arXiv subjects

Dillon Dong

Publications and source records attributed to Dillon Dong.

14 recordsLinked to original sources

arXiv2026

VLASS Discovery of a Luminous Galactic Radio Transient Evolving on Decade Timescales

We present a multiwavelength analysis of the radio transient VT J1906+0849, discovered as a 70 mJy source in Epoch 1 of the Very Large Array Sky Survey (VLASS), 21 yr after an NRAO VLA Sky Survey (NVSS) non-detection. Radio observations reveal the source was first detected in 2005, peaking at $\gtrsim200$ mJy in 2014, then declining until a late 2025 rebrightening. The transient sits at a Galactic latitude of $\approx0.74^\circ$ and the properties of its optical-infrared counterpart support a Galactic origin. At $d\gtrsim15$ kpc, the extreme radio luminosity is likely powered by sustained accretion onto a compact object. However, a Swift-XRT non-detection shows it is X-ray faint relative to the Galactic X-ray binary population, and the radio emission is distinct from X-ray binaries in its temporal and spectral behavior. Broadband radio spectra suggest synchrotron self-absorption, but size constraints from equipartition and very long baseline interferometry show little to no expansion in the radio-emitting region over 5+ yr, despite significant spectral evolution. Near-infrared spectroscopy reveals a single broad emission line with a stable centroid but variable width and luminosity. We attribute this feature to blueshifted Br$\gamma$ tracing a persistent asymmetric $\approx2000$ km s$^{-1}$ outflow. These properties are unlike any previously identified Galactic radio source. One possible interpretation is that VT J1906+0849 is a young analog of the microquasar SS 433, with a dense disk wind confining a continuously powered synchrotron outflow. This jet-wind interaction explains the compact, slowly expanding radio source and may contribute to the absence of bright X-ray emission.

astro-ph.HE↗

arXiv2025

Radio Variability in Recently-Quenched Galaxies: The Impact of TDE or AGN Driven Outflows

Outflows and jets launched from the nuclei of galaxies emit radio synchrotron emission that can be used to study the impact of accretion energy on the host galaxy. The decades-long baseline now enabled by large radio surveys allows us to identify cases where new outflows or jets have been launched. Here, we present the results of a targeted VLA program observing four post-starburst galaxies that have brightened significantly in radio emission over the past ~20 years. We obtain quasi-simultaneous observations in five bands (1-18 GHz) for each source. We find peaked spectral energy distributions, indicative of self-absorbed synchrotron emission. While all four sources have risen significantly over the past ~20 years in the 1-2 GHz band, two also show clear recent flares in the 2-4 GHz band. These sources are less luminous than typical peaked spectrum radio AGN. It remains unclear whether these sources are low luminosity analogs of the peaked radio AGN from accreted gas, or driven by tidal disruption events with missed optical flares. Regardless of the source of the accreted material, these newly-launched outflows contain sufficient energy to drive the molecular gas outflows observed in post-starburst galaxies and to drive turbulence suppressing star formation.

astro-ph.GA↗

arXiv2024

The Symbiotic Recurrent Nova V745 Sco at Radio Wavelengths

V745 Sco is a Galactic symbiotic recurrent nova with nova eruptions in 1937, 1989 and 2014. We study the behavior of V745 Sco at radio wavelengths (0.6-37,GHz), covering both its 1989 and 2014 eruptions and informed by optical, X-ray, and $\gamma$-ray data. The radio light curves are synchrotron-dominated. Surprisingly, compared to expectations for synchrotron emission from explosive transients such as radio supernovae, the light curves spanning 0.6-37 GHz all peak around the same time ($\sim$18-26 days after eruption) and with similar flux densities (5-9 mJy).We model the synchrotron light curves as interaction of the nova ejecta with the red giant wind, but find that simple spherically symmetric models with wind-like circumstellar material (CSM) cannot explain the radio light curve. Instead, we conclude that the shock suddenly breaks out of a dense CSM absorbing screen around 20 days after eruption, and then expands into a relatively low density wind ($\dot{M}_{out} \approx 10^{-9}-10^{-8}$ M$_{\odot}$ yr$^{-1}$ for $v_w = 10$ km s$^{-1}$) out to $\sim$1 year post-eruption. The dense, close-in CSM may be an equatorial density enhancement or a more spherical red giant wind with $\dot{M}_{in} \approx [5-10] \times 10^{-7}$ M$_{\odot}$ yr$^{-1}$, truncated beyond several $\times 10^{14}$ cm. The outer lower-density CSM would not be visible in typical radio observations of Type Ia supernovae: V745 Sco cannot be ruled out as a Type Ia progenitor based on CSM constraints alone.Complementary constraints from the free-free radio optical depth and the synchrotron luminosity imply the shock is efficient at accelerating relativistic electrons and amplifying magnetic fields, with $\epsilon_e$ and $\epsilon_B \approx 0.01-0.1$.

