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Interacting binaries in which a white dwarf accretes material from a companion-cataclysmic variables (CVs) in which the mass donor is a Roche-lobe filling star on or near the main sequence, and symbiotic stars in which the mass donor is a late type giant-are relatively commonplace. They display a wide range of behaviors in the optical, X-rays, and other wavelengths, which still often baffle observers and theorists alike. Here I review the existing body of research on X-ray emissions from these objects for the benefits of both experts and newcomers to the field. I provide introductions to the past and current X-ray observatories, the types of known X-ray emissions from these objects, and the data analysis techniques relevant to this field. I then summarize of our knowledge regarding the X-ray emissions from magnetic CVs, non-magnetic CVs and symbiotic stars, and novae in eruption. I also discuss space density and the X-ray luminosity functions of these binaries and their contribution to the integrated X-ray emission from the Galaxy. I then discuss open questions and future prospects.

The presence of a nearby companion alters the evolution of massive stars in binary systems, leading to phenomena such as stellar mergers, x-ray binaries, and gamma-ray bursts. Unambiguous constraints on the fraction of massive stars affected by binary interaction were lacking. We simultaneously measured all relevant binary characteristics in a sample of Galactic massive O stars and quantified the frequency and nature of binary interactions. More than 70% of all massive stars will exchange mass with a companion, leading to a binary merger in one-third of the cases. These numbers greatly exceed previous estimates and imply that binary interaction dominates the evolution of massive stars, with implications for populations of massive stars and their supernovae.

Novae are thermonuclear explosions on a white dwarf surface fueled by mass accreted from a companion star. Current physical models posit that shocked expanding gas from the nova shell can produce x-ray emission, but emission at higher energies has not been widely expected. Here, we report the Fermi Large Area Telescope detection of variable γ-ray emission (0.1 to 10 billion electron volts) from the recently detected optical nova of the symbiotic star V407 Cygni. We propose that the material of the nova shell interacts with the dense ambient medium of the red giant primary and that particles can be accelerated effectively to produce π° decay γ-rays from proton-proton interactions. Emission involving inverse Compton scattering of the red giant radiation is also considered and is not ruled out.

The bright and distant past A dwarf nova is a type of cataclysmic variable containing a collapsed white dwarf star that accretes matter from its close companion in a binary system, a red dwarf. An instability periodically dumps material onto the white dwarf, increasing the luminosity by up to a hundredfold. Classical novae are thousands of times brighter than dwarf novae, and are accompanied by the formation of shells around the system. Theory predicts that dwarf novae will eventually gain sufficient mass to undergo classical nova eruptions. This suspected link between dwarf and classical novae now has an observational basis with the discovery of an ancient nova shell around the dwarf nova Z Camelopardalis. The nature of the shell suggests that a few thousand years ago, Z Cam underwent a classical nova eruption and for some days was one of the brightest stars in the sky. Classical novae are thousands of times brighter than dwarf novae, and are accompanied by the formation of shells around the system. This paper reports the discovery of a shell an order of magnitude more extended than those detected around many other classical novae surrounding the prototypical dwarf nova Z Camelopardalis, thereby observationally linking the objects. Cataclysmic variables (classical novae and dwarf novae) are binary star systems in which a red dwarf transfers hydrogen-rich matter, by way of an accretion disk, to its white dwarf companion 1 . In dwarf novae, an instability 2 is believed to episodically dump much of the accretion disk onto the white dwarf. The liberation of gravitational potential energy then brightens these systems by up to 100-fold every few weeks or months 2 . Thermonuclear-powered eruptions thousands of times more luminous 3 , 4 occur in classical novae 5 , accompanied by significant mass ejection 6 and formation of clearly visible shells 7 , 8 from the ejected material. Theory predicts that the white dwarfs in all dwarf novae must eventually accrete enough mass to undergo classical nova eruptions 9 . Here we report a shell, an order of magnitude more extended than those detected around many classical novae, surrounding the prototypical dwarf nova Z Camelopardalis. The derived shell mass matches that of classical novae, and is inconsistent with the mass expected from a dwarf nova wind or a planetary nebula. The shell observationally links the prototypical dwarf nova Z Camelopardalis with an ancient nova eruption and the classical nova process.

Novae are caused by runaway thermonuclear burning in the hydrogen-rich envelopes of accreting white dwarfs, which leads to a rapid expansion of the envelope and the ejection of most of its mass 1 , 2 . Theory has predicted the existence of a ‘fireball’ phase following directly on from the runaway fusion, which should be observable as a short, bright and soft X-ray flash before the nova becomes visible in the optical 3 – 5 . Here we report observations of a bright and soft X-ray flash associated with the classical Galactic nova YZ Reticuli 11 h before its 9 mag optical brightening. No X-ray source was detected 4 h before and after the event, constraining the duration of the flash to shorter than 8 h. In agreement with theoretical predictions 4 , 6 – 8 , the source’s spectral shape is consistent with a black-body of 3.27 +0.11 −0.33  × 10 5  K (28.2 +0.9 −2.8  eV), or a white dwarf atmosphere, radiating at the Eddington luminosity, with a photosphere that is only slightly larger than a typical white dwarf. Novae are caused by runaway thermonuclear burning in the hydrogen-rich envelopes of accreting white dwarfs, which leads to a rapid expansion of the envelope and the ejection of most of its mass 1 , 2 . Theory has predicted the existence of a ‘fireball’ phase following directly on from the runaway fusion, which should be observable as a short, bright and soft X-ray flash before the nova becomes visible in the optical 3 – 5 . Here we report observations of a bright and soft X-ray flash associated with the classical Galactic nova YZ Reticuli 11 h before its 9 mag optical brightening. No X-ray source was detected 4 h before and after the event, constraining the duration of the flash to shorter than 8 h. In agreement with theoretical predictions 4 , 6 – 8 , the source’s spectral shape is consistent with a black-body of 3.27 +0.11 −0.33  × 10 5  K (28.2 +0.9 −2.8  eV), or a white dwarf atmosphere, radiating at the Eddington luminosity, with a photosphere that is only slightly larger than a typical white dwarf.

