We propose a wind-driven model for peculiar transients, and apply the model to AT2018cow and iPTF14hls. In the wind-driven model, we assume that a continuous outflow like a stellar wind is injected from a central system. While these transients have different observational properties, this model can explain their photometric properties which are not reproduced by a supernova-like instantaneous explosion. Furthermore, the model predicts characteristic spectral features and evolution, which are well in line with those of AT2018cow and iPTF14hls. Despite the different observational properties, the wind model shows that they have some common features; the large mass-loss rates (up to $\sim 20M_{\odot}{\rm ~yr^{-1}}$ for AT2018cow and $\sim 75M_{\odot}{\rm ~yr^{-1}}$ for iPTF14hls), and the characteristic radii of $\sim 10^{13}{\rm ~cm}$ for the launch of the wind. It would indicate that both may be related to events involving a red super giant (RSG), in which the RSG envelope is rapidly ejected by an event at a stellar core scale. On the other hand, the main differences are the kinetic energies, the total ejected mass, and time scales. We then suggest that iPTF14hls may represent a dynamical common-envelope evolution induced by a massive binary system ($\sim 120M_{\odot}+100M_{\odot}$). AT2018cow may be either a tidal disruption event of a low-mass RSG by a black hole (BH), or a BH-forming failed supernova.
We present optical spectroscopy, ultraviolet to infrared imaging and X-ray observations of the intermediate luminosity optical transient (ILOT) SN 2010da in NGC 300 (d=1.86 Mpc) spanning from -6 to +6 years relative to the time of outburst in 2010. Based on the light curve and multi-epoch SEDs of SN 2010da, we conclude that the progenitor of SN 2010da is a ~10-12 Msol yellow supergiant possibly transitioning into a blue loop phase. During outburst, SN 2010da had a peak absolute magnitude of M<-10.4 mag, dimmer than other ILOTs and supernova impostors. We detect multi-component hydrogen Balmer,…
We consider the situation where the luminosity from a transient event is reprocessed by an optically thick wind. Potential applications are the tidal disruption of stars by black holes, engine-powered supernovae, and unique fast transients found by current and future wide-field surveys. We derive relations between the injected and observed luminosity for steady and time dependent winds, and discuss how the temperature is set for scattering-dominated radiative transport. We apply this framework to specific examples of tidal disruption events and the formation of a black hole by a massive star, …
The hard X-ray source IGR J11215-5952 is a peculiar transient, displaying very short X-ray outbursts every 165 days. We obtained high-resolution spectra of the optical counterpart, HD 306414, at different epochs, spanning a total of three months, before and around the 2007 February outburst with the combined aims of deriving its astrophysical parameters and searching for orbital modulation. We fit model atmospheres generated with the fastwind code to the spectrum. We also cross-correlated each individual spectrum to the best-fit model to derive radial velocities. From its spectral features, we…
Context: During the ongoing all-sky survey, the Mikhail Pavlinsky ART-XC telescope on board the SRG observatory should discover new X-ray sources, many of which can be transient. Here we report on the discovery and multiwavelength follow-up of a peculiar X-ray source SRGA J043520.9+552226=SRGe J043523.3+552234 - the high-energy counterpart of the optical transient AT2019wey. Aims: Thanks to its sensitivity and the survey strategy, the Mikhail Pavlinsky ART-XC telescope uncovers poorly studied weak transient populations. Using a synergy with current public optical surveys, we are aiming at reve…
We report on a first census of Galactic black hole X-ray binary (BHXRB) properties with the second data release (DR2) of {\em Gaia}, focusing on dynamically confirmed and strong candidate black hole transients. DR2 provides five-parameter astrometric solutions including position, parallax and proper motion for 11 of a sample of 24 systems. Distance estimates are tested with parallax inversion as well as Bayesian inference. We derive an empirically motivated characteristic scale length of $L$=2.17$\pm$0.12 kpc for this BHXRB population to infer distances based upon an exponentially decreasing s…
We present the first direct evidence for dense clumps of matter in the companion wind in a Supergiant Fast X-ray Transient (SFXT) binary. This is seen as a brief period of enhanced absorption during one of the bright, fast flares that distinguish these systems. The object under study was IGR J17544-2619, and a total of 236 ks of data were accumulated with the Japanese satellite Suzaku. The activity in this period spans a dynamic range of almost 10000 in luminosity and gives a detailed look at SFXT behavior.