astro-ph.HE↗

arXiv2024

Detection of Radio Emission from Super-flaring Solar-Type Stars in the VLA Sky Survey

Solar-type stars have been observed to flare at optical wavelengths to energies much higher than observed for the Sun. To date, no counterparts have been observed at longer wavelengths. We have searched the the VLA Sky Survey (VLASS) for radio emission associated with a sample of 150 single, solar-type stars previously been observed to exhibit superflares in the Transiting Exoplanet Survey Satellite (TESS). Counterparts to six of these stars were present in VLASS as transient or highly variable radio sources. One of the stars is detected in all three epochs, exhibiting an unprecedented level of apparently persistent radio emission. The engine for this radio emission is unclear, but may be related to accretion, a binary companion, or the presence of large-scale magnetic field. Two stars show radio emission with >50 circular polarization fraction, indicating a coherent emission process likely being present. We find that the six VLASS-detected stars tend to have higher flare rates and higher flare energies of our TESS sample. This, in addition to the VLASS-detected stars adhering to the Gudel-Benz relation, suggest that the radio emission may be directly associated with superflares. These results confirm that the superflare phenomenon on solar-type stars extends to radio wavelengths, in this instance tracing particle acceleration. These data provide the first window on the luminosity function of radio superflares for solar-type stars and highlights the need for coordinated, multi-wavelength monitoring of such stars to fully illustrate the stellar flare-particle relation.

astro-ph.SR↗

arXiv2023

NANCY: Next-generation All-sky Near-infrared Community surveY

The Nancy Grace Roman Space Telescope is capable of delivering an unprecedented all-sky, high-spatial resolution, multi-epoch infrared map to the astronomical community. This opportunity arises in the midst of numerous ground- and space-based surveys that will provide extensive spectroscopy and imaging together covering the entire sky (such as Rubin/LSST, Euclid, UNIONS, SPHEREx, DESI, SDSS-V, GALAH, 4MOST, WEAVE, MOONS, PFS, UVEX, NEO Surveyor, etc.). Roman can uniquely provide uniform high-spatial-resolution (~0.1 arcsec) imaging over the entire sky, vastly expanding the science reach and precision of all of these near-term and future surveys. This imaging will not only enhance other surveys, but also facilitate completely new science. By imaging the full sky over two epochs, Roman can measure the proper motions for stars across the entire Milky Way, probing 100 times fainter than Gaia out to the very edge of the Galaxy. Here, we propose NANCY: a completely public, all-sky survey that will create a high-value legacy dataset benefiting innumerable ongoing and forthcoming studies of the universe. NANCY is a pure expression of Roman's potential: it images the entire sky, at high spatial resolution, in a broad infrared bandpass that collects as many photons as possible. The majority of all ongoing astronomical surveys would benefit from incorporating observations of NANCY into their analyses, whether these surveys focus on nearby stars, the Milky Way, near-field cosmology, or the broader universe.

