V404 Cyg Research Articles

Complex Organics Surrounding the FU Ori–type Object V1057 Cyg Indicative of Sublimated Ices

FU Ori– and EX Lup–type objects present natural experiments for understanding a critical stage in the star and planet formation process. These objects offer insight into the diversity of molecules available to forming planetary systems due to a sudden increase in accretion, and central luminosity causes the disk and surrounding material to increase in temperature. This allows for volatiles to sublimate off of grains and exist in the gas phase for tens to hundreds of years after initial outburst. While this dynamic stage may be common for solar-type protostars, observations of the chemical impact of these bursts are rare. In this article, we present observations from the NOrthern Extended Millimeter Array of five young stellar objects (YSOs) that have undergone outbursts within the past 100 yr and catalog the volatile chemistry found within ∼1000 au of the YSO. Only one source clearly shows a line-rich spectra with >11 molecules detected, including complex organics and water, as is expected for a spectra signature for a postoutburst source. This source is V1057 Cyg, and we present it as the northern analog to the well-studied and molecule-rich FU Ori source, V883 Ori. Our conclusions on the chemical inventory of the other four sources in our sample are sensitivity limited, as V1057 Cyg contains the highest disk/envelope gas mass.

The black hole low-mass X-ray binary V404 Cygni is part of a wide triple.

Evidence suggests that, when compact objects such as black holes and neutron stars form, they may receive a 'natal kick', during which the stellar remnant gains momentum. Observational evidence for neutron star kicks is substantial1,2, yet is limited for black hole natal kicks, and some proposed black hole formation scenarios result in very small kicks3-5. Here we report that the canonical black hole low-mass X-ray binary (LMXB) V404 Cygni is part of a wide hierarchical triple with a tertiary companion at least 3,500 astronomical units (AU) away from the inner binary. Given the orbital configuration, the black hole probably received a sub-5 km s-1 kick to have avoided unbinding the tertiary. This discovery lends support to the idea that at least some black holes form with nearly no natal kick. Furthermore, the tertiary in this system lends credence to evolutionary models of LMXBs involving a hierarchical triple structure6. Remarkably, the tertiary is evolved, indicating that the system formed 3-5 billion years ago and that the black hole has removed at least half a solar mass of matter from its evolved secondary companion. During the event in which the black hole formed, it is required that at least half of the mass of the black hole progenitor collapsed into the black hole; it may even have undergone a complete implosion, enabling the tertiary to remain loosely bound.

Spectral and Timing Study of V404 Cygni with CHANDRA Observations

We present the spectral and timing study of V404 Cygni from all its available Chandra observations and which recently come up in public domain of Chandra data archive. The data reduction and analysis were done using CIAO 4.14 and HEASOFT 6.30.1. The spectral analysis was done using spectral fitting package XSPEC version 12.12.1, available in the Heasoft package. The spectra of the source is fitted in the energy range 0.3 -8.0 keV using two empirical spectral models - the absorbed power law and an absorbed disk-blackbody. The X-ray binary source V404 Cygni is found to be in the quiescent state having the X-ray luminosity in the range withfew times 1032 erg s−1. The source is found to be in the hard state and is well explained by power-law model with a powerlaw photon index Γ ∼ 2 with nH in the range ∼ (0.7 -1.2) × 1022 cm−2. From timing analysis, Src-1 (V404 Cygni), in all the time bins- 0.5, 1 and 2 ks, the probability for the count rate to be constant is 0.17 ×10−33 in all the observations in the year 2021 and 2023 (ObsID 23421, ObsID 23422, ObsID 23423 & ObsID 28927). However, in the year 2017 observation it is found to be less variable. This clearly shows the presence of short-term variability in kilo-seconds time-scales with the currently available Chandra data. So, it is indicative that the binary source V404 Cygni is more likely to be variable source both in long-term (years) as well as short-term (kiloseconds) scales.

Misalignment of jet, orbital plane, and accretion disk observed in black hole binaries

Black holes are interesting objects and prove to be the leading edge of testing and verifying of our understandings in gravity. Among some of the most sought after quantities is the spin of the black hole: it is closely related to how the black hole system behaves. Various attempts have been made to derive the spin of black holes, in which the orientation in space could be deduced using the spectrum produced by the accretion disk, observation of the jets produced by black holes, and calculation of the orbit of black hole binaries using light and velocity curves. These observations, however, can sometimes lead to disagreeing results. In this paper, conflicts of the orientations of the spins of black holes measured using these different methods are presented. Sources including XTE J1550-564, V404 Cyg, and V4641 Sgr display misaligned accretion disk with orbital plane and/or jets. Explanations of how such misalignments could arise are also given, including warped accretion disks and precessing jets.