The black hole transient MAXI J1820+070 displayed optical P-Cyg profiles and other wind-related emission line features during the hard state of its discovery outburst. We present near-infrared (nIR) spectroscopy covering the different accretion states of the system during this event. Our 8-epoch data set (VLT/X-shooter) reveals strong variability in the properties of the nIR emission lines. This includes the presence of absorption troughs and extended emission line wings with kinetic properties that are remarkably similar to those inferred from the wind signatures observed in optical emission …
The metal rich globular cluster RZ 2109 in the massive Virgo elliptical galaxy NGC 4472 (M49) harbours the ultra luminous X-ray source XMMU 122939.7+075333. Previous studies showed that this source varies between bright and faint phases on timescales of just a few hours. Here, we report the discovery of two peculiar X-ray bursting events that last for about 8 and 3.5 hours separated by about 3 days. Such a recurring X-ray burst-like behaviour has never been observed before. We argue that type-I X-ray bursts or super bursts as well as outburst scenarios requiring a young stellar object are high…
We present the discovery of an optical accretion disk wind in the X-ray transient Swift J1858.6-0814. Our 90-spectrum data set, taken with the 10.4m GTC telescope over 8 different epochs and across five months, reveals the presence of conspicuous P-Cyg profiles in He I at 5876 Angs and Halpha. These features are detected throughout the entire campaign, albeit their intensity and main observational properties are observed to vary on time-scales as short as five minutes. In particular, we observe significant variations in the wind velocity, between a few hundreds and ~ 2400 km/s. In agreement wi…
We present an optical spectroscopic study of the black hole X-ray transient V4641 Sgr (=SAX J1819.3-2525) covering the 1999, 2002 and 2004 outbursts. The spectra were taken over 22 different epochs during the low luminosity phases that follow the sharp and bright outburst peaks displayed by the system. The data reveal the frequent presence of wind-related features in H (Balmer) and He I emission lines in the form of P-Cygni profiles and strong emission lines with broad wings. The terminal velocity of the wind, as measured in the blue-shifted absorption (P-Cygni) components, is in the range of …
The progenitor systems of the class of "Ca-rich transients" is a key open issue in time domain astrophysics. These intriguing objects exhibit unusually strong calcium line emissions months after explosion, fall within an intermediate luminosity range, are often found at large projected distances from their host galaxies, and may play a vital role in enriching galaxies and the intergalactic medium. Here we present multi-wavelength observations of iPTF15eqv in NGC 3430, which exhibits a unique combination of properties that bridge those observed in Ca-rich transients and Type Ib/c supernovae. iP…
We report on the X-ray properties of the new transient Swift J0840.7$-$3516, discovered with Swift/BAT in 2020 February, using extensive data of Swift, MAXI, NICER, and NuSTAR. The source flux increased for $\sim 10^3$ s after the discovery, decayed rapidly over $\sim$ 5 orders of magnitude in 5 days, and then remained almost constant over 9 months. Large-amplitude short-term variations on time scales of 1--$10^4$ s were observed throughout the decay. In the initial flux rise, the source showed a hard power-law shaped spectrum with a photon index of $\sim 1.0$ extending up to $\sim 30$ keV, ab…
We present a high-cadence short term photometric and spectroscopic monitoring campaign of a type Ibn SN 2019wep, which is one of the rare SN Ibn after SNe 2010al and 2019uo to display signatures of flash ionization (\ion{He}{2}, \ion{C}{3}, \ion{N}{3}). We compare the decline rates and rise time of SN 2019wep with other SNe Ibn and fast transients. The post-peak decline in all bands (0.1 mag d$^{-1}$) are consistent with SNe Ibn but less than the fast transients. On the other hand, the $Δ$m$_{15}$ values are slightly lower than the average values for SNe Ibn but consistent with the fast transi…
Is the Peculiar Galactic Center Transient Swift J174610.4-290018 A Nova Outburst?