Astrophysical jets seem to occur in nearly all types of accreting objects, from supermassive black holes to young stellar objects. On the basis of x-ray binaries, a unified scenario describing the disc/jet coupling has evolved and been extended to many accreting objects. The only major exceptions are thought to be cataclysmic variables: Dwarf novae, weakly accreting white dwarfs, show similar outburst behavior to x-ray binaries, but no jet has yet been detected. Here we present radio observations of a dwarf nova in outburst showing variable flat-spectrum radio emission that is best explained as synchrotron emission originating in a transient jet. Both the inferred jet power and the relation to the outburst cycle are analogous to those seen in x-ray binaries, suggesting that the disc/jet coupling mechanism is ubiquitous.

Nova explosions are caused by global thermonuclear runaways triggered in the surface layers of accreting white dwarfs 1 – 3 . It has been predicted 4 – 6 that localized thermonuclear bursts on white dwarfs can also take place, similar to type-I X-ray bursts observed in accreting neutron stars. Unexplained rapid bursts from the binary system TV Columbae, in which mass is accreted onto a moderately strong magnetized white dwarf from a low-mass companion, have been observed on several occasions in the past 40 years 7 – 11 . During these bursts, the optical/ultraviolet luminosity increases by a factor of more than  three in less than an hour and fades in around ten hours. Fast outflows have been observed in ultraviolet spectral lines 7 , with velocities of more than 3,500 kilometres per second, comparable to the escape velocity from the white dwarf surface. Here we report on optical bursts observed in TV Columbae and in two additional accreting systems, EI Ursae Majoris and ASASSN-19bh. The bursts have a total energy of approximately 10 −6  times than those of classical nova explosions (micronovae) and bear a strong resemblance to type-I X-ray bursts 12 – 14 . We exclude accretion or stellar magnetic reconnection events as their origin and suggest thermonuclear runaway events in magnetically confined accretion columns as a viable explanation. The identification and characterization of rapid bursts in three accreting white dwarfs have shown that magnetically confined thermonuclear runaways resembling type-I X-ray bursts may occur in the surface layers of white dwarf atmospheres.

Nova explosions occur on the white dwarf component of a cataclysmic variable binary stellar system that is accreting matter lost by its companion. When sufficient material has been accreted by the white dwarf, a thermonuclear runaway occurs and ejects material in what is observed as a classical nova explosion. We describe both the recent advances in our understanding of the progress of the outburst and outline some of the puzzles that are still outstanding. We report on the effects of improving both the nuclear reaction rate library and including a modern nuclear reaction network in our one-dimensional, fully implicit, hydrodynamic computer code. In addition, there has been progress in observational studies of supernovae Ia with implications about the progenitors, and we discuss that in this review.

We describe a dynamic science portal called the GROWTH Marshal that allows time-domain astronomers to define science programs; program filters to save sources from different discovery streams; coordinate follow-up with various robotic or classical telescopes; analyze the panchromatic follow-up data; and generate summary tables for publication. The GROWTH marshal currently serves 137 scientists, 38 science programs, and 67 telescopes. Every night, in real time, several science programs apply various customized filters to the 105 nightly alerts from the Zwicky Transient Facility. Here, we describe the schematic and explain the functionality of the various components of this international collaborative platform.

A star is reborn In February this year the recurrent nova RS Ophiuchi (RS Oph) burst into life. Every 20 years or so the white dwarf component of this binary accumulates sufficient material from its red giant companion to power a thermonuclear explosion that we see as an increase in magnitude from a very dim 12.5 to magnitude 5. Two groups report observations of the recent outburst. Satellite X-ray observations by Sokoloski et al . reveal an initial phase in which the blast wave expanded freely. Within two days the outbound wave started to slow, suggesting that there was much less debris than had been expected from such an event. O'Brien et al . trained the largest terrestrial radio telescope arrays on RS Oph and were able to directly image a shock wave in a nova explosion for the first time, 14 days after its initial discovery. The structures revealed show an evolution to a remnant similar to that of a type II supernova — but evolving over months rather than millennia. Stellar explosions such as novae and supernovae produce most of the heavy elements in the Universe. The onset of a nova is well understood 1 as driven by runaway thermonuclear fusion reactions on the surface of a white dwarf in a binary star system; but the structure, dynamics and mass of the ejecta are not well known. In rare cases, the white dwarf is embedded in the wind nebula of a red-giant companion, and the explosion products plough through the nebula and produce X-ray emission. Here we report X-ray observations of such an event, from the eruption of the recurrent nova RS Ophiuchi 2 , 3 . The hard X-ray emission from RS Ophiuchi early in the eruption emanates from behind a blast wave, or outward-moving shock wave, that expanded freely for less than 2 days and then decelerated owing to interaction with the nebula. The X-rays faded rapidly, suggesting that the blast wave deviates from the standard spherical shell structure 4 , 5 , 6 . The early onset of deceleration indicates that the ejected shell had a low mass, the white dwarf has a high mass 7 , and that RS Ophiuchi is therefore a progenitor of the type of supernova (type Ia) integral to studies of the expansion of the Universe.