astro-ph.IM↗

arXiv2022

Deep Synoptic Array science I: discovery of the host galaxy of FRB 20220912A

We report the detection and interferometric localization of the repeating fast radio burst (FRB) source FRB 20220912A during commissioning observations with the Deep Synoptic Array (DSA-110). Two bursts were detected from FRB 20220912A, one each on 2022 October 18 and 2022 October 25. The best-fit position is (R.A. J2000, decl. J2000) = (23:09:04.9, +48:42:25.4), with a 90% confidence error ellipse of $\pm2$ arcsec and $\pm1$ arcsec in right ascension and declination respectively. The two bursts have disparate polarization properties and temporal profiles. We find a Faraday rotation measure that is consistent with the low value of $+0.6$ rad m$^{-2}$ reported by CHIME/FRB. The DSA-110 localization overlaps with the galaxy PSO J347.2702+48.7066 at a redshift $z=0.0771$, which we identify as the likely host. PSO J347.2702$+$48.7066 has a stellar mass of approximately $10^{10}M_{\odot}$, modest internal dust extinction, and a star-formation rate likely in excess of $0.1\,M_{\odot}$ yr$^{-1}$. The host-galaxy contribution to the dispersion measure is likely $\lesssim50$ pc cm$^{-3}$. The FRB 20220912A source is therefore likely viewed along a tenuous plasma column through the host galaxy.

astro-ph.HE↗

arXiv2022

A Flat-Spectrum Radio Transient at 122 Mpc consistent with an Emerging Pulsar Wind Nebula

We report the discovery and follow-up observations of VT 1137-0337: an unusual radio transient found in our systematic search for extragalactic explosions in the VLA Sky Survey (VLASS). VT 1137-0337 is located in the brightest region of a dwarf starburst galaxy (stellar mass $\sim 10^{8.3} M_{\odot}$, star formation rate $\sim 0.5 M_{\odot}$ yr$^{-1}$) at a luminosity distance of 121.6 Mpc. Its 3 GHz luminosity of $\sim 2.5 \times 10^{28}$ erg s$^{-1}$ Hz$^{-1}$ is comparable to luminous radio supernovae associated with dense circumstellar interaction and relativistic outflows. However, its broadband radio spectrum - a featureless power law $\propto \nu^{-0.35 \pm 0.02}$ over a range of $\gtrsim$10$\times$ in frequency and fading at a rate of $\sim$ 5% per year over 4 years - cannot be directly explained by the shock of a stellar explosion. Jets launched by various classes of accreting black holes also struggle to account for VT 1137-0337's combination of observational properties. Instead, we propose that VT 1137-0337 is a $\sim$decades old pulsar wind nebula that has recently emerged from within the free-free opacity of its surrounding supernova ejecta. If the nebula is powered by spindown, the central neutron star should be highly magnetized, with a surface dipole field of $\sim 10^{13} - 10^{14}$ G and a present-day spin period of $\sim 10 - 100$ ms. Alternatively, the nebula may be powered by the release of magnetic energy from a magnetar. Magnetar nebulae have been proposed to explain the persistent radio sources associated with the repeating fast radio bursts FRB 121102 and FRB 190520B. These FRB persistent sources have not previously been observed as transients, but do bear a striking resemblance to VT 1137-0337 in their radio luminosity, spectral index, and host galaxy properties.

astro-ph.HE↗

arXiv2021

The nascent milliquasar VT J154843.06+220812.6: tidal disruption event or extreme accretion-state change?

We present detailed multiwavelength follow up of a nuclear radio flare, VT J154843.06+220812.6, hereafter VT J1548. VT J1548 was selected as a ${\sim}1$ mJy radio flare in 3 GHz observations from the VLA Sky Survey (VLASS). It is located in the nucleus of a low mass ($\log M_{\rm BH}/M_\odot \sim6$) host galaxy with weak or no past AGN activity. VT J1548 is associated with a slow rising (multiple year), bright mid IR flare in the WISE survey, peaking at ${\sim}10\%L_{\rm edd.}$. No associated optical transient is detected, although we cannot rule out a short, early optical flare given the limited data available. Constant late time (${\sim}3$ years post-flare) X-ray emission is detected at ${\sim}10^{42}$ erg s$^{-1}$. The radio SED is consistent with synchrotron emission from an outflow incident on an asymmetric medium. A follow-up, optical spectrum shows transient, bright, high-ionization coronal line emission ($[{\rm Fe\,X}]\,\lambda 6375,[{\rm Fe\,XI}]\,\lambda 7894,[{\rm S\,XII}]\,\lambda 7612$). Transient broad H$\alpha$ is also detected but without corresponding broad H$\beta$ emission, suggesting high nuclear extinction. We interpret this event as either a tidal disruption event or an extreme flare of an active galactic nucleus, in both cases obscured by a dusty torus. Although these individual properties have been observed in previous transients, the combination is unprecedented. This event highlights the importance of searches across all wave bands for assembling a sample of nuclear flares that spans the range of observable properties and possible triggers.