School of Astronomy and Space Science, Nanjing University（南京大学天文与空间科学学院）；Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education（南京大学现代天文与天体物理教育部重点实验室）；Institute of Science and Technology for Deep Space Exploration, Suzhou Campus, Nanjing University（南京大学苏州校区深空探测科学与技术研究院）
Supergiant fast X-ray transient (SFXT) is a new class of the high mass X-ray binary that shows short X-ray flares. The physical mechanism of SFXT short flares is still open for discussion. The accretion process of dense clumps in stellar wind onto neutron star (NS) has been proposed as the origin of such short flares. In order to examine the applicability of the clumpy wind scenario, we focus on the accretion mode that depends on orbital parameters. Our goal is to impose restrictions on the orbital parameters of SFXT. Assuming a simple analytic model of clumpy wind, we investigate the conditio…
We studied the transient Galactic black hole candidate MAXI J0637-430 with data from Insight-HXMT, Swift and XMM-Newton. The broad-band X-ray observations from Insight-HXMT help us constrain the power-law component. MAXI J0637-430 is located at unusually high Galactic latitude; if it belongs to the Galactic thick disk, we suggest a most likely distance <7 kpc. Compared with other black hole transients, MAXI J0637-430 is also unusual for other reasons: a fast transition to the thermal dominant state at the start of the outburst; a low peak temperature and luminosity (we estimate them at ~ 0.7 k…
BD+6073 is the optical counterpart of the X-ray source IGR J00370+6122, a probable accretion-powered X-ray pulsar. The X-ray light curve of this binary system shows clear periodicity at 15.7 d, which has been interpreted as repeated outbursts around the periastron of an eccentric orbit. We obtained high-resolution spectra of BD+6073 at different epochs. We used the FASTWind code to generate a stellar atmosphere model to fit the observed spectrum and obtain physical magnitudes. The synthetic spectrum was used as a template for cross-correlation with the observed spectra to measure radial veloci…
The nature of the peculiar `Ca-rich' SN 2019ehk in the nearby galaxy M100 remains unclear. Its origin has been debated as either a stripped core-collapse supernova or a thermonuclear helium detonation event. Here, we present very late-time photometry of the transient obtained with the Keck I telescope at $\approx280$ days from peak light. Using the photometry to perform accurate flux calibration of a contemporaneous nebular phase spectrum, we measure an [O I] luminosity of $(0.19-1.08)\times10^{38}$ erg s$^{-1}$ and [Ca II] luminosity of $(2.7-15.6)\times10^{38}$ erg s$^{-1}$ over the range of…
We present our photometric and spectroscopic observations on the peculiar transient AT2018cow. The multi-band photometry covers from peak to $\sim$70 days and the spectroscopy ranges from 5 to $\sim$50 days. The rapid rise ($t_{\mathrm{r}}$$\lesssim$2.9 days), high luminosity ($M_{V,\mathrm{peak}}\sim-$20.8 mag) and fast decline after peak make AT2018cow stand out of any other optical transients. While we find that its light curves show high resemblance to those of type Ibn supernovae. Moreover, the spectral energy distribution remains high temperature of $\sim$14,000 K after $\sim$15 days sin…
Accretion of matter onto black holes is universally associated with strong radiative feedback and powerful outflows. In particular, black hole transients show outflows whose properties are strongly coupled to those of the accretion flow. This includes X-ray winds of ionized material, expelled from the accretion disc encircling the black hole, and collimated radio jets. Very recently, a distinct optical variability pattern has been reported in the transient black hole transient V404 Cyg, and interpreted as disrupted mass flow into the inner regions of its large accretion disc. Here, we report o…
Super-Chandrasekhar remnants of double white dwarf mergers could sometimes collapse into a rapidly rotating neutron star (NS), accompanying with a mass ejection of a few times $0.01M_{\odot}$. Bright optical transient emission can be produced by the ejecta due to heating by radioactivities and particularly by energy injection from the NS. Since the merger remnants before collapse resemble a star evolving from the asymptotic giant branch phase to the planetary nebula phase, an intense dusty wind is considered to be driven about several thousand years ago before the collapse and surround the rem…
