arXiv subjects

Brian D. Metzger

Publications and source records attributed to Brian D. Metzger.

At least 127 records · Page 7Linked to original sources

arXiv2021

A Program for Multi-Messenger Standard Siren Cosmology in the Era of LIGO A+, Rubin Observatory, and Beyond

The most promising variation of the standard siren technique combines gravitational-wave (GW) data for binary neutron star (BNS) mergers with redshift measurements enabled by their electromagnetic (EM) counterparts, to constrain cosmological parameters such as $H_0$, $Ω_m$, and $w_0$. Here we evaluate the near- and long-term prospects of multi-messenger cosmology in the era of future GW observatories: Advanced LIGO Plus (A+, 2025), Voyager-like detectors (2030s), and Cosmic Explorer-like detectors (CE, 2035 and beyond). We show that the BNS horizon distance of $\approx 700$ Mpc for A+ is well-matched to the sensitivity of the Vera C. Rubin Observatory (VRO) for kilonova detections. We find that one year of joint A+ and VRO observations will constrain the value of $H_0$ to percent-level precision, given a small investment of VRO time dedicated to target-of-opportunity GW follow-up. In the Voyager era, the BNS-kilonova observations begin to constrain $Ω_m$ with an investment of a few percent of VRO time. With the larger BNS horizon distance in the Cosmic Explorer era, on-axis short gamma-ray bursts (SGRBs) and their afterglows (though accompanying only some of the GW-detected mergers) supplant kilonovae as the most promising counterparts for redshift identification. We show that five years of joint observations with Cosmic Explorer-like facilities and a next-generation gamma-ray satellite with localization capabilities similar to that presently possible with Swift could constrain both $Ω_m$ and $w_0$ to $15-20\%$. We therefore advocate for a robust target-of-opportunity (ToO) program with VRO, and a wide-field gamma-ray satellite with improved sensitivity in the 2030s, to enable standard siren cosmology with next-generation gravitational wave facilities.

astro-ph.CO↗

arXiv2021

High-Energy Neutrinos and Gamma-Rays from Non-Relativistic Shock-Powered Transients

Shock interaction has been argued to play a role in powering a range of optical transients, including supernovae (particularly the superluminous class), classical novae, stellar mergers, tidal disruption events, and fast blue optical transients. These same shocks can accelerate relativistic ions, generating high-energy neutrino and gamma-ray emission via hadronic pion production. The recent discovery of time-correlated optical and gamma-ray emission in classical novae has revealed the important role of radiative shocks in powering these events, enabling an unprecedented view of the properties of ion acceleration, including its efficiency and energy spectrum, under similar physical conditions to shocks in extragalactic transients. Here we introduce a model for connecting the radiated optical fluence of non-relativistic transients to their maximal neutrino and gamma-ray fluence. We apply this technique to a wide range of extragalactic transient classes in order to place limits on their contributions to the cosmological high-energy gamma-ray and neutrino backgrounds. Based on a simple model for diffusive shock acceleration at radiative shocks, calibrated to novae, we demonstrate that several of the most luminous transients can accelerate protons up to energies $E_{\rm max} \gtrsim 10^{16}$ eV, sufficient to contribute to the IceCube astrophysical background. Furthermore, several of the considered sources$-$particularly hydrogen-poor supernovae$-$may serve as "hidden" gamma-ray sources due to the high gamma-ray opacity of their ejecta, evading constraints imposed by the non-blazar Fermi-LAT background. However, adopting an ion acceleration efficiency $\sim$ 0.3-1$\%$ motivated by nova observations, we find that currently known classes of non-relativistic, potentially shock-powered transients contribute at most a few percent of the total IceCube background.

