Non-relativistic Outflows Research Articles

The Peculiar Radio Evolution of the Tidal Disruption Event ASASSN-19bt

We present detailed radio observations of the tidal disruption event (TDE) ASASSN-19bt/AT 2019ahk, obtained with the Australia Telescope Compact Array, the Atacama Large Millimeter/submillimeter Array, and the MeerKAT radio telescopes, spanning 40–1464 days after the onset of the optical flare. We find that ASASSN-19bt displays unusual radio evolution compared to other TDEs, as the peak brightness of its radio emission increases rapidly until 457 days post-optical discovery and then plateaus. Using a generalized approach to standard equipartition techniques, we estimate the energy and corresponding physical parameters for two possible emission geometries: a nonrelativistic spherical outflow and a relativistic outflow observed from a range of viewing angles. We find that the nonrelativistic solution implies a continuous energy rise in the outflow from E ∼ 1046 to E ∼ 1049 erg with outflow speed β ≈ 0.05, while the off-axis relativistic jet solution instead suggests E ≈ 1052 erg with Lorentz factor Γ ∼ 10 at late times in the maximally off-axis case. We find that neither model provides a holistic explanation for the origin and evolution of the radio emission, emphasizing the need for more complex models. ASASSN-19bt joins the population of TDEs that display unusual radio emission at late times. Conducting long-term radio observations of these TDEs, especially during the later phases, will be crucial for understanding how these types of radio emission in TDEs are produced.

Radio observations of the tidal disruption event AT2020opy: a luminous non-relativistic outflow encountering a dense circumnuclear medium

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.

Quasi-perpendicular Shock Acceleration and Tidal Disruption Event Radio Flares

Delayed radio flares of optical tidal disruption events (TDEs) indicate the existence of nonrelativistic outflows accompanying TDEs. The interaction of TDE outflows with the surrounding circumnuclear medium creates quasi-perpendicular shocks in the presence of toroidal magnetic fields. Because of the large shock obliquity and large outflow velocity, we find that the shock acceleration induced by TDE outflows generally leads to a steep particle energy spectrum, with the power-law index significantly larger than the “universal” index for a parallel shock. The measured synchrotron spectral indices of recently detected TDE radio flares are consistent with our theoretical expectation. It suggests that the particle acceleration at quasi-perpendicular shocks can be the general acceleration mechanism accounting for the delayed radio emission of TDEs.

Radio emission from simulated tidal disruption events

ABSTRACT Several tidal disruption events such as ASASSN-14li and XMMSL1 J0740-85 have recently been observed in the radio. While the radio emission of some tidal disruption events are attributed to a relativistic jet, a few others are associated with a non-relativistic outflow. This outflow can either be due to a spherical wind or unbound tidal debris. We explore this latter hypothesis in this paper. We show that the maximum velocity of the unbound debris is a function of the impact parameter, such that smaller impact parameters (closer approaches) produce larger maximum velocities. We then model this outflow which expands and shocks the local interstellar medium and compute the peak radio flux and frequency as functions of the impact parameter. Moreover, multiple epochs of observations can put additional constraints on the profile of the local interstellar medium. We apply this analysis to four tidal disruption events whose radio emission is attributed to a non-relativistic outflow and show that the velocities of the unbound material are consistent with our simulated events. We also place constraints on the density profile of three of the four tidal disruption events with multiple epochs of observations.