Classical Supergiant X-ray Binaries (SGXBs) and Supergiant Fast X-ray Transients (SFXTs) are two types of High-mass X-ray Binaries (HMXBs) that present similar donors but, at the same time, show very different behavior in the X-rays. The reason for this dichotomy of wind-fed HMXBs is still a matter of debate. Among the several explanations that have been proposed, some of them invoke specific stellar wind properties of the donor stars. Only dedicated empiric analysis of the donors' stellar wind can provide the required information to accomplish an adequate test of these theories. However, such…
A fraction of high-mass X-ray binaries are supergiant fast X-ray transients. These systems have on average low X-ray luminosities, but display short flares during which their X-ray luminosity rises by a few orders of magnitude. The leading model for the physics governing this X-ray behaviour suggests that the winds of the donor OB supergiants are magnetized. In agreement with this model, the first spectropolarimetric observations of the SFXT IGR J11215-5952 using the FORS2 instrument at the Very Large Telescope indicate the presence of a kG longitudinal magnetic field. Based on these results, …
The origin of the so-called $p$-isotopes $^{92,94}\mathrm{Mo}$ and $^{96,98}\mathrm{Ru}$ in the solar system remains a mystery as several astrophysical scenarios fail to account for them. In addition, data on presolar silicon carbide grains of type X (SiC X) exhibit peculiar Mo patterns, especially for $^{95,97}\mathrm{Mo}$. We examine production of Mo and Ru isotopes in neutrino-driven winds associated with core-collapse supernovae (CCSNe) over a wide range of conditions. We find that proton-rich winds can make dominant contributions to the solar abundance of $^{98}\mathrm{Ru}$, significant c…
A number of astrophyical transients originating from stellar explosions are powered by the collision of the ejected material with the circumstellar medium, which efficiently produces thermal radiation via shock dissipation. We investigate how such interaction-powered transients are distributed in the peak bolometric luminosity vs the rise time phase space. Taking the advantage of less time-consuming one-dimensional simulations with spherical symmetry, we calculated more than 500 models with different circumstellar mass and radius, ejecta mass and energy, and chemical compositions. The peak bol…
Radiation-ionization hydrodynamic simulations of AGN line-driven winds lead to transient shielding and BAL/UFO signatures
School of Physics and Astronomy, University of Southampton（南安普顿大学物理与天文学院）；Eureka Scientific Inc.（尤里卡科学公司）；Univ. Grenoble Alpes, CNRS, IPAG（格勒诺布尔阿尔卑斯大学，法国国家科学研究中心，天体物理学与行星学研究所）；Space Telescope Science Institute（太空望远镜科学研究所）；Department of Physics, Astrophysics, University of Oxford（牛津大学物理与天体物理系）；School of Mathematics and Physics, Queen’s University Belfast（贝尔法斯特女王大学数学与物理学院）
AT 2019avd is a nuclear transient detected from infrared to soft X-rays, though its nature is yet unclear. The source has shown two consecutive flaring episodes in the optical and the infrared bands and its second flare was covered by X-ray monitoring programs. During this flare, the UVOT/Swift photometries revealed two plateaus: one observed after the peak and the other one appeared ~240 days later. Meanwhile, our NICER and XRT/Swift campaigns show two declines in the X-ray emission, one during the first optical plateau and one 70-90 days after the optical/UV decline. The evidence suggests th…
We present an optical spectrum of the energetic Type Ib supernova (SN) 2012au obtained at an unprecedented epoch of 6.2 years after explosion. Forbidden transition emission lines of oxygen and sulfur are detected with expansion velocities of 2300 km/s. The lack of narrow H Balmer lines suggests that interaction with circumstellar material is not a dominant source of the observed late-time emission. We also present a deep Chandra observation that reveals no X-ray emission down to a luminosity of L_X < 2 x 10^{38} erg/s (0.5-10 keV). Our findings are consistent with the notion that SN 2012au is …
