disk-like circumstellar medium (CSM) with inner radius $\sim2\times 10^{16}$ cm and extending to $\sim 10^{17}$ cm. Based on the declining $NH_{\rm int}(t)$ and X-ray luminosity evolution, we infer a CSM mass of $\sim(1.2\,f$--2.0$\sqrt{f}) \rm{M_{\odot}}$, where $f$ is the volume filling factor. Finally, we place SN\,2014C in the context of 119 core-collapse SNe with evidence for strong shock interaction with a thick circumstellar medium and we highlight the challenges that the current mass-loss theories (including wave-driven mass loss, binary interaction and line-driven winds) face when interpreting the wide dynamic ranges of CSM parameters inferred from observations.

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arXiv2022

Radio Analysis of SN 2004C Reveals an Unusual CSM Density Profile as a Harbinger of Core Collapse

We present extensive multi-frequency VLA and VLBA observations of the radio-bright supernova (SN) IIb SN 2004C that span $\sim(40-2793)$ days post-explosion. We interpret the temporal evolution of the radio spectral energy distribution (SED) in the context of synchrotron self-absorbed (SSA) 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 $\sim0.06c$ that carries an energy of $\sim 10^{49}$ erg. Our modeling further reveals a flat CSM density profile $ρ_{\rm{CSM}} \propto R^{-0.03 \pm0.22}$ up to a break radius $R_{br} \approx (1.96 \pm 0.10) \times 10^{16}$ cm, with a steep density gradient following $ρ_{\rm{CSM}} \propto R^{-2.3 \pm 0.5}$ at larger radii. We infer that the flat part of the density profile corresponds to a CSM shell with mass $\sim0.021 M_{\odot}$, and that the progenitor's effective mass-loss rate varied with time over the range $(50-500) \times 10^{-5} M_{\odot} \rm{yr}^{-1}$ 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.

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arXiv2021

Searching for high-z radio galaxies with the MGCLS

We present results from a search for high-redshift radio galaxy (H$z$RG) candidates using 1.28 GHz data in the Abell 2751 field drawn from the MeerKAT Galaxy Cluster Legacy Survey (MGCLS). We use the H$z$RG criteria that a radio source is undetected in all-sky optical and infrared catalogues, and has a very steep radio spectrum. We cross-match the radio catalogue against multi-wavelength galaxy catalogues from DECaLS and AllWISE. For those radio sources with no multi-wavelength counterpart, we further implement a radio spectral index criterium of $α< -1$, using in-band spectral index measurements from the wide-band MeerKAT data. Using a 5$σ$ signal-to-noise cut on the radio flux densities, we find a total of 274 HzRG candidates: 179 ultra-steep spectrum sources, and 95 potential candidates which cannot be ruled out as they have no spectral information available. The spectral index assignments in this work are complete above a flux density of 0.3 mJy, at least an order of magnitude lower than existing studies in this frequency range or when extrapolating from lower frequency limits. Our faintest HzRG candidates with and without an in-band spectral index measurement have a 1.28\,GHz flux density of 57 $\pm$ 8 $μ$Jy and 68 $\pm$ 13 $μ$Jy, respectively. Although our study is not complete down to these flux densities, our results indicate that the sensitivity and bandwidth of the MGCLS data makes them a powerful radio resource to search for H$z$RG candidates in the Southern sky, with 20 of the MGCLS pointings having similar image quality as the Abell~2751 field and full coverage in both DECaLS and AllWISE. Data at additional radio frequencies will be needed for the faintest source populations, which could be provided in the near future by the MeerKAT UHF band (580 -- 1015 MHz) at a similar resolution ($\sim$ 8-10 arcsec).