Spectroscopic searches for evolutionary orbital period changes in WR+OB binaries: The case of WR 127 (Hen 3-1772)

Aims. We aim to determine the secular evolution of the orbital period of the short-period binary system WR 127 (WN3b+O9.5V, P ≈ 9.555d). Methods. We performed new low-resolution spectroscopic observations of WR 127 with the 2.5 m CMO SAI telescope to construct the radial velocity curves of the components. Our results suggest component masses of MWRsin3(i) = 11.8 ± 1.4 M⊙ and MOsin3(i) = 17.2 ± 1.4 M⊙. By comparing these values with archival radial velocity curves we were able to create an (O − C) plot with an accuracy sufficient to search for the orbital period change in WR 127. Results. We report the reliable detection of a secular increase in the orbital period of WR 127 at a rate of Ṗ = 0.83 ± 0.14 s yr−1, which corresponds to a dynamical mass-loss rate from the Wolf-Rayet (WR) star of ĖWR = (2.6 ± 0.5) × 10−5 M⊙ yr−1. Conclusions. The mass-loss rate from WR stars in three Wolf-Rayet+OB binaries (WR 127, CX Cep, and V444 Cyg) as inferred from spectroscopic and photometric measurements suggests a preliminary empirical correlation between a WR star’s mass and its dynamical mass-loss rate of ṀWR ∼ MWR1.8. This relation is important for the understanding of the evolution of massive close binaries that include WR stars as such an evolution is a precursor of gravitational-wave binary merging events with neutron stars and black holes.

Correction to: A survey of molecular line emission towards Herbig Be star V645 Cyg

Correction to: A survey of molecular line emission towards Herbig Be star V645 Cyg

CO and Atomic Line Emission from the Young Stellar Object V1331 Cygni

The infrared spectrum of the young disk star V1331 Cygni shows rare atomic emission lines from several species including C i, O i, Na i, Mg i, Al i, Si i, Ca i, Fe i, and probably S i. The presence of H i emission but the absence of N i recombination lines indicates excitation by a comparatively cool source unable to ionize nitrogen. Measurements of both the fundamental and first overtone emission of carbon monoxide indicate a hot (T ≈ 3000 K), optically thick (optical depth ≈100 in the strongest transitions of the fundamental) for the molecular gas.

A survey of molecular line emission towards Herbig Be star V645 Cyg

ABSTRACT We present a survey of molecular line emission towards the molecular cloud surrounding Herbig Be star V645 Cyg. The survey was performed with the 20-m Onsala space telescope at 3 and 4 mm. We detected emission lines of 33 molecules and their isotopologues from diatomic molecules to four COMs up to seven atoms. Using detected lines, we estimated molecular column densities and abundances relative to molecular hydrogen in local thermodynamic equilibrium (LTE) approximation for all molecules except for methanol, for which we obtained physical parameters using a non-LTE model. Moreover, in the basement of the non-LTE model of methanol line emission, we consider that there is a weak maser effect in the additional spectral component of 51–40 E methanol line at 84.521 GHz. We compared the molecular abundances with values found in several astrochemical templates: molecular clouds, hot cores, and photodissociation regions, and found that signatures of these different types can be found towards V645 Cyg. We also obtained maps of the cloud in several molecular emission lines. The peaks of CO and CH3OH emission are shifted from the direction of the star, but the CS, HCO+, HNC, HCN, and N2H+ emission peaks are observed directly towards the star. Exploring the gas kinematics around V645 Cyg, we found that velocity structure in the ambient molecular cloud on the scale ≈1.6–2.0 pc is not the same as within ≈0.5 pc found previously by other authors.

Unveiling optical signatures of outflows in accreting white dwarfs

Accreting white dwarfs are known to show signatures of wind-type outflows in the ultraviolet. However, at optical wavelengths, wind detections have only been reported for a few sources. We present GTC-10.4 m optical spectroscopy of four accreting white dwarfs (BZ Cam, V751 Cyg, MV Lyr, and V425 Cas) observed during luminous epochs, when their optical emission is expected to be dominated by the accretion disc. Our analysis focuses on four emission lines: Hα and He Iλ5876, λ6678, and λ7065. Line profiles are complex and variable on short (minutes) and long (days to weeks) timescales, with transient absorption and emission components. Among them, we detect strong blueshifted absorptions at ≳1000 km s−1. These high-velocity components, present only in the blue wing of the emission lines, are observed in all four sources and could be associated with accretion disc winds. For MV Lyr and V425 Cas, these would represent the first detection of optical outflows in these objects, while in the cases of BZ Cam and V751 Cyg, the presence of outflows has been previously reported. This study suggests that, in addition to ultraviolet winds, optical outflows might also be common in accreting white dwarfs. We discuss the observational properties of these winds and their possible similarity to those detected in accreting black holes and neutrons stars.