We propose that superluminous transients that appear at central regions of active galactic nuclei (AGNs) such as CSS100217:102913+404220 (CSS100217) and PS16dtm, which reach near or super-Eddington luminosities of the central black holes, are powered by the interaction between accretion disk winds and clouds in broad-line regions (BLRs) surrounding them. If the disk luminosity temporary increases by, e.g., limit-cycle oscillations, leading to a powerful radiatively driven wind, strong shock waves propagate in the BLR. Because the dense clouds in the AGN BLRs typically have similar densities to…
We present the results of hydrodynamical simulations of the disk photosphere irradiated by strong X-rays produced in the inner most part of the disk. As expected, the irradiation heats the photosphere and drives a thermal wind. To apply our results to the well-studied X-ray transient source GRO J1655-40, we adopted the observed mass of its black hole, and the observed properties of its X-ray radiation. To compare the results with the observations, we also computed transmitted X-ray spectra based on the wind solution. Our main finding is: the density of the fast moving part of the wind is more …
All disc-accreting astrophysical objects produce powerful outflows. In binaries containing neutron stars (NS) or black holes, accretion often takes place during violent outbursts. The main disc wind signatures during these eruptions are blue-shifted X-ray absorption lines, which are preferentially seen in disc-dominated "soft states". By contrast,optical wind-formed lines have recently been detected in "hard states", when a hot corona dominates the luminosity. The relationship between these signatures is unknown, and no erupting system has revealed wind-formed lines between the X-ray and optic…
Evidence for a luminosity-class dichotomy in the supergiant donors of persistent high-mass X-ray binaries and supergiant fast X-ray transients
INAF-IASF Milano, Istituto di Astrofisica Spaziale e Fisica Cosmica（意大利国家天体物理研究所米兰空间天体物理学与宇宙物理学研究所）；INAF-OAS, Osservatorio di Astrofisica e Scienza dello Spazio（意大利国家天体物理研究所太空天体物理学与空间科学观测站）
We present an optical and infrared (IR) study of IC 10 X-2, a high-mass X-ray binary in the galaxy IC 10. Previous optical and X-ray studies suggest X-2 is a Supergiant Fast X-ray Transient: a large-amplitude (factor of $\sim$ 100), short-duration (hours to weeks) X-ray outburst on 2010 May 21. We analyze R- and g-band light curves of X-2 from the intermediate Palomar Transient Factory taken between 2013 July 15 and 2017 Feb 14 show high-amplitude ($\gtrsim$ 1 mag), short-duration ($\lesssim8$ d) flares and dips ($\gtrsim$ 0.5 mag). Near-IR spectroscopy of X-2 from Palomar/TripleSpec show He I…
The dense wind environment (or circumstellar medium) may be ubiquitous for the regular Type II supernovae (SNe) before the explosion, the interaction of which with the SN ejecta could result in a wind breakout event. The shock generated by the interaction of the SN ejecta and the wind can accelerate the protons and subsequently the high-energy gamma-rays and neutrinos could arise from the inelastic pp collisions. In this work, we present the detailed calculations of gamma-ray and neutrino production for the regular Type II SNe. The calculation is executed by applying time-dependent evolutions …
Tycho's supernova remnant (SNR) has a periphery which clearly deviates from spherical shape bash on the X-ray and radio observations. The forward shock from the southeast to the north of the remnant has a relatively deformed outline with a depression in the east, although in the west it is generally round and smooth. Moreover, at some locations of the shell, the supernova ejecta distributes closely to the forward shock to produce protrusions. Using 3D hydrodynamical simulation, the dynamical evolution of the supernova ejecta in an inhomogeneous medium and the formation process of the profile o…
We discuss the electromagnetic radiation from newborn binary black holes (BBHs). As a consequence of the evolution of massive stellar binaries, a binary consisting of a primary black hole (BH) and a secondary Wolf-Rayet star is expected as a BBH progenitor system. We investigate optical transients from the birth of BBHs powered by the Bondi-Hoyle-Lyttleton accretion onto the primary BH, which occur $\sim1-10$ Gyr earlier than gravitational wave signals at the BH-BH merger. When the secondary massive star collapses into a BH, it may eject a fraction of its outer material and may form a disk aro…