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arXiv2020

A "blue ring nebula" surrounding a thousands of years old stellar merger

Stellar mergers are a brief-lived but common phase in the evolution of binary star systems. Among the many astrophysical implications of these events include the creation of atypical stars (e.g. magnetic stars, blue stragglers, rapid rotators), interpretation of stellar populations, and formation channels of LIGO-detected compact object mergers. Although stellar mergers are thus commonly invoked phenomena, observations of these events and details of their evolution remain elusive. While a handful of stellar mergers have been directly observed in recent years, the central remnants of these events remain shrouded by an opaque shell of dust and molecules, making it impossible to observe their final state (e.g. as a single merged star or a tighter surviving binary). Here we report observations of an unusual, ring-shaped ultraviolet nebula and the star at its center, TYC 2597-735-1. The nebula shows two opposing fronts, suggesting a bipolar outflow from TYC 2597-735-1. TYC 2597-735-1's spectrum and proximity above the Galactic plane suggest it is an old star, yet it shows abnormally low surface gravity and a detectable long-term luminosity decay, uncharacteristic for its evolutionary stage. TYC 2597-735-1 also exhibits H-alpha emission, radial velocity variations, enhanced ultraviolet radiation, and excess infrared emission, common signposts of dusty circumstellar disks, stellar activity, and accretion. The combined observations, paired with stellar evolution models, suggests TYC 2597-735-1 merged with a lower-mass companion several thousand years ago. TYC 2597-735-1 provides a look at an unobstructed stellar merger found at an evolutionary stage between its dynamic onset and the theorized final equilibrium state, directly inferring how two stars merge into a single star.

astro-ph.SR↗

arXiv2020

New Insights into Classical Novae

We survey our understanding of classical novae: non-terminal, thermonuclear eruptions on the surfaces of white dwarfs in binary systems. The recent and unexpected discovery of GeV gamma-rays from Galactic novae has highlighted the complexity of novae and their value as laboratories for studying shocks and particle acceleration. We review half a century of nova literature through this new lens, and conclude: --The basics of the thermonuclear runaway theory of novae are confirmed by observations. The white dwarf sustains surface nuclear burning for some time after runaway, and until recently, it was commonly believed that radiation from this nuclear burning solely determines the nova's bolometric luminosity. --The processes by which novae eject material from the binary system remain poorly understood. Mass loss from novae is complex (sometimes fluctuating in rate, velocity, and morphology) and often prolonged in time over weeks, months, or years. --The complexity of the mass ejection leads to gamma-ray producing shocks internal to the nova ejecta. When gamma-rays are detected (around optical maximum), the shocks are deeply embedded and the surrounding gas is very dense. --Observations of correlated optical and gamma-ray light curves confirm that the shocks are radiative and contribute significantly to the bolometric luminosity of novae. Novae are therefore the closest and most common "interaction-powered" transients.

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arXiv2020

Fermi-LAT Observations of V549 Vel 2017: a Sub-Luminous Gamma-Ray Nova?

We report on the Fermi-LAT detection (with ~5.7 sigma significance) as well as the multi-wavelength analysis of the 2017 nova eruption V549 Vel. Unlike the recent shock-powered novae ASASSN-16ma and V906 Car, the optical and gamma-ray light curves of V549 Vel show no correlation, likely implying relatively weak shocks in the eruption. Gaia detected a candidate progenitor of V549 Vel and found a parallax measurement of 1.91+/-0.39 mas, equivalent to a mode distance of d=560 pc (90% credible interval of 380-1050 pc). The progenitor was also observed by the 2MASS and WISE surveys. When adopting the Gaia distance, the spectral energy distribution of the progenitor is close to that of a G-type star. Swift/XRT detected the supersoft X-ray emission of the nova (kT=30-40 keV) since day 236, and the inferred blackbody size is comparable to that of other novae assuming d=560 pc (i.e., R_bb~5*10^8 cm). However, there is also an unknown astrometric excess noise of 3.2 mas found in the Gaia data, and the inferred distance becomes controversial. If the Gaia distance is accurate, the gamma-ray luminosity of V549 Vel will be as low as L_gamma~4*10^33 erg/s, making it the least luminous gamma-ray nova known so far. This may imply that the shock properties responsible for the gamma-ray emission in V549 Vel are different from those of the more luminous events. If the nova is located farther away, it is likely a symbiotic system with a giant companion as the observed progenitor.