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arXiv2021

A tidal disruption event coincident with a high-energy neutrino

Cosmic neutrinos provide a unique window into the otherwise-hidden mechanism of particle acceleration in astrophysical objects. A flux of high-energy neutrinos was discovered in 2013, and the IceCube Collaboration recently reported the likely association of one high-energy neutrino with a flare from the relativistic jet of an active galaxy pointed towards the Earth. However a combined analysis of many similar active galaxies revealed no excess from the broader population, leaving the vast majority of the cosmic neutrino flux unexplained. Here we present the likely association of a radio-emitting tidal disruption event, AT2019dsg, with a second high-energy neutrino. AT2019dsg was identified as part of our systematic search for optical counterparts to high-energy neutrinos with the Zwicky Transient Facility. The probability of finding any coincident radio-emitting tidal disruption event by chance is 0.5%, while the probability of finding one as bright in bolometric energy flux as AT2019dsg is 0.2%. Our electromagnetic observations can be explained through a multi-zone model, with radio analysis revealing a central engine, embedded in a UV photosphere, that powers an extended synchrotron-emitting outflow. This provides an ideal site for PeV neutrino production. Assuming that the association is genuine, our observations suggest that tidal disruption events with mildly-relativistic outflows contribute to the cosmic neutrino flux.

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arXiv2021

SN 2014C: VLBI image shows a shell structure and decelerated expansion

We report on new Very Long Baseline Interferometry radio measurements of supernova 2014C in the spiral galaxy NGC 7331, made with the European VLBI Network ~5 yr after the explosion, as well as on flux density measurements made with the Jansky Very Large Array (VLA). SN 2014C was an unusual supernova, initially of Type Ib, but over the course of ~1 yr it developed strong H$α$ lines, implying the onset of strong interaction with some H-rich circumstellar medium (CSM). The expanding shock-front interacted with a dense shell of circumstellar material during the first year, but has now emerged from the dense shell and is expanding into the lower density CSM beyond. Our new VLBI observations show a relatively clear shell structure and continued expansion with some deceleration, with a suggestion that the deceleration is increasing at the latest times. Our multi-frequency VLA observations show a relatively flat powerlaw spectrum with $S_ν\propto ν^{-0.56 \pm 0.03}$, and show no decline in the radio luminosity since $t\sim1$ yr.

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arXiv2021

The Radio Luminosity-Risetime Function of Core-Collapse Supernovae

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 the SNe were detected. We characterize the SN lightcurves near the peak using a two-parameter model, with $t_{\rm pk}$ being the time to rise to a peak and $L_{\rm pk}$ the spectral luminosity at that peak. Over all SNe in our sample at $D<100$ Mpc, we find that $t_{\rm pk} = 10^{1.7\pm0.9}$ d, and that $L_{\rm pk} = 10^{25.5\pm1.6}$ erg s$^{-1}$ Hz$^{-1}$, and therefore that generally, 50% of SNe will have $L_{\rm pk} < 10^{25.5}$ erg s$^{-1}$ Hz$^{-1}$. These $L_{\rm pk}$ values are ~30 times lower than those for only detected SNe. Types I b/c and II (excluding IIn's) have similar mean values of $L_{\rm pk}$ but the former have a wider range, whereas Type IIn SNe have ~10 times higher values with $L_{\rm pk} = 10^{26.5\pm1.1}$ erg s$^{-1}$ Hz$^{-1}$. As for $t_{\rm pk}$, Type I b/c have $t_{\rm pk}$ of only $10^{1.1\pm0.5}$ d while Type II have $t_{\rm pk} = 10^{1.6\pm1.0}$ and Type IIn the longest timescales with $t_{\rm pk} = 10^{3.1\pm0.7}$ d. We also estimate the distribution of progenitor mass-loss rates, $\dot M$, and find the mean and standard deviation of log$_{10}(\dot M/$Msol) yr$^{-1}$ are $-5.4\pm1.2$ (assuming $v_{\rm wind}=1000$ km s$^{-1}$) for Type I~b/c SNe, and $-6.9\pm1.4$ (assuming $v_{\rm wind} = 10$ km s$^{-1}$ for Type II SNe excluding Type IIn.

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arXiv2019

AT 2018cow VLBI: No Long-Lived Relativistic Outflow

We report on 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, ~$1.4 \times 10^{43}$ erg s$^{-1}$, larger than those of ordinary 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 ~$4 \times 10^{28}$ 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 <0.49$c$ by t = 98 d (3$σ$ limit). We also constrain the proper motion of AT 2018cow to be <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 one month very unlikely.