Adiabatic–radiative shock systems in YSO jets and novae outflows

Context. The termination regions of non-relativistic jets in protostars and supersonic outflows in classical novae are non-thermal emitters. This has been confirmed by radio and gamma-ray detection, respectively. A two-shock system is expected to be formed in the termination region where the jet, or the outflow material, and the ambient medium impact. Radiative shocks are expected to form in these systems given their high densities. However, in the presence of high velocities, the formation of adiabatic shocks is also possible. A case of interest is when the two types of shocks occur simultaneously. Adiabatic shocks are more efficient at particle acceleration while radiative shocks strongly compress the gas. Furthermore, a combined adiabatic–radiative shock system is very prone to developing instabilities in the contact discontinuity, leading to mixing, turbulence, and density enhancement. Additionally, these dense non-relativistic jets and outflows are excellent candidates for laboratory experiments as demonstrated by magnetohydrodynamics scaling. Aims. We aim to study the combination of adiabatic and radiative shocks in protostellar jets and novae outflows. We focus on determining the conditions under which this combination is feasible together with its physical implications. Methods. We performed an analytical study of the shocks in both types of sources for a set of parameters by comparing cooling times and propagation velocities. We also estimated the timescales for the growth of instabilities in the contact discontinuity separating both shocks. We studied the hydrodynamical evolution of a jet colliding with an ambient medium with 2D numerical simulations, confirming our initial theoretical estimates. Results. We show that for a wide set of observationally constrained parameters, the combination of an adiabatic and a radiative shock is possible at the working surface of the termination region in jets from young stars and novae outflows. We find that instabilities are developed at the contact discontinuity, mixing the shocked materials. Additionally, we explore the magnetohydrodynamic parameter scaling required for studying protostellar jets and novae outflows using laboratory experiments on laser facilities. Conclusions. The coexistence of an adiabatic and a radiative shock is expected at the termination region of protostellar jets and novae outflows. This scenario is very promising for particle acceleration and gamma-ray emission. The parameters for scaled laboratory experiments are very much in line with plasma conditions achievable in currently operating high-power laser facilities. This provides a new means for studying novae outflows that has never been considered before.

Radio emission from outflow–cloud interaction and its constraint on tidal disruption event outflow

ABSTRACT Tidal disruption event (TDE) can launch an ultrafast outflow. If the black hole is surrounded by large amounts of clouds, outflow–cloud interaction will generate bow shocks, accelerate electrons, and produce radio emission. Here, we investigate the interaction between a non-relativistic outflow and clouds in active galaxies, which is manifested as outflow–BLR (broad-line region) interaction, and can be extended to outflow–torus interaction. This process can generate considerable radio emission, which may account for the radio flares appearing a few months later after TDE outbursts. Benefitting from efficient energy conversion from outflow to shocks and the strong magnetic field, outflow–cloud interaction may play a non-negligible, or even dominating role in generating radio flares in a cloudy circumnuclear environment if the CNM density is no more than 100 times the Sgr A*-like one. In this case, the evolution of radio spectra can be used to directly constrain the properties of outflows.

The Major Role of Eccentricity in the Evolution of Colliding Pulsar-Stellar Winds

Binary systems that host a massive star and a non-accreting pulsar can be powerful non-thermal emitters. The relativistic pulsar wind and the non-relativistic stellar outflows interact along the orbit, producing ultrarelativistic particles that radiate from radio to gamma rays. To properly characterize the physics of these sources, and better understand their emission and impact on the environment, careful modeling of the outflow interactions, spanning a broad range of spatial and temporal scales, is needed. Full three-dimensional approaches are very computationally expensive, but simpler approximate approaches, while still realistic at the semi-quantitative level, are available. We present here the results of calculations done with a quasi three-dimensional scheme to compute the evolution of the interacting flows in a region spanning in size up to a thousand times the size of the binary. In particular, we analyze for the first time the role of different eccentricities in the large scale evolution of the shocked flows. We find that the higher the eccentricity, the closer the flows behave like a one-side outflow, which becomes rather collimated for eccentricity values ≳0.75. The simulations also unveil that the pulsar and the stellar winds become fully mixed within the grid for low eccentricity systems, presenting a more stochastic behavior at large scales than in the highly eccentric systems.

