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We present analyses of an Intermediate Polar, IGR J15094-6649, based on the archival optical data obtained from the Transiting Exoplanet Survey Satellite (TESS) and X-ray data obtained from the Suzaku, NuSTAR, and Neil Gehrels Swift Observatory (Swift). Present analysis confirms and refines the previously reported spin period of IGR J15094-6649 as 809.49584$\pm$0.00075 s. Clear evidence of a beat period of 841.67376$\pm$0.00082 s is found during the long-term TESS optical observations, which was not evident in the earlier studies. The dominance of X-ray and optical spin pulse unveils the disc-fed dominance accretion, however, the presence of an additional beat frequency indicates that part of the accreting material also flows along the magnetic field lines. The energy-dependent spin pulsations in the low (< 10 keV) energy band are due to the photoelectric absorption in the accretion flow. However, the complex absorbers may be responsible to produce low amplitude spin modulations via Compton scattering in the hard ( > 10 keV) energy band and indicate that the height of the X-ray emitting region may be negligible. The observed double-humped X-ray profiles with a pronounced dip are indicative of the photoelectric absorption in the intervening accretion stream. Analysis of the X-ray spectra reveals the complexity of the X-ray emission, being composed of multi-temperature plasma components with a soft excess, reflection, and suffers from strong absorption., 15 pages, 10 figures, and 5 Tables, Accepted for publication in MNRAS. arXiv admin note: text overlap with arXiv:2112.06270

SU Lyn is a binary system composed of a white dwarf and a red giant star. Although it is known to be bright and variable at X-ray wavelengths, the optical counterpart of the source appeared as a single red giant without prominent emission lines. Because of the lack of optical features typical for interacting systems, the system was classified as a hidden symbiotic star. We present the results of optical monitoring of the system. While SU Lyn did not show substantial photometric variability, the spectroscopic observations revealed a complex behavior. The system showed strong emission line variability, including P Cygni profiles, changing line emission environments, and variable reddening. Both X-ray and optical observations indicate that the components of SU Lyn were interacting only for a short time during the last twelve years of monitoring. For the first time we showed that SU Lyn resembled a classical symbiotic star when it was X-ray bright, and remained hidden afterwards. We also discuss the current evolutionary status of the red giant, as well as possible future evolution of the system. We suggest that SU Lyn could be a progenitor of a classical, persistent symbiotic system., 15 pages, 10 figures, accepted for publication in MNRAS

Accreting white dwarfs are often found in close binary systems with orbital periods ranging from tens of minutes to several hours. In most cases, the accretion process is relatively steady, with significant modulations only occurring on time-scales of ~days or longer. Here, we report the discovery of abrupt drops in the optical luminosity of the accreting white dwarf binary system TW Pictoris by factors up to 3.5 on time-scales as short as 30 minutes. The optical light curve of this binary system obtained by the Transiting Exoplanet Survey Satellite (TESS) clearly displays fast switches between two distinct intensity modes that likely track the changing mass accretion rate onto the white dwarf. In the low mode, the system also displays magnetically-gated accretion bursts, which implies that a weak magnetic field of the white dwarf truncates the inner disk at the co-rotation radius in this mode. The properties of the mode switching observed in TW Pictoris appear analogous to those observed in transitional millisecond pulsars, where similar transitions occur, although on timescales of ~tens of seconds. Our discovery establishes a previously unrecognised phenomenon in accreting white dwarfs and suggests a tight link to the physics governing magnetic accretion onto neutron stars., Comment: Submitted on 17 June 2021. Accepted for publication in Nature Astronomy on 18 August 2021

Based on the X-ray observations from XMM-Newton and Swift, and optical observations from Transiting Exoplanet Survey Satellite (TESS) and AAVSO, we present temporal and spectral properties of probable intermediate polar SWIFT J0503.7-2819. The X-ray light curve shows two distinctive features, where possibly the second pole seems to be active during the middle of the XMM-Newton observations. Present analysis confirms and also refines the previously reported orbital period of SWIFT J0503.7-2819 as 81.65$\pm$0.04 min. The X-ray and optical variations of this target have been found to occur at the period of $\sim$ 65 min, which we propose as the spin period of the white dwarf (WD). The energy-dependent modulation at this period, which are due to the photoelectric absorption in the accretion flow, also assures this conjecture. Two temperature thermal plasma model well explains the X-ray spectra with temperatures of $\sim$ 150 eV and $\sim$ 18.5 keV, which is absorbed by a dense material with an average equivalent hydrogen column density of 3.8 $\times$ 10$^{22}$ cm$^{-2}$ that partially covers $\sim$ 27% of the X-ray source. An attempt is made to understand the accretion flow in this system using the present data of SWIFT J0503.7-2819. If the proposed spin period is indeed the actual period, then SWIFT J0503.7-2819 could be the first nearly synchronous intermediate polar below the period gap., Accepted for publication in MNRAS