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arXiv2019

Recent VLBI Results on SN 1986J and the Possibility of FRBs Originating from Inside the Expanding Ejecta of Supernovae

We discuss our VLA and VLBI observations of supernova 1986J, which is characterized by a compact radio-bright component within the expanding shell of ejecta. No other supernova (SN) has such a central component at cm wavelengths. The central component therefore provides a unique probe of the propagation of radio signals at cm wavelengths through the ejecta of a young SN. Such a probe is important in the context of Fast Radio Bursts (FRB), which, in many models, are thought to be produced by young magnetars or neutron stars. The FRB signals would therefore have to propagate through the expanding SN ejecta. Our observations of the Type II SN 1986J show that the ejecta would remain opaque to cm-wave emission like FRBs for at least several decades after the explosion, and by the time the ejecta have become transparent, the contribution of the ejecta to the dispersion measure is likely small.

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arXiv2017

SN 2014C: VLBI Images of a Supernova Interacting with a Circumstellar Shell

We report on VLBI measurements of supernova 2014C at several epochs between $t = 384$ and 1057 days after the explosion. SN 2014C was an unusual supernova that initially had Type Ib optical spectrum, but after $t = 130$ d it developed a Type IIn spectrum with prominent H$α$ lines, suggesting the onset of strong circumstellar interaction. Our first VLBI observation was at $t = 384$ d, and we find that the outer radius of SN 2014C was $(6.40 \pm 0.26) \times 10^{16}$ cm (for a distance of 15.1 Mpc), implying an average expansion velocity of $19300 \pm 790$ \kms\ up to that time. At our last epoch, SN 2014C was moderately resolved and shows an approximately circular outline but with an enhancement of the brightness on the W side. The outer radius of the radio emission at $t = 1057$ d is $(14.9 \pm 0.6) \times 10^{16}$ cm. We find that the expansion between $t = 384$ and 1057 d is well described by a constant velocity expansion with $v = 13600 \pm 650$ \kms. SN 2014C had clearly been substantially decelerated by $t = 384$ d. Our measurements are compatible with a scenario where the expanding shock impacted upon a shell of dense circumstellar material during the first year, as suggested by the observations at other wavelengths, but had progressed through the dense shell by the time of the VLBI observations.

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arXiv2017

On the Possibility of Fast Radio Bursts from Inside Supernovae: The Case of SN 1986J

We discuss the possibility of obtaining Fast Radio Bursts (FRBs) from the interior of supernovae, in particular SN 1986J. Young neutron stars are involved in many of the possible scenarios for the origin of FRBs, and it has been suggested that the high dispersion measures observed in FRBs might be produced by the ionized material in the ejecta of associated supernovae. Using VLA and VLBI measurements of the Type IIn SN 1986J, which has a central compact component not so far seen in other supernovae, we can directly observe for the first time radio signals which originate in the interior of a young (~30 yr old) supernova. We show that at age 30 yr, any FRB signal at ~1 GHz would still be largely absorbed by the ejecta. By the time the ejecta have expanded so that a 1-GHz signal would be visible, the internal dispersion measure due to the SN ejecta would be below the values typically seen for FRBs. The high dispersion measures seen for the FRBs detected so far could of course be due to propagation through the intergalactic medium provided that the FRBs are at distances much larger than that of SN 1986J, which is 10 Mpc. We conclude that if FRBs originate in Type II SNe/SNRs, they would likely not become visible till 60 ~ 200 yr after the SN explosion.

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arXiv2017

SN 1986J VLBI. IV. The Nature of the Central Component

We report on VLA measurements between 1 and 45 GHz of the evolving radio spectral energy distribution (SED) of SN 1986J, made in conjunction with VLBI imaging. The SED of SN 1986J is unique among supernovae, and shows an inversion point and a high-frequency turnover. Both are due to the central component seen in the VLBI images, and both are progressing downward in frequency with time. The optically-thin spectral index of the central component is almost the same as that of the shell. We fit a simple model to the evolving SED consisting of an optically-thin shell and a partly-absorbed central component. The evolution of the SED is consistent with that of a homologously expanding system. Both components are fading, but the shell more rapidly. We conclude that the central component is physically inside the expanding shell, and not a surface hot-spot central only in projection. Our observations are consistent with the central component being due to interaction of the shock with the dense and highly-structured circumstellar medium that resulted from a period of common-envelope evolution of the progenitor. However a young pulsar-wind nebula or emission from an accreting black hole can also not be ruled out at this point.