Spectroscopic searches for evolutionary orbital period changes in WR+OB binaries: the case of V444 Cyg

ABSTRACT We present the results of new photometric and spectroscopic observations of the WN5+O6 binary V444 Cyg and a detailed analysis of extant spectroscopy and photometry. Using elements of the spectroscopic orbit and assuming e ≈ 0, i ≈ 78° we determined the masses and orbit sizes of the components of V444 Cyg as MO6 ≈ 26.4 M⊙, MWN5 ≈ 10.7 M⊙, aO6 ≈ 10.6 R⊙, aWN5 ≈ 26.1 R⊙. Based on new and archival light curves and using the Hertzsprung method, we improved the photometric estimate of the secular increase rate of the orbital period in V444 Cyg, obtaining $\dot{P}_{\mathrm{ph}} = 0.119\pm 0.003$ s yr−1. From a comparison of the new and archival radial velocity curves of V444 Cyg, we independently derived the secular orbital period change rate as $\dot{P}_{\mathrm{sp}} = 0.147\pm 0.032$ s yr−1, in agreement with the photometric $\dot{P}_{\mathrm{ph}}$. The obtained secular increase rate of the binary orbital period $\dot{P}$ and the mean radii of the components enabled us to estimate the stellar wind mass-loss rate from the WR star as $\dot{M}_{\mathrm{WN5}} = -(6.0\pm 0.4)\times 10^{-6}~{\rm M}_{\odot }\mbox{ yr}^{-1}$.

Colliding winds in WR21 and WR31 – I. The X-ray view

ABSTRACT WR21 and WR31 are two WR + O binaries with short periods, quite similar to the case of V444 Cyg. The XMM-Newton observatory has monitored these two objects and clearly revealed phase-locked variations as expected from colliding winds. The changes are maximum in the soft band (0.5–2.0 keV, variations by a factor 3–4) where they are intrinsically linked to absorption effects. The increase in absorption due to the dense WR wind is confirmed by the spectral analysis. The flux maximum is however not detected exactly at conjunction with the O star in front but slightly afterwards, suggesting Coriolis deflection of the collision zone as in V444 Cyg. In the hard band (2–10 keV), the variations (by a factor of 1.5–2.0) are much more limited. Because of the lower orbital inclinations, eclipses as observed for V444 Cyg are not detected in these systems.

Probing the jet size of two black hole X-ray binaries in the hard state

ABSTRACT Using multifrequency very long baseline interferometry (VLBI) observations, we probe the jet size in the optically thick hard state jets of two black hole X-ray binary (BHXRB) systems, MAXI J1820+070 and V404 Cygni. Because of optical depth effects, the phase-referenced VLBI core positions move along the jet axis of the BHXRB in a frequency-dependent manner. We use this ‘core shift’ to constrain the physical size of the hard state jet. We place an upper limit of 0.3 au on the jet size measured between the 15 and 5 GHz emission regions of the jet in MAXI J1820+070, and an upper limit of 1.0 au between the 8.4 and 4.8 GHz emission regions of V404 Cygni. Our limit on the jet size in MAXI J1820+070 observed in the low-hard state is a factor of 5 smaller than the values previously observed in the high-luminosity hard state (using time lags between multifrequency light curves), thus showing evidence of the BHXRB jet scaling in size with jet luminosity. We also investigate whether motion of the radio-emitting region along the jet axis could affect the measured VLBI parallaxes for the two systems, leading to a mild tension with the parallax measurements of Gaia. Having mitigated the impact of any motion along the jet axis in the measured astrometry, we find the previous VLBI parallax measurements of MAXI J1820+070 and V404 Cygni to be unaffected by jet motion. With a total time baseline of 8 yr, due to having incorporated 14 new epochs in addition to the previously published ones, our updated parallax measurement of V404 Cygni is 0.450 ± 0.018 mas (2.226 ± 0.091 kpc).