astro-ph.HE↗

arXiv2020

Reconstructing Masses of Merging Neutron Stars from Stellar $R$-Process Abundance Signatures

Neutron star mergers (NSMs) are promising astrophysical sites for the rapid neutron-capture ("$r$-") process, but can their integrated yields explain the majority of heavy-element material in the Galaxy? One method to address this question has utilized a forward approach that propagates NSM rates and yields along with stellar formation rates, in the end comparing those results with observed chemical abundances of $r$-process-rich, metal-poor stars. In this work, we take the inverse approach by utilizing $r$-process-element abundance ratios of metal-poor stars as input to reconstruct the properties---especially the masses---of the neutron star (NS) binary progenitors of the $r$-process stars. This novel analysis provides an independent avenue for studying the population of the original neutron star binary systems that merged and produced the $r$-process material incorporated in Galactic metal-poor halo stars. We use ratios of elements typically associated with the limited-$r$ process and the actinide region to those in the lanthanide region (i.e., Zr/Dy and Th/Dy) to probe the NS masses of the progenitor merger. We find that NSMs can account for all $r$-process material in metal-poor stars that display $r$-process signatures, while simultaneously reproducing the present-day distribution of double-NS (DNS) systems. However, the most $r$-process enhanced stars (the $r$-II stars) on their own would require progenitor NSMs of very asymmetric systems that are distinctly different from present ones in the Galaxy. As this analysis is model-dependent, we also explore variations in line with future expectation regarding potential theoretical and observational updates, and comment on how these variations impact our results.

astro-ph.HE↗

arXiv2020

A Late-time Radio Survey of Short GRBs at $z<0.5$: New Constraints on the Remnants of Neutron Star Mergers

Massive, rapidly-spinning magnetar remnants produced as a result of binary neutron star (BNS) mergers may deposit a fraction of their energy into the surrounding kilonova ejecta, powering a synchrotron radio signal from the interaction of the ejecta with the circumburst medium. We present 6.0 GHz Very Large Array (VLA) observations of nine, low-redshift short gamma-ray bursts (SGRBs; $z<0.5$) on rest-frame timescales of $\approx2.4-13.9$ yr following the bursts. We place $3σ$ limits on radio continuum emission of $F_ν\lesssim6-20\,μ$Jy at the burst positions, or $L_ν\lesssim(0.6-8.3)\times10^{28}$erg s$^{-1}$Hz$^{-1}$. Comparing these limits with new light curve modeling which properly incorporates relativistic effects, we obtain limits on the energy deposited into the ejecta of $E_{ej}\lesssim(0.6-6.7)\times 10^{52}$erg ($E_{ej}\lesssim(1.8-17.6)\times10^{52}$erg) for an ejecta mass of $0.03\,M_{\odot}$ ($0.1\,M_{\odot}$). We present a uniform re-analysis of 27 SGRBs with $5.5-6.0$ GHz observations, and find that $\gtrsim50\%$ of SGRBs did not form stable magnetar remnants in their mergers. Assuming SGRBs are produced by BNS mergers drawn from the Galactic BNS population plus an additional component of high-mass GW194025-like mergers in a fraction $f_{GW190425}$ of cases, we place constraints on the maximum mass of a non-rotating neutron star (NS) ($M_{TOV}$), finding $M_{TOV}\lesssim2.23\,M_{\odot}$ for $f_{GW190425}=0.4$; this limit increases for larger values of $f_{GW190425}$. The detection (or lack thereof) of radio remnants in untargeted surveys such as the VLA Sky Survey (VLASS) could provide more stringent constraints on the fraction of mergers that produce stable remnants. If $\gtrsim30-300$ radio remnants are discovered in VLASS, this suggests that SGRBs are a biased population of BNS mergers in terms of the stability of the remnants they produce.