astro-ph.HE↗

arXiv2021

FIRST J153350.8+272729: the radio afterglow of a decades-old tidal disruption event

We present the discovery of the fading radio transient FIRST J153350.8+272729. The source had a maximum observed 5-GHz radio luminosity of $8\times10^{39}$ erg s$^{-1}$ in 1986, but by 2019 had faded by a factor of nearly 400. It is located 0.15 arcsec from the center of a galaxy (SDSS J153350.89+272729) at 147 Mpc, which shows weak Type II Seyfert activity. We show that a tidal disruption event (TDE) is the preferred scenario for FIRST J153350.8+272729, although it could plausibly be interpreted as the afterglow of a long-duration gamma-ray burst. This is only the second TDE candidate to be first discovered at radio wavelengths. Its luminosity fills a gap between the radio afterglows of sub-relativistic TDEs in the local universe, and relativistic TDEs at high redshifts. The unusual properties of FIRST J153350.8+272729 (ongoing nuclear activity in the host galaxy, high radio luminosity) motivate more extensive TDE searches in untargeted radio surveys.

astro-ph.HE↗

arXiv2020

Multi-wavelength Observations of AT2019wey: a New Candidate Black Hole Low-mass X-ray Binary

AT2019wey (SRGA J043520.9+552226, SRGE J043523.3+552234) is a transient first reported by the ATLAS optical survey in 2019 December. It rose to prominence upon detection, three months later, by the Spektrum-Roentgen-Gamma (SRG) mission in its first all-sky survey. X-ray observations reported in Yao et al. suggest that AT2019wey is a Galactic low-mass X-ray binary (LMXB) with a black hole (BH) or neutron star (NS) accretor. Here we present ultraviolet, optical, near-infrared, and radio observations of this object. We show that the companion is a short-period (P < 16 hr) low-mass (< 1 Msun) star. We consider AT2019wey to be a candidate BH system since its locations on the L_radio--L_X and L_opt--L_X diagrams are closer to BH binaries than NS binaries. We demonstrate that from 2020 June to August, despite the more than 10 times brightening at radio and X-ray wavelengths, the optical luminosity of AT2019wey only increased by 1.3--1.4 times. We interpret the UV/optical emission before the brightening as thermal emission from a truncated disk in a hot accretion flow and the UV/optical emission after the brightening as reprocessing of the X-ray emission in the outer accretion disk. AT2019wey demonstrates that combining current wide-field optical surveys and SRG provides a way to discover the emerging population of short-period BH LMXB systems with faint X-ray outbursts.

astro-ph.HE↗

arXiv2020

Localized FRBs are Consistent with Magnetar Progenitors Formed in Core-Collapse Supernovae

With the localization of fast radio bursts (FRBs) to galaxies similar to the Milky Way and the detection of a bright radio burst from SGR J1935+2154 with energy comparable to extragalactic radio bursts, a magnetar origin for FRBs is evident. By studying the environments of FRBs, evidence for magnetar formation mechanisms not observed in the Milky Way may become apparent. In this paper, we use a sample of FRB host galaxies and a complete sample of core-collapse supernova (CCSN) hosts to determine whether FRB progenitors are consistent with a population of magnetars born in CCSNe. We also compare the FRB hosts to the hosts of hydrogen-poor superluminous supernovae (SLSNe-I) and long gamma-ray bursts (LGRBs) to determine whether the population of FRB hosts is compatible with a population of transients that may be connected to millisecond magnetars. After using a novel approach to scale the stellar masses and star-formation rates of each host galaxy to be statistically representative of $z=0$ galaxies, we find that the CCSN hosts and FRBs are consistent with arising from the same distribution. Furthermore, the FRB host distribution is inconsistent with the distribution of SLSNe-I and LGRB hosts. With the current sample of FRB host galaxies, our analysis shows that FRBs are consistent with a population of magnetars born through the collapse of giant, highly magnetic stars.