Radio Monitoring of the Tidal Disruption Event Swift J164449.3+573451. IV. Continued Fading and Non-relativistic Expansion

Abstract We present continued radio and X-ray observations of the previously relativistic tidal disruption event (TDE) Swift J164449.3+573451 (Sw 1644+57) extending to about 9.4 yr post disruption, as part of ongoing campaigns with the Jansky Very Large Array (VLA) and the Chandra X-ray observatory. We find that the X-ray emission has faded below detectable levels, with an upper limit of ≲3.5 × 10−15 erg cm−2 s−1 in a 100 ks observation, while the radio emission continues to be detected and steadily fade. Both are consistent with forward shock emission from a non-relativistic outflow, although we find that the radio spectral energy distribution is better fit at these late times with an electron power-law index of p ≈ 3 (as opposed to p ≈ 2.5 at earlier times). With the revised spectral index we find ϵ B ≈ 0.01 using the radio and X-ray data, and a density of ≈0.04 cm3 at a radius of R ≈ 0.65 pc (R sch ≈ 2 × 106 R ⊙) from the black hole. The energy scale of the blastwave is ≈1052 erg. We also report detections of Sw 1644+57 at 3 GHz from the first two epochs of the VLA Sky Survey (VLASS), and find that ∼102 off-axis Sw 1644+57-like events to z ∼ 0.5 may be present in the VLASS data. Finally, we find that Sw 1644+57 itself will remain detectable for decades at radio frequencies, although observations at sub-GHz frequencies will become increasingly important to characterize its dynamical evolution.

A study of radial self-similar non-relativistic MHD outflow models: parameter space exploration and application to the water fountain W43A

ABSTRACT Outflows, spanning a wide range of dynamical properties and spatial extensions, have now been associated with a variety of accreting astrophysical objects, from supermassive black holes at the core of active galaxies to young stellar objects. The role of such outflows is key to the evolution of the system that generates them, for they extract a fraction of the orbiting material and angular momentum from the region close to the central object and release them in the surroundings. The details of the launching mechanism and their impact on the environment are fundamental to understand the evolution of individual sources and the similarities between different types of outflow-launching systems. We solve semi-analytically the non-relativistic, ideal, magnetohydrodynamics equations describing outflows launched from a rotating disc threaded with magnetic fields using our new numerical scheme. We present here a parameter study of a large sample of new solutions. We study the different combinations of forces that lead to a successfully launched jet and discuss their global properties. We show how these solutions can be applied to the outflow of the water fountain W43A for which we have observational constraints on magnetic field, density and velocity of the flow at the location of two symmetrical water maser emitting regions.

Radio Properties of Tidal Disruption Events

Radio observations of tidal disruption events (TDEs) probe material ejected by the disruption of stars by supermassive black holes (SMBHs), uniquely tracing the formation and evolution of jets and outflows, revealing details of the disruption hydrodynamics, and illuminating the environments around previously-dormant SMBHs. To date, observations reveal a surprisingly diverse population. A small fraction of TDEs (at most a few percent) have been observed to produce radio-luminous mildly relativistic jets. The remainder of the population are radio quiet, producing less luminous jets, non-relativistic outflows or, possibly, no radio emission at all. Here, we review the radio observations that have been made of TDEs to date and discuss possible explanations for their properties, focusing on detected sources and, in particular, on the two best-studied events: Sw J1644+57 and ASASSN-14li. We also discuss what we have learned about the host galaxies of TDEs from radio observations and review constraints on the rates of bright and faint radio outflows in TDEs. Upcoming X-ray, optical, near-IR, and radio surveys will greatly expand the sample of TDEs, and technological advances open the exciting possibility of discovering a sample of TDEs in the radio band unbiased by host galaxy extinction.