astro-ph.HE↗

arXiv2020

Neutrino Counterparts of Fast Radio Bursts

The discovery of a luminous radio burst, FRB 200428, with properties similar to those of fast radio bursts (FRB), in coincidence with an X-ray flare from the Galactic magnetar SGR 1935+2154, supports magnetar models for cosmological FRBs. The burst's X-ray to radio fluence ratio, as well as the X-ray spectral shape and peak energy, are consistent with FRB 200428 being the result of an ultra-relativistic shock (powered, e.g., by an ejected plasmoid) propagating into a magnetized baryon-rich external medium; the shock simultaneously generates X-ray/gamma-rays via thermal synchrotron emission from electrons heated behind the shock, and coherent radio emission via the synchrotron maser mechanism. Here, we point out that a unique consequence of this baryon-loaded shock scenario is the generation of a coincident burst of high-energy neutrinos, generated by photo-hadronic interaction of relativistic ions - heated or accelerated at the shock - with thermal synchrotron photons. We estimate the properties of these neutrino burst FRB counterparts and find that a fraction ~1e-8-1e-5 of the flare energy (or ~1e-4-0.1 of the radio isotropic energy) is channeled into production of neutrinos with typical energies ~ TeV-PeV. We conclude by discussing prospects for detecting this signal with IceCube and future high-energy neutrino detectors.

astro-ph.HE↗

arXiv2020

Collapsars as a major source of r-process elements

The production of elements by rapid neutron capture (r-process) in neutron-star mergers is expected theoretically and is supported by multimessenger observations of gravitational-wave event GW170817: this production route is in principle sufficient to account for most of the r-process elements in the Universe. Analysis of the kilonova that accompanied GW170817 identified delayed outflows from a remnant accretion disk formed around the newly born black hole as the dominant source of heavy r-process material from that event. Similar accretion disks are expected to form in collapsars (the supernova-triggering collapse of rapidly rotating massive stars), which have previously been speculated to produce r-process elements. Recent observations of stars rich in such elements in the dwarf galaxy Reticulum II, as well as the Galactic chemical enrichment of europium relative to iron over longer timescales, are more consistent with rare supernovae acting at low stellar metallicities than with neutron-star mergers. Here we report simulations that show that collapsar accretion disks yield sufficient r-process elements to explain observed abundances in the Universe. Although these supernovae are rarer than neutron-star mergers, the larger amount of material ejected per event compensates for the lower rate of occurrence. We calculate that collapsars may supply more than 80 per cent of the r-process content of the Universe.

astro-ph.HE↗

arXiv2020

Nuclear Burning in Collapsar Accretion Disks

The core collapse of massive, rapidly-rotating stars are thought to be the progenitors of long-duration gamma-ray bursts (GRB) and their associated hyper-energetic supernovae (SNe). At early times after the collapse, relatively low angular momentum material from the infalling stellar envelope will circularize into an accretion disk located just outside the black hole horizon, resulting in high accretion rates necessary to power a GRB jet. Temperatures in the disk midplane at these small radii are sufficiently high to dissociate nuclei, while outflows from the disk can be neutron-rich and may synthesize r-process nuclei. However, at later times, and for high progenitor angular momentum, the outer layers of the stellar envelope can circularize at larger radii $\gtrsim 10^{7}$ cm, where nuclear reaction can take place in the disk midplane ((e.g.~$^{4}$He + $^{16}$O $\rightarrow$ $^{20}$Ne + $γ$).. Here we explore the effects of nuclear burning on collapsar accretion disks and their outflows by means of hydrodynamical $α$-viscosity torus simulations coupled to a 19-isotope nuclear reaction network, which are designed to mimic the late infall epochs in collapsar evolution when the viscous time of the torus has become comparable to the envelope fall-back time. Our results address several key questions, such as the conditions for quiescent burning and accretion versus detonation and the generation of $^{56}$Ni in disk outflows, which we show could contribute significantly to powering GRB supernovae. Being located in the slowest, innermost layers of the ejecta, the latter could provide the radioactive heating source necessary to make the spectral signatures of r-process elements visible in late-time GRB-SNe spectra.

astro-ph.SR↗

arXiv2020

X-ray Spectroscopy of the gamma-ray Brightest Nova V906 Car (ASASSN-18fv)