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arXiv2017

SN 1986J VLBI. III. The Central Component Becomes Dominant

We present a new 5-GHz global-VLBI image of supernova 1986J, observed in 2014 at $t=31.6$ yr after the explosion, and compare it to previous images to show the evolution of the supernova. Our new image has a dynamic range of ~100 and a background rms noise level of 5.9 $μ$Jy beam$^{-1}$. There is no significant linear polarization, with the image peak being $<$3% polarized. The latest image is dominated by the compact central component, whose flux density is now comparable to that of the extended supernova shell. This central component is marginally resolved with a FWHM width of $900_{-500}^{+100} \; μ$as, corresponding to a radius of $r_{\rm comp}=6.7 _{-3.7}^{+0.7} \times 10^{16}$ cm for a distance of 10 Mpc. Using VLBI observations between 2002 and 2014, we measured the proper motions of both the central component and a hot-spot to the NE in the shell relative to the quasar 3C66A. The central component is stationary to within the uncertainty of 12 $μ$as yr$^{-1}$, corresponding to 570 km s$^{-1}$. Our observations argue in favor of the central component being located near the physical center of SN 1986J. The shell hot-spot had a mean velocity of 2810+-750 km s$^{-1}$ to the NE, which is consistent with it taking part in the homologous expansion of the shell seen earlier. The shell emission is evolving in a non-selfsimilar fashion, with the brightest emission shifting inwards within the structure, and with only relatively faint emission being seen near the outer edge and presumed forward shock. An animation is available.

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arXiv2015

New expansion rate measurements of the Crab Nebula in radio and optical

We present new radio measurements of the expansion rate of the Crab nebula's synchrotron nebula over a ~30-yr period. We find a convergence date for the radio synchrotron nebula of CE 1255 +- 27. We also re-evaluated the expansion rate of the optical line emitting filaments, and we show that the traditional estimates of their convergence dates are slightly biased. Using an un-biased Bayesian analysis, we find a convergence date for the filaments of CE 1091 +- 34 (~40 yr earlier than previous estimates). Our results show that both the synchrotron nebula and the optical line-emitting filaments have been accelerated since the explosion in CE 1054, but that the synchrotron nebula has been relatively strongly accelerated, while the optical filaments have been only slightly accelerated. The finding that the synchrotron emission expands more rapidly than the filaments supports the picture that the latter are the result of the Rayleigh-Taylor instability at the interface between the pulsar-wind nebula and the surrounding freely-expanding supernova ejecta, and rules out models where the pulsar wind bubble is interacting directly with the pre-supernova wind of the Crab's progenitor.

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arXiv2014

The variability of the Crab Nebula in radio: No radio counterpart to gamma-ray flares

We present new Jansky Very Large Array (VLA) radio images of the Crab Nebula at 5.5 GHz, taken at two epochs separated by 6 days about two months after a gamma-ray flare in 2012 July. We find no significant change in the Crab's radio emission localized to a region of <2 light-months in radius, either over the 6-day interval between our present observations or between the present observations and ones from 2001. Any radio counterpart to the flare has a radio luminosity of <~ $2 \times 10^{-4}$ times that of the nebula. Comparing our images to one from 2001, we do however find changes in radio brightness, up to 10% in amplitude, which occur on decade timescales throughout the nebula. The morphology of the changes is complex suggesting both filamentary and knotty structures. The variability is stronger, and the timescales likely somewhat shorter, nearer the centre of the nebula. We further find that even with the excellent uv~coverage and signal-to-noise of the VLA, deconvolution errors are much larger than the noise, being up to 1.2% of peak brightness of the nebula in this particular case.

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arXiv2013

VLBI Constraints on Type I b/c Supernovae

VLBI observations of supernovae and gamma-ray bursts provide almost the only way of obtaining spatially resolved information about the sources. In particular, a determination of the expansion velocity of the forward shock, as well as the geometry of the fireball and its evolution with time are possible for relatively nearby events, provided they are radio bright. Monitoring the expansion of the shock front can provide information on the density profiles of both the circumstellar material and on the ejecta. VLBI observations can also potentially resolve GRB jets which are not directed along the line of sight, providing crucial confirmation of relativistic expansion in such objects. This review gives an overview of recent results from supernovae, including the Type I b/c SNe 2011dh, 2009bb, and 2007gr, and discusses the prospects for future observations.

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