Laboratory study of non-ideal effects in magnetically collimated astrophysical outflows

Central outflow’s collimation by magnetic field is an important theoretical mechanism for explaining the astrophysical objects’ morphology formation, and its credibility has been tested in many laser plasma experiments in a dimensionless manner. This article introduces integrated simulation and experiment work based on the present laboratory magnetically collimated jet framework, to explore how non-ideal terms’ strength including radiative cooling and magnetic diffusion from different targets can affect the outflow shape. The interaction between outflow from a target with low atomic number and external field satisfies the ideal magneto-hydrodynamic conditions, and the outflow shape results in diamagnetic cavity and jet; on the other hand, a heavy element target brings strong magnetic diffusion that destroys the collimation structure, together with the stagnation of outflow introduced by radiative cooling, and outflow shape results in weakly collimated hemisphere near the target and a detached magnetized density clump. The detailed dimensionless analysis shows that the large-scale dissipation of jets in young stellar objects can possibly be an analog of the laboratory jet’s magnetic diffusion breakup, also similar structures like the loosely collimated lobes and bright ansaes in planetary nebula can be observed in highly diffusive laboratory outflows. This article shows for the first time that a series of non-relativistic astronomical outflows’ dynamic behaviors can be explained by the non-ideal magneto-hydrodynamic evolution of laboratory plasmas.

Generation of compact plasma objects in plasma focus discharge

This paper provides the results of studies of plasma flows generated in the plasma focus facility PF-1000U. Regimes are found, involving the formation of compact plasma objects propagating in the variable-density background environment to the distances exceeding the initial crosswise dimensions of these objects by dozens of times. It is shown that the flow spreads with its proper captured toroidal magnetic field. The plasma flow front structure is studied. The conclusions about the role of radiation cooling and magnetic confinement in the collimation and stability of outflows are drawn. The results obtained in this work can be used to analyze the existing and build new models describing the collimation and stability of non-relativistic outflows of young stellar objects.

Binary Neutron Star (BNS) Merger: What We Learned from Relativistic Ejecta of GW/GRB 170817A

Gravitational Waves (GW) from coalescence of a Binary Neutron Star (BNS) and its accompanying short Gamma-Ray Burst (GRB) GW/GRB 170817A confirmed the presumed origin of these puzzling transients and opened up the way for relating properties of short GRBs to those of their progenitor stars and their surroundings. Here we review an extensive analysis of the prompt gamma-ray and late afterglows of this event. We show that a fraction of polar ejecta from the merger had been accelerated to ultra-relativistic speeds. This structured jet had an initial Lorentz factor of about 260 in our direction, which was O ( 10 ∘ ) from the jet’s axis, and was a few orders of magnitude less dense than in typical short GRBs. At the time of arrival to circum-burst material the ultra-relativistic jet had a close to Gaussian profile and a Lorentz factor ≳ 130 in its core. It had retained in some extent its internal collimation and coherence, but had extended laterally to create mildly relativistic lobes—a cocoon. Its external shocks on the far from center inhomogeneous circum-burst material and low density of colliding shells generated slowly rising afterglows, which peaked more than 100 days after the prompt gamma-ray. The circum-burst material was somehow correlated with the merger. As non-relativistic outflows or tidally ejected material during BNS merger could not have been arrived to the location of the external shocks before the relativistic jet, circum-burst material might have contained recently ejected materials from resumption of internal activities, faulting and mass loss due to deformation and breaking of stars crusts by tidal forces during latest stages of their inspiral but well before their merger. By comparing these findings with the results of relativistic Magneto-Hydro-Dynamics (MHD) simulations and observed gravitational waves we conclude that progenitor neutron stars were most probably old, had close masses and highly reduced magnetic fields.

Unveiling the origin of the gamma-ray emission in NGC 1068 with the Cherenkov Telescope Array