Shocks in gamma-ray emitting classical novae are expected to produce bright thermal and non-thermal X-rays. We test this prediction with simultaneous NuSTAR and Fermi/LAT observations of nova V906 Car, which exhibited the brightest GeV gamma-ray emission to date. The nova is detected in hard X-rays while it is still gamma-ray bright, but contrary to simple theoretical expectations, the detected 3.5-78 keV emission of V906 Car is much weaker than the simultaneously observed >100 MeV emission. No non-thermal X-ray emission is detected, and our deep limits imply that the gamma-rays are likely hadronic. After correcting for substantial absorption (N_H ~ 2 x 10^23 cm^-2), the thermal X-ray luminosity (from a 9 keV optically-thin plasma) is just ~2% of the gamma-ray luminosity. We consider possible explanations for the low thermal X-ray luminosity, including the X-rays being suppressed by corrugated, radiative shock fronts or the X-rays from the gamma-ray producing shock are hidden behind an even larger absorbing column (N_H >10^25 cm^-2). Adding XMM-Newton and Swift/XRT observations to our analysis, we find that the evolution of the intrinsic X-ray absorption requires the nova shell to be expelled 24 days after the outburst onset. The X-ray spectra show that the ejecta are enhanced in nitrogen and oxygen, and the nova occurred on the surface of a CO-type white dwarf. We see no indication of a distinct super-soft phase in the X-ray lightcurve, which, after considering the absorption effects, may point to a low mass of the white dwarf hosting the nova.

astro-ph.HE↗

arXiv2020

Variability in Short Gamma-ray Bursts: Gravitationally Unstable Tidal Tails

Short gamma-ray bursts are thought to result from the mergers of two neutron stars or a neutron star and stellar mass black hole. The final stages of the merger are generally accompanied by the production of one or more tidal "tails" of ejecta, which fall back onto the remnant-disc system at late times. Using the results of a linear stability analysis, we show that if the material comprising these tails is modeled as adiabatic and the effective adiabatic index satisfies $γ\ge 5/3$, then the tails are gravitationally unstable and collapse to form small-scale knots. We analytically estimate the properties of these knots, including their spacing along the tidal tail and the total number produced, and their effect on the mass return rate to the merger remnant. We perform hydrodynamical simulations of the disruption of a polytropic (with the polytropic and adiabatic indices $γ$ equal), $γ=2$ neutron star by a black hole, and find agreement between the predictions of the linear stability analysis and the distribution of knots that collapse out of the instability. The return of these knots to the black hole induces variability in the fallback rate, which can manifest as variability in the lightcurve of the GRB and -- depending on how rapidly the instability operates -- the prompt emission. The late-time variability induced by the return of these knots is also consistent with the extended emission observed in some GRBs.

astro-ph.HE↗

arXiv2020

Periodicity in recurrent fast radio bursts and the origin of ultra long period magnetars

The recurrent fast radio burst FRB 180916 was recently shown to exhibit a 16 day period (with possible aliasing) in its bursting activity. Given magnetars as widely considered FRB sources, this period has been attributed to precession of the magnetar spin axis or the orbit of a binary companion. Here, we make the simpler connection to a {\it rotational period}, an idea observationally motivated by the 6.7 hour period of the Galactic magnetar candidate, 1E 161348--5055. We explore three physical mechanisms that could lead to the creation of ultra long period magnetars: (i) enhanced spin-down due to episodic mass-loaded charged particle winds (e.g. as may accompany giant flares), (ii) angular momentum kicks from giant flares and (iii) fallback leading to long lasting accretion disks. We show that particle winds and fallback accretion can potentially lead to a sub-set of the magnetar population with ultra long periods, sufficiently long to accommodate FRB 180916 or 1E 161348--5055. If confirmed, such periods implicate magnetars in relatively mature states (ages $1-10$ kyr) and which possessed large internal magnetic fields at birth $B_{\rm int}\gtrsim 10^{16}$ G. In the low-twist magnetar model for FRBs, such long period magnetars may dominate FRB production for repeaters at lower isotropic-equivalent energies and broaden the energy distribution beyond that expected for a canonical population of magnetars which terminate their magnetic activity at shorter periods $P \lesssim 10$ s.