Post-outburst Evolution of Bona Fide FU Ori-type V2493 Cygnus: A Spectro-photometric Monitoring

We present here the results of 8 yr of our near-simultaneous optical–near-infrared spectro-photometric monitoring of the bona fide FU Ori (FUor)-type candidate V2493 Cygnus (V2493 Cyg) starting from 2013 September to 2021 June. During our optical monitoring period (between 2015 October 16 and 2019 December 30), the V2493 Cyg is slowly dimming with an average dimming rate of ∼26.6 ± 5.6 mmag yr−1 in the V band. Our optical photometric colors show a significant reddening of the source post the second outburst pointing toward a gradual expansion of the emitting region post the second outburst. The mid-infrared colors, on the contrary, exhibit a blueing trend, which can be attributed to the brightening of the disk due to the outburst. Our spectroscopic monitoring shows a dramatic variation of the Hα line as it transitioned from absorption feature to the emission feature and back. Such transition can possibly be explained by the variation in the wind structure in combination with accretion. Combining our time evolution spectra of the Ca ii infrared triplet lines with the previously published spectra of V2493 Cyg, we find that the accretion region has stabilized compared to the early days of the outburst. The evolution of the O i λ7773 Å line also points toward the stabilization of the circumstellar disk post the second outburst.

Ask the machine: systematic detection of wind-type outflows in low-mass X-ray binaries

ABSTRACT The systematic discovery of outflows in the optical spectra of low-mass X-ray binaries opened a new avenue for the study of the outburst evolution in these extreme systems. However, the efficient detection of such features in a continuously growing data base requires the development of new analysis techniques with a particular focus on scalability, adaptability, and automatization. In this pilot study, we explore the use of machine learning algorithms to perform the identification of outflows in spectral line profiles observed in the optical range. We train and test the classifier on a simulated data base constructed through a combination of disc emission line profiles and outflow signatures, emulating typical observations of low-mass X-ray binaries. The final, trained classifier is applied to two sets of spectra taken during two bright outbursts that were particularly well covered, those of V404 Cyg (2015) and MAXI J1820+070 (2018). The resulting classification gained by this novel approach is overall consistent with that obtained through traditional techniques, while simultaneously providing a number of key advantages over the latter, including the access to low-velocity outflows. This study sets the foundations for future studies on large samples of spectra from low-mass X-ray binaries and other compact binaries.

An evolutionary model for the V404 Cyg system

V404 Cyg is a low mass X-Ray binary (LMXB) system that has undergone outbursts in 1938, 1989, and 2015. During these events, it has been possible to make determinations for the relevant data of the system. This data include the mass of the compact object (i.e., a black hole; BH) and its companion, the orbital period, the companion spectral type, and luminosity class. Remarkably, the companion star has a metallicity value that is appreciably higher than solar. All these data allow for the construction of theoretical models to account for its structure, determine its initial configuration, and predict its fate. Assuming that the BH is already formed when the primary star reaches the zero age main sequence, we used our binary evolution code for this purpose. We find that the current characteristics of the system are nicely accounted for by a model with initial masses of 9 M⊙ for the BH, 1.5 M⊙ for the companion star and an initial orbital period of 1.5 d, while also considering that at most 30% of the mass transferred by the donor is accreted by the BH. The metallicity of the donor for our best fit is Z = 0.028 (twice solar metallicity). We also studied the evolution of the BH spin parameter, assuming that is not rotating initially. Remarkably, the spin of the BHs in our models is far from reaching the available observational determination. This may indicate that the BH in V404 Cyg was initially spinning, a result that may be relevant for understanding the formation BHs in the context of LMXB systems.

XMM-Newton observation of V1504 Cyg as a probe for the existence of an evaporated corona

Aims. We present an analysis of an XMM-Newton observation of the dwarf nova V1504 Cyg during the decline from an outburst. Our goal is to search for evidence of an evaporated X-ray corona. Such a corona can be understood as an optically thin and geometrically thick disc around the central part of an optically thick and geometrically thin disc. Methods. We study the X-ray spectra of a dwarf nova using a cooling-flow model and the evolution in the amplitude of variability and power density spectra in the UV and X-ray. Results. The X-ray (pn) count rate increases from initially around 0.03 cps to 0.17 cps, with a harder spectrum and a higher degree of variability. Meanwhile, the OM/UVW1 light curve follows a slow decline with a decreasing amplitude of variability. Next, we split the X-ray data into two segments and analysed them separately. Both were described by a cooling-flow model, while the first low-luminosity segment required an additional power-law component, suggesting the presence of a wind. A spectral fitting revealed a higher temperature for the second brighter segment. A timing analysis revealed a potential break frequency at log(f/Hz) = −3.02 during the decline towards the quiescence. This detection is in agreement with optical data from Kepler observations. Conclusions. The X-ray nature of the break frequency supports the innermost parts of the disc as source of the variability. Moreover, a similar frequency was observed in several other cataclysmic variables. Thus, a sandwich model where a geometrically thick corona surrounds the geometrically thin disc is a plausible accretion configuration.