Flickering, and more specifically aperiodic broad-band variability, is an important phenomenon used in understanding the geometry and dynamics of accretion flows. Although the inner regions of accretion flows are known to generate variability on relatively fast timescales, the broad-band variability generated in the outer regions have mostly remained elusive due to their long intrinsic variability timescales. Ultra-compact AM CVn systems are relatively small when compared to other accreting binaries and are well suited to search and characterise low frequency variability. Here we present the first low frequency power spectral analysis of the ultracompact accreting white dwarf system SDSS J1908$+$3940. The analysis reveals a low frequency break at $\sim 6.8 \times 10^{-7} $ Hz in the time-averaged power spectrum as well as a second higher frequency component with characteristic frequency of $\sim 1.3 \times 10^{-4} $ Hz. We associate both components to the viscous timescales within the disc through empirical fits to the power spectrum as well as analytical fits using the fluctuating accretion disk model. Our results show that the low frequency break can be associated to the disk outer regions of a geometrically thin accretion flow. The detection of the low frequency break in SDSS J1908$+$3940 provides a precedent for further detection of similar features in other ultracompact accreting systems. More importantly, it provides a new observable that can help constrain simulations of accretion flows., 16 pages, 13 figures, 4 tables, accepted for publication in MNRAS

AQ Men is a nova-like variable which is presumed to have a tilted, precessing accretion disc. Grazing eclipses in this system have been speculated to be useful in exploring the geometry of its accretion disc. In this work we analysed TESS observations of AQ Men, which provide the best light curve of this object thus far. We show that the depths of the eclipses are changing with the orientation of the accretion disc, which means that they can serve as a direct test of the tilted accretion disc models. The precession period of the accretion disc is increasing during the TESS observations. However, it is still shorter than the period determined in the previous studies. The amplitude of the variability related to the precession of the accretion disc varies, and so does the shape of this variability. Moreover, we have detected a positive superhump that was previously unseen in AQ Men. Interestingly, the positive superhump has a strongly non-sinusoidal shape, which is not expected for a nova-like variable., Comment: 11 pages, 11 figures, accepted for publication in MNRAS

One-pot reactions elaborated around transition metal-catalyzed isomerization of alkenes not only offer the inherent advantages of atom-, step- and redox-economy but also enable the preparation of value-added products that would be difficult to access by conventional methods. In this Review, we cover seminal and recent examples of tandem, sequential and domino processes, which incorporate the most attractive features of olefin isomerization.

We present the Transiting Exoplanet Surveying Satellite (TESS) light curve of the intermediate polar YY Draconis (YY Dra, also known as DO Dra). The power spectrum indicates that while there is stream-fed accretion for most of the observational period, there is a day-long, flat-bottomed low state at the beginning of 2020 during which the only periodic signal is ellipsoidal variation and there is no appreciable flickering. We interpret this low state to be a complete cessation of accretion, a phenomenon that has been observed only once before in an intermediate polar. Simultaneous ground-based observations of this faint state establish that when accretion is negligible, YY Dra fades to $g=17.37\pm0.12$, which we infer to be the magnitude of the combined photospheric contributions of the white dwarf and its red dwarf companion. Using survey photometry, we identify additional low states in 2018-2019 during which YY Dra repeatedly fades to -- but never below -- this threshold. This implies relatively frequent cessations in accretion. Spectroscopic observations during future episodes of negligible accretion can be used to directly measure the field strength of the white dwarf by Zeeman splitting. Separately, we search newly available catalogs of variable stars in an attempt to resolve the long-standing dispute over the proper identifier of this system., 12 pages, 4 figures, submitted to the Astronomical Journal and reviewed favorably

Flickering is a fast variability observed in all accreting systems. It has been shown that in most cataclysmic variables flickering originates in the accretion disc. However, in polars the strong magnetic field of the white dwarf prevents the formation of an accretion disc. Therefore, the origin of flickering in polars is not clear. We analyzed the changes of flickering amplitude with orbital phase in seven polars in order to reveal its site of origin. We show that at least in some polars there are two separate sources of flickering. Moreover, at least one of the sources is located at a large distance from the main source of light in the system., Comment: 7 pages, 9 figures, accepted for publication on MNRAS

The preparation of symmetrical (hetero)biaryls via arylazo sulfones has been successfully carried out upon visible light irradiation in the presence of PPh3AuCl as the catalyst. The present protocol led to the efficient synthesis of a wide range of target compounds in an organic-aqueous solvent under photocatalyst-free conditions.