astro-ph.HE↗

arXiv2020

Implications of a "Fast Radio Burst" from a Galactic Magnetar

A luminous radio burst was recently detected in temporal coincidence with a hard X-ray flare from the Galactic magnetar SGR 1935+2154 with a time and frequency structure consistent with cosmological fast radio bursts (FRB) and a fluence within a factor of $\lesssim 10$ of the least energetic extragalactic FRB previously detected. Although active magnetars are commonly invoked FRB sources, several distinct mechanisms have been proposed for generating the radio emission which make different predictions for the accompanying higher frequency radiation. We show that the properties of the coincident radio and X-ray flares from SGR 1935+2154, including their approximate simultaneity and relative fluence $E_{\rm radio}/E_{\rm X} \sim 10^{-5}$, as well as the duration and spectrum of the X-ray emission, are consistent with extant predictions for the synchrotron maser shock model. Rather than arising from the inner magnetosphere, the X-rays are generated by (incoherent) synchrotron radiation from thermal electrons heated at the same shocks which produce the coherent maser emission. Although the rate of SGR 1935+2154-like bursts in the local universe is not sufficient to contribute appreciably to the extragalactic FRB rate, the inclusion of an additional population of more active magnetars with stronger magnetic fields than the Galactic population can explain both the FRB rate as well as the repeating fraction, however only if the population of active magnetars are born at a rate that is at least two-orders of magnitude lower than that of SGR 1935+2154-like magnetars. This may imply that the more active magnetar sources are not younger magnetars formed in a similar way to the Milky Way population (e.g. via ordinary supernovae), but instead through more exotic channels such as superluminous supernovae, accretion-induced collapse or neutron star mergers.

astro-ph.HE↗

arXiv2020

Electromagnetic transients and gravitational waves from white dwarf disruptions by stellar black holes in triple systems

Mergers of binaries comprised of compact objects can give rise to explosive transient events, heralding the birth of exotic objects which cannot be formed through single star evolution. Using a large number of direct N-body simulations, we explore the possibility that a white dwarf (WD) is dynamically driven to tidal disruption by a stellar-mass black hole (BH) as a consequence of the joint effects of gravitational wave (GW) emission and Lidov-Kozai oscillations imposed by the tidal field of a outer tertiary companion orbiting the inner BH-WD binary. We explore the sensitivity of our results to the distributions of natal kick velocities imparted to the BH and WD upon formation, adiabatic mass loss, semi-major axes and eccentricities of the triples, and stellar mass ratios. We find rates of WD-TDEs in the range $1.2\times 10^{-3}-1.4$ Gpc$^{-3}$ yr$^{-1}$ for $z\leq 0.1$, rarer than stellar TDEs in triples by a factor of $\sim 3$--$30$. The uncertainty in the TDE rates may be greatly reduced in the future using gravitational wave (GW) observations of Galactic binaries and triples with LISA. WD-TDEs may give rise to high energy X-ray or gamma-ray transients of duration similar to long gamma-ray bursts but lacking the signatures of a core-collapse supernova, while being accompanied by a supernova-like optical transient which lasts for only days. WD--BH and WD--NS binaries will also emit GWs in the LISA band before the TDE. The discovery and identification of triple-induced WD-TDE events by future time domain surveys and/or GWs could enable the study of the demographics of BHs in nearby galaxies.