astro-ph.HE↗

arXiv2020

Variable Radio AGN at High Redshift Identified in the VLA Sky Survey

As part of an on-going study of radio transients in Epoch 1 (2017-2019) of the Very Large Array Sky Survey (VLASS), we have discovered a sample of 0.2<z<3.2 active galactic nuclei (AGN) selected in the optical/infrared that have recently brightened dramatically in the radio. These sources would have previously been classified as radio-quiet based on upper limits from the Faint Images of the Radio Sky at Twenty-centimeters (FIRST; 1993-2011) survey; however, they are now consistent with radio-loud quasars. We present a quasi-simultaneous, multi-band (1-18 GHz) VLA follow-up campaign of our sample of AGN with extreme radio variability. We conclude that the radio properties are most consistent with AGN that have recently launched jets within the past few decades, potentially making them among the youngest radio AGN known.

astro-ph.GA↗

arXiv2018

Science with an ngVLA: Radio Continuum Emission from Galaxies: An Accounting of Energetic Processes

Radio continuum observations have proven to be a workhorse in our understanding of the star formation process (i.e., stellar birth and death) from galaxies both in the nearby universe and out to the highest redshifts. In this article we focus on how the ngVLA will transform our understanding of star formation by enabling one to map and decompose the radio continuum emission from large, heterogeneous samples of nearby galaxies on $\gtrsim 10$\,pc scales to conduct a proper accounting of the energetic processes powering it. At the discussed sensitivity and angular resolution, the ngVLA will simultaneously be able to create maps of current star formation activity at $\sim$100\,pc scales, as well as detect and characterize (e.g., size, spectral shape, density, etc.) discrete H{\sc ii} regions and supernova remnants on 10\,pc scales in galaxies out to the distance of the Virgo cluster. Their properties can then be used to see how they relate to the local and global ISM and star formation conditions. Such investigations are essential for understanding the astrophysics of high-$z$ measurements of galaxies, allowing for proper modeling of galaxy formation and evolution.

astro-ph.GA↗

arXiv2015

Microwave Continuum Emission and Dense Gas Tracers in NGC 3627: Combining Jansky VLA and ALMA Observations

We present Karl G. Jansky Very Large Array (VLA) Ka band (33 GHz) and Atacama Large Millimeter Array (ALMA) Band 3 (94.5 GHz) continuum images covering the nucleus and two extranuclear star-forming regions within the nearby galaxy NGC 3627 (M 66), observed as part of the Star Formation in Radio Survey (SFRS). Both images achieve an angular resolution of $\lesssim$2\arcsec, allowing us to map the radio spectral indices and estimate thermal radio fractions at a linear resolution of $\lesssim$90 pc at the distance of NGC 3627. The thermal fraction at 33 GHz reaches unity at and around the peaks of each HII region; we additionally observed the spectral index between 33 and 94.5 GHz to become both increasingly negative and positive away from the peaks of the HII regions, indicating an increase of non-thermal extended emission from diffusing cosmic-ray electrons and the possible presence of cold dust, respectively. While the ALMA observations were optimized for collecting continuum data, they also detected line emission from the $J=1\rightarrow0$ transitions of HCN and HCO$^{+}$. The peaks of dense molecular gas traced by these two spectral lines are spatially offset from the peaks of the 33 and 94.5 GHz continuum emission for the case of the extranuclear star-forming regions, indicating that our data reach an angular resolution at which one can spatially distinguish sites of recent star formation from the sites of future star formation. Finally, we find trends of decreasing dense gas fraction and velocity dispersion with increasing star formation efficiency among the three regions observed, indicating that the dynamical state of the dense gas, rather than its abundance, plays a more significant role in the star formation process.

astro-ph.GA↗