We present a comprehensive nucleosynthesis study of the neutrino-driven wind in the aftermath of a binary neutron star merger. Our focus is the initial remnant phase when a massive central neutron star is present. Using tracers from a recent hydrodynamical simulation, we determine total masses and integrated abundances to characterize the composition of unbound matter. We find that the nucleosynthetic yields depend sensitively on both the life time of the massive neutron star and the polar angle. Matter in excess of up to $9 \cdot 10^{-3} M_\odot$ becomes unbound until $\sim 200~{\rm ms}$. Due…
We study the radioactively-powered transients produced by accretion disk winds following a compact object merger. Starting with the outflows generated in two-dimensional hydrodynamical disk models, we use wavelength-dependent radiative transfer calculations to generate synthetic light curves and spectra. We show that the brightness and color of the resulting kilonova transients carry information about the merger physics. In the regions of the wind where neutrino irradiation raises the electron fraction to Ye > 0.25, r-process nucleosynthesis halts before producing high-opacity, complex ions (t…
Accretion disk winds are revealed in Chandra gratings spectra of black holes. The winds are hot and highly ionized (typically composed of He-like and H-like charge states), and show modest blue-shifts. Similar line spectra are sometimes seen in "dipping" low-mass X-ray binaries, which are likely viewed edge-on; however, that absorption is tied to structures in the outer disk, and blue-shifts are not typically observed. Here we report the detection of blue-shifted He-like Fe XXV (3100 +/- 400 km/s) and H-like Fe XXVI (1000 +/- 200 km/s) absorption lines in a Chandra/HETG spectrum of the transie…
I will review the main observational properties of the new Galactic High Mass X-ray Binaries discovered by the INTEGRAL satellite in the hard energy range 17-100 keV. About 70% of the newly discovered HMXBs host OB supergiant companions and show peculiar properties with respect to classical HMXBs detected with previous missions: some of them display huge local absorptions, in excess of 1E23 cm^-2 (the so-called obscured sources), while others show fast transient X-ray emission, leading to the definition of a new sub-class of HMXBs, the so-called Supergiant Fast X-ray Transients. Their peculiar…
Supergiant Fast X-ray Transients are a class of Galactic High Mass X-ray Binaries with supergiant companions. Their extreme transient X-ray flaring activity was unveiled thanks to INTEGRAL/IBIS observations. The SFXTs dynamic range, with X-ray luminosities from 1E32 erg/s to 1E37 erg/s, and long time intervals of low X-ray emission, are puzzling, given that both their donor star properties and their orbital and spin periodicities seem very similar to those displayed by massive binaries with persistent X-ray emission. Clumpy supergiant winds, accretion barriers, orbital geometries and wind anis…
Recent wide-field surveys discovered new types of peculiar optical transients that showed diverse behaviors of the evolution of photospheric properties. We develop a general theory of homologous explosions with constant opacity, paying special attention on the evolution of the photospheric radius $R_{\rm ph}$. We find that regardless of the density distribution profile, $R_{\rm ph}$ always increases early on and decreases at late times. This result does not depend on the radiation and cooling processes inside the ejecta.The general rising/falling behavior of $R_{\rm ph}$ can be used to quickly…
We present a detailed, 3D hydrodynamics study of the neutrino-driven winds that emerge from the remnant of a NS merger. Our simulations are performed with the Newtonian, Eulerian code FISH, augmented by a detailed, spectral neutrino leakage scheme that accounts for heating due to neutrino absorption in optically thin conditions. Consistent with the 2D study of Dessart et al. (2009), we find that a strong baryonic wind is blown out along the original binary rotation axis within $100$ ms after the merger. We compute a lower limit on the expelled mass of $3.5 \times 10^{-3} M_{\odot}$, large enou…