astro-ph.HE↗

arXiv2020

Constraints on the Engines of Fast Radio Bursts

We model the sample of fast radio bursts (FRB), including the newly discovered CHIME repeaters, using the synchrotron blast wave model of Metzger, Margalit & Sironi (2019). This model postulates that FRBs are precursor radiation from ultra-relativistic magnetized shocks generated as flare ejecta from a central engine collide with an effectively stationary external medium. Downward drifting of the burst frequency structure naturally arises from deceleration of the blast-wave. The data are consistent with FRBs being produced by flares of energy $E_{\rm flare} \sim 10^{43}-10^{46}(f_ξ/10^{-3})^{-4/5}$ erg, where $f_ξ$ is the maser efficiency, and minimum bulk Lorentz factors $Γ\approx 10^2-10^3$, which generate the observed FRBs at shock radii $r_{\rm sh} \sim 10^{12}-10^{13}$ cm. We infer upstream densities $n_{\rm ext}(r_{\rm sh}) \sim 10^{2}-10^{4}$ cm$^{-3}$ and radial profiles $n_{\rm ext} \propto r^{-k}$ showing a range of slopes $k \approx [-2,1]$ (which are seen to evolve between bursts), broadly consistent with the upstream medium being the inner edge of an ion-loaded shell released by a recent energetic flare. The burst timescales, energetics, rates, and external medium properties are consistent with repeating FRBs arising from young, hyper-active flaring magnetars, but the methodology presented is generally applicable to any central engine which injects energy impulsively into a dense magnetized medium. Uncertainties and variations of the model are discussed, including the effects of the strong electric field of the FRB wave (strength parameter $a \gg 1$) on the upstream medium. One-dimensional particle-in-cell simulations of magnetized shocks into a pair plasma are presented which demonstrate that high maser efficiency can be preserved, even in the limit $a \gg 1$ in which the FRB wave accelerates the upstream electrons to ultra-relativistic speeds.

astro-ph.HE↗

arXiv2020

Direct evidence for shock-powered optical emission in a nova

Classical novae are thermonuclear explosions that occur on the surfaces of white dwarf stars in interacting binary systems (Bode & Evans 2008). It has long been thought that the luminosity of classical novae is powered by continued nuclear burning on the surface of the white dwarf after the initial runaway (Gallaher & Starrfield 1978). However, recent observations of GeV $γ$-rays from classical novae have hinted that shocks internal to the nova ejecta may dominate the nova emission. Shocks have also been suggested to power the luminosity of events as diverse as stellar mergers (Metzger & Pejcha 2017), supernovae (Moriya et al. 2018), and tidal disruption events (Roth et al. 2016), but observational confirmation has been lacking. Here we report simultaneous space-based optical and $γ$-ray observations of the 2018 nova V906 Carinae (ASASSN-18fv), revealing a remarkable series of distinct correlated flares in both bands. The optical and $γ$-ray flares occur simultaneously, implying a common origin in shocks. During the flares, the nova luminosity doubles, implying that the bulk of the luminosity is shock-powered. Furthermore, we detect concurrent but weak X-ray emission from deeply embedded shocks, confirming that the shock power does not appear in the X-ray band and supporting its emergence at longer wavelengths. Our data, spanning the spectrum from radio to $γ$-ray, provide direct evidence that shocks can power substantial luminosity in classical novae and other optical transients.

astro-ph.HE↗

arXiv2019

Internal Shocks from Variable Outflows in Classical Novae

We present one-dimensional hydrodynamical simulations including radiative losses, of internal shocks in the outflows from classical novae, to explore the role of shocks in powering multi-wavelength emission from radio to gamma-ray wavelengths. Observations support a picture in which the initial phases of some novae generate a slow, equatorially-focused outflow (directly from the outer Lagrange point, or from a circumbinary disk), which then transitions to, or is overtaken by, a faster more isotropic outflow from the white dwarf which collides and shocks the slower flow, powering gamma-ray and optical emission through reprocessing by the ejecta. However, the common occurence of multiple peaks in nova light curves suggests that the outflow's acceleration need not be monotonic, but instead can involve successive transitions between "fast" and "slow" modes. Such a time-fluctuating outflow velocity naturally can reproduce several observed properties of nova, such as correlated gamma-ray and optical flares, expansion of the photosphere coincident with (though lagging slightly) the peak flare luminosity, and complex time-evolution of spectral lines (including accelerating, decelerating, and merging velocity components). While the shocks are still deeply embedded during the gamma-ray emission, the onset of $\sim$ keV X-ray and $\sim 10$ GHz radio synchrotron emission is typically delayed until the forward shock of the outermost monolithic shell (created by merger of multiple internal shock-generated shells) reaches a sufficiently low column through the dense external medium generated by the earliest phase of the outburst.

astro-ph.HE↗