Several observations are revealing the widespread occurrence of mildly relativistic wide-angle AGN winds strongly interacting with the gas of their host galaxy. Such winds are potential cosmic-ray accelerators, as supported by gamma-ray observations of the nearby Seyfert galaxy NGC 1068 with the Fermi gamma-ray space telescope. The non-thermal emission produced by relativistic particles accelerated by the AGN-driven wind observed in the circum-nuclear molecular disk of such galaxy is invoked to produce the gamma-ray spectrum. The AGN wind model predicts a hard spectrum that extend in the very high energy band which differs significantly from those corresponding to other models discussed in the literature, like starburst or AGN jet. We present dedicated simulations of observations through the Cherenkov Telescope Array (CTA), the next-generation ground based gamma-ray observatory, of the very high energy spectrum of the Seyfert galaxy NGC 1068 assuming the AGN wind and the AGN jet models. We demonstrate that, considering 50 hours of observations, CTA can be effectively used to constrain the two different emission models, providing important insight into the physics governing the acceleration of particles in non-relativistic AGN-driven outflows. This analysis strongly motivates observations of Seyfert and starburst galaxies with CTA in order to test source population models of the extragalactic gamma-ray and neutrino backgrounds.

Jets in Hydrogen-poor Superluminous Supernovae: Constraints from a Comprehensive Analysis of Radio Observations

Abstract The energy source powering the extreme optical luminosity of hydrogen-stripped superluminous supernovae (SLSNe-I) is not known, but recent studies have highlighted the case for a central engine. Radio and/or X-ray observations are best placed to track the fastest ejecta and probe the presence of outflows from a central engine. We compile all the published radio observations of SLSNe-I to date and present three new observations of two new SLSNe-I. None were detected. Through modeling the radio emission, we constrain the subparsec environments and possible outflows in SLSNe-I. In this sample, we rule out on-axis collimated relativistic jets of the kind detected in gamma-ray bursts (GRBs). We constrain off-axis jets with opening angles of 5° (30°) to energies of E k < 4 × 10 50 erg ( E k < 10 50 erg ) in environments shaped by progenitors with mass-loss rates of M ˙ < 10 − 4 M ⊙ yr − 1 ( M ˙ < 10 − 5 M ⊙ yr − 1 ) for all off-axis angles, assuming fiducial values ϵ e = 0.1 and ϵ B = 0.01 . The deepest limits rule out emission of the kind seen in faint uncollimated GRBs (with the exception of GRB 060218) and from relativistic SNe. Finally, for the closest SLSN-I, SN 2017egm, we constrain the energy of an uncollimated nonrelativistic outflow like those observed in normal SNe to E k ≲ 10 48 erg.

Radio Observations of the Tidal Disruption Event XMMSL1 J0740–85

Abstract We present radio observations of the tidal disruption event candidate (TDE) XMMSL1 J0740−85 spanning 592 to 875 days post X-ray discovery. We detect radio emission that fades from an initial peak flux density at 1.6 GHz of 1.19 ± 0.06 mJy to 0.65 ± 0.06 mJy, suggesting an association with the TDE. This makes XMMSL1 J0740−85 at d = 75 Mpc the nearest TDE with detected radio emission to date and only the fifth TDE with radio emission overall. The observed radio luminosity rules out a powerful relativistic jet like that seen in the relativistic TDE Swift J1644+57. Instead, we infer from an equipartition analysis that the radio emission most likely arises from a non-relativistic outflow similar to that seen in the nearby TDE ASASSN-14li, with a velocity of about 104 km s−1 and a kinetic energy of about 1048 erg, expanding into a medium with a density of about 102 cm−3. Alternatively, the radio emission could arise from a weak initially relativistic but decelerated jet with an energy of erg, or (for an extreme disruption geometry) from the unbound debris. The radio data for XMMSL1 J0740−85 continues to support the previous suggestion of a bimodal distribution of common non-relativistic isotropic outflows and rare relativistic jets in TDEs (in analogy with the relation between Type Ib/c supernovae and long-duration gamma-ray bursts). The radio data also provide a new measurement of the circumnuclear density on a sub-parsec scale around an extragalactic supermassive black hole.