Short time-scale evolution of the polarized radio jet during V404 Cygni’s 2015 outburst

ABSTRACTWe present a high time resolution, multifrequency linear polarization analysis of very large array (VLA) radio observations during some of the brightest radio flaring (${\sim } 1 \,{\rm Jy}\,$) activity of the 2015 outburst of V404 Cygni. The VLA simultaneously captured the radio evolution in two bands (each with two 1 GHz base-bands), recorded at 5/7 GHz and 21/26 GHz , allowing for a broadband polarimetric analysis. Given the source’s high flux densities, we were able to measure polarization on time-scales of ${\sim }13\,$ min, constituting one of the highest temporal resolution radio polarimetric studies of a black hole X-ray binary outburst to date. Across all base-bands, we detect variable, weakly linearly polarized emission (${\lt } 1{{ \rm per\ cent}}$) with a single, bright peak in the time-resolved polarization fraction, consistent with an origin in an evolving, dynamic jet component. We applied two independent polarimetric methods to extract the intrinsic electric vector position angles and rotation measures from the 5 and 7 GHz base-band data and detected a variable intrinsic polarization angle, indicative of a rapidly evolving local environment or a complex magnetic field geometry. Comparisons to the simultaneous, spatially-resolved observations taken with the Very Long Baseline Array at 15.6 GHz , do not show a significant connection between the jet ejections and the polarization state.

О природе фотометрической активности молодой звезды V2492 Cyg

The article provides a brief overview of the work carried out over several years to study the photometric activity of the young star V2492 Cyg. After the discovery of the flare of this star in August 2010, intensive multi-wave photometric and spectral observations were carried out, on the basis of which contradictory results were obtained regarding the classification of the object and, as a consequence, the determination of the main mechanism responsible for the variability of radiation. The initial statement that this is uxor and its variability is due to extinction on the line of view contradicted the spectroscopy data, which indicated an obvious outflow of matter caused by accretion. Currently, it is believed that photometric activity is due to both the accretion of matter from the disk onto the star and variable extinction caused by various gas-dust structures on the line of view.

Interpretation of the light curve of gamma-ray emission from the 2021 outburst of the recurrent nova RS Ophiuchi

Nova outbursts take place in binary star systems comprising a white dwarf (WD) and either a low-mass Sun-like star (classical novae) or, a red giant. GeV gamma-ray emission has been detected from a dozen of classical novae and from one novae in symbiotic system (V407 Cyg) by Fermi-LAT. For classical novae, gamma-ray emission is generally thought to be related to internal shocks formed as fast outflow collides with the slow outflow. However, for V407 Cyg, the origin of the gamma-ray emission has been debated, as both internal shock and external shock, resulted from the collision between the nova ejecta and the ambient wind of the giant companion, were suggested to explain the gamma-ray data. Recently, bright GeV and TeV gamma-ray emission has been detected from a nova in symbiotic system, RS Ophiuchi, during its 2021 outburst, which shows a remarkably smooth power-law decay in time up to about one month after the outburst. We show that this temporal decay behavior can be interpreted as arising from an adiabatic external shock expanding in the red giant wind. In this interpretation, the gamma-rays are produced by shock-accelerated protons interacting with the dense wind through the hadronic process. We also derive the scaling relations for the decay slopes for both adiabatic and radiative nova shocks in the self-similar deceleration phase.

X-Ray Spectra from Weakly Magnetized Accretion Flows

In this paper, we expand upon previous work that argued for the possibility of a subequipartition magnetic field in the accretion flow of a black hole binary system. Using X-ray observations of the three well-known sources A0620-00, XTE J1118+480, and V404 Cyg during the quiescent state, we compare the theoretically expected spectral shape with the observed data in order to verify that the parameters of the subequipartition model are plausible. In all three cases, we find that it is possible to reproduce the spectral shape of the X-ray observations with a subequipartition flow. These findings support the idea that the quiescent state spectrum of X-ray binary systems is produced by a weakly magnetized accretion flow. A subequipartition flow would pose a significant challenge to our current understanding of jet launching, which relies on the presence of a strong magnetic field to power the jet.