We present TESS photometry of the asynchronous polar BY Cam, which undergoes a beat cycle between the 199.384 min white dwarf (WD) spin period and the 201.244 min orbital period. This results in changes in the flow of matter onto the WD. The TESS light curve covers 92% of the beat cycle once and 71% of the beat cycle twice. The strongest photometric signal, at 197.560 min, is ascribed to a side-band period. During times of light-curve stability, the photometry modulates at the spin frequency, supporting our WD spin-period identification. Both one-pole and two-pole accretion configurations repeat from one beat cycle to the next with clear and repeatable beat-phase-dependent intensity variations. To explain these, we propose the operation of a magnetic valve at L1. The magnetic valve modulates the mass-transfer rate, as evidenced by a factor of 5 variation in orbital-averaged intensity, over the course of the beat cycle in a repeatable manner. The accretion stream threading distance from the WD is also modulated at the beat period, because of the variation of the WD magnetic field with respect to the stream and because of changes in the mass transfer rate due to the operation of the magnetic valve. Changes in the threading distance result in significant shifts in the position of accreting spots around the beat cycle. As a consequence, only the faintest photometric minima allow for an accurate ephemeris determination. Three regions on the WD appear to receive most of the accretion flow, suggestive of a complex WD magnetic field.

Accretion disks around supermassive black holes in active galactic nuclei produce continuum radiation at ultraviolet and optical wavelengths. Physical processes in the accretion flow lead to stochastic variability of this emission on a wide range of timescales. We measure the optical continuum variability observed in 67 active galactic nuclei and the characteristic timescale at which the variability power spectrum flattens. We find a correlation between this timescale and the black hole mass, extending over the entire mass range of supermassive black holes. This timescale is consistent with the expected thermal timescale at the ultraviolet-emitting radius in standard accretion disk theory. Accreting white dwarfs lie close to this correlation, suggesting a common process for all accretion disks., Author's version; 49 pages; Science (published Aug 13)

All disc-accreting astrophysical objects also produce powerful disc winds and/or jets. In compact binaries containing neutron stars or black holes, accretion often takes place during violent outbursts. The main disc wind signatures seen during these eruptions are blue-shifted X-ray absorption lines. However, these signatures are only observed during "soft states", when the accretion disc generates most of the luminosity. By contrast, optical wind-formed absorption lines have recently been detected in "hard states", when the luminosity is dominated by a hot corona. The relationship between these disc wind signatures is unknown, and no erupting compact binary has so far been observed to display wind-formed lines between the X-ray and optical bands, despite the many strong resonance transitions in this ultraviolet (UV) region of the spectrum. In turn, the impact of disc winds on the overall mass and energy budget of these systems remains a key open question. Here, we show that the transient neutron star X-ray binary Swift J1858.6-0814 exhibits wind-formed, blue-shifted absorption features associated with C IV, N V and He II in time-resolved, UV spectroscopy obtained with the Cosmic Origins Spectrograph on board the Hubble Space Telescope during a luminous hard state. In simultaneous ground-based observations, the optical H and He I lines also display transient blue-shifted absorption troughs. By decomposing our UV data into constant and flaring components, we demonstrate that the blue-shifted absorption is associated with the former, which implies that the outflow is always present. The joint presence of UV and optical wind features in the hard state reveals a multi-phase and/or spatially stratified evaporative outflow from the outer disc. This type of persistent mass loss across all accretion states has been predicted by radiation-hydrodynamic simulations and is required to account for the shorter-than-expected outburst durations.

We present a catalogue of point-like H{\alpha}-excess sources in the Northern Galactic Plane. Our catalogue is created using a new technique that leverages astrometric and photomeric information from Gaia to select H{\alpha}-bright outliers in the INT Photometric H{\alpha} Survey of the Northern Galactic Plane (IPHAS), across the colour-absolute magnitude diagram. To mitigate the selection biases due to stellar population mixing and to extinction, the investigated objects are first partitioned with respect to their positions in the Gaia colour-absolute magnitude space, and in the Galactic coordinates space, respectively. The selection is then performed on both partition types independently. Two significance parameters are assigned to each target, one for each partition type. These represent a quantitative degree of confidence that the given source is a reliable H{\alpha}-excess candidate, with