Ultrahigh energy cosmic rays from nonrelativistic quasar outflows

It has been suggested that non-relativistic outflows from quasars can naturally account for the missing component of the extragalactic $\gamma$-ray background and explain the cumulative neutrino background through pion decay in collisions between protons accelerated by the outflow shock and interstellar protons. Here we show that the same quasar outflows are capable of accelerating protons to energies of $\sim 10^{20}$ eV during the early phase of their propagation. The overall quasar population is expected to produce a cumulative ultra high energy cosmic ray flux of $\sim10^{-7}\,\rm GeV\,cm^{-2}s^{-1}sr^{-1}$ at $E_{\rm CR}\gtrsim10^{18}$ eV. The spectral shape and amplitude is consistent with recent observations for outflow parameters constrained to fit secondary $\gamma$-rays and neutrinos without any additional parameter tuning. This indicates that quasar outflows simultaneously account for all three messengers at their observed levels.

RADIO PROPERTIES OF THE BAT AGNs: THE FIR–RADIO RELATION, THE FUNDAMENTAL PLANE, AND THE MAIN SEQUENCE OF STAR FORMATION

ABSTRACT We conducted 22 GHz 1″ JVLA imaging of 70 radio-quiet active galactic nuclei (AGNs) from the Swift-BAT survey. We find radio cores in all but three objects. The radio morphologies of the sample fall into three groups: compact and core-dominated, extended, and jet-like. We spatially decompose each image into core flux and extended flux, and compare the extended radio emission with that predicted from previous Herschel observations using the canonical FIR–radio relation. After removing the AGN contribution to the FIR and radio flux densities, we find that the relation holds remarkably well despite the potentially different star formation physics in the circumnuclear environment. We also compare our core radio flux densities with predictions of coronal models and scale-invariant jet models for the origin of radio emission in radio-quiet AGNs, and find general consistency with both models. However, we find that the L R/L X relation does not distinguish between star formation and non-relativistic AGN-driven outflows as the origin of radio emission in radio-quiet AGNs. Finally, we examine where objects with different radio morphologies fall in relation to the main sequence (MS) of star formation, and conclude that those AGNs that fall below the MS, as X-ray selected AGNs have been found to do, have core-dominated or jet-like 22 GHz morphologies.

DISCOVERY OF AN OUTFLOW FROM RADIO OBSERVATIONS OF THE TIDAL DISRUPTION EVENT ASASSN-14li

ABSTRACT We report the discovery of transient radio emission from the nearby optically discovered tidal disruption event (TDE) ASASSN-14li (distance of 90 Mpc), making it the first typical TDE detected in the radio, and unambiguously pointing to the formation of a non-relativistic outflow with a kinetic energy of ≈(4–10) × 1047 erg, a velocity of ≈12,000–36,000 km s−1, and a mass of ≈3 × 10−5–7 × 10−4 M ⊙. We show that the outflow was ejected on 2014 August 11–25, in agreement with an independent estimate of the timing of super-Eddington accretion based on the optical, ultraviolet, and X-ray observations, and that the ejected mass corresponds to about 1%–10% of the mass accreted in the super-Eddington phase. The temporal evolution of the radio emission also uncovers the circumnuclear density profile, &rgr; ( R ) ∝ R − 2.5 ?> on a scale of about 0.01 pc, a scale that cannot be probed via direct measurements even in the nearest supermassive black holes. Our discovery of radio emission from the nearest well-studied TDE to date, with a radio luminosity lower than all previous limits, indicates that non-relativistic outflows are ubiquitous in TDEs, and that future, more sensitive, radio surveys will uncover similar events.

RAPIDLY EVOLVING AND LUMINOUS TRANSIENTS DRIVEN BY NEWLY BORN NEUTRON STARS

We provide a general analysis on the properties of emitting material of some rapidly evolving and luminous transients discovered recently with the Pan-STARRS1 Medium Deep Survey. It is found that these transients are probably produced by a low-mass non-relativistic outflow that is continuously powered by a newly born, rapidly spinning, and highly magnetized neutron star. Such a system could originate from an accretion-induced collapse of a white dwarf or a merger of a neutron star-neutron star binary. Therefore, observations to these transients would be helpful for constraining white dwarf and neutron star physics and/or for searching and identifying gravitational wave signals from the mergers.