From the first two seasons of the Palomar Transient Factory, we identify three peculiar transients (PTF09dav, PTF10iuv, PTF11bij) with five distinguishing characteristics: peak luminosity in the gap between novae and supernovae (M_R = 15.5 to -16.5), rapid photometric evolution (rise-time ~12--15 days), large photospheric velocities (~6000 to 11000 km/s), early spectroscopic evolution into nebular phase (~1 to 3 months) and peculiar nebular spectra dominated by Calcium. We also culled the extensive decade-long Lick Observatory Supernova Search database and identified an additional member of th…
Very recently \citet{XueYQ2019} reported an important detection of the X-ray transient, CDF-S XT2, whose light curve is analogous to X-ray plateau features of gamma-ray burst afterglows. They suggested that this transient is powered by a remnant stable magnetar from a binary neutron star merger since several pieces of evidence (host galaxy, location, and event rate) all point toward such an assumption. In this paper, we revisit this scenario and confirm that this X-ray emission can be well explained by the internal gradual magnetic dissipation process in an ultra-relativistic wind of the newbo…
We use 3D hydrodynamic simulations of the long-term evolution of neutron star merger ejecta to predict the light curves of electromagnetic transients that are powered by the decay of freshly produced r-process nuclei. For the dynamic ejecta that are launched by tidal and hydrodynamic interaction, we adopt grey opacities of 10 cm$^2$/g, as suggested by recent studies. For our reference case of a 1.3-1.4 $M_\odot$ merger, we find a broad IR peak 2-4 d after the merger. The peak luminosity is $\approx 2\times 10^{40}$ erg/s for an average orientation, but increased by up to a factor of 4 for more…
Supergiant fast X-ray transients (SFXTs) are a sub-class of supergiant high mass X-ray binaries hosting a neutron star accreting from the stellar wind of a massive OB companion. Compared to the classical systems, SFXTs display a pronounced variability in X-rays that has long been (at least partly) ascribed to the presence of clumps in the stellar wind. We report here on the first set of results of an on-going XMM-Newton observational program aimed at searching for spectroscopic variability during the X-ray flares and outbursts of the SFXTs. The goal of the paper is to present the observational…
We consider implications of a possible presence of a Thomson-thick, low-temperature, plasma cloud surrounding the compact object in the binary system Cyg X-3. The presence of such a cloud was earlier inferred from the energy-independent orbital modulation of the X-ray flux and the lack of high frequencies in its power spectra. Here, we study the effect of Compton scattering by the cloud on the X-ray energy and power spectra, concentrating on the hard spectral state. The process reduces the energy of the high-energy break/cut-off in the energy spectra, which allows us to determine the Thomson o…
The accretion-induced collapse (AIC) of a white dwarf in a binary with a nondegenerate companion can sometimes lead to the formation of a rapidly rotating and highly magnetized neutron star (NS). The spin-down of this NS can drive a powerful pulsar wind (PW) and bring out some detectable multi-wavelength emissions. On the one hand, the PW can evaporate the companion in a few days to form a torus surrounding the NS. Then, due to the blockage of the PW by the torus, a reverse shock can be formed in the wind to generate intense hard X-rays. This emission component disappears in a few weeks' time,…
The six known highly dispersed fast radio bursts are attributed to extragalactic radio sources, of unknown origin but extremely energetic. We propose here a new explanation - not requiring an extreme release of energy - involving a body (planet, asteroid, white dwarf) orbiting an extragalactic pulsar. We investigate a theory of radio waves associated to such pulsar-orbiting bodies. We focus our analysis on the waves emitted from the magnetic wake of the body in the pulsar wind. After deriving their properties, we compare them with the observations of various transient radio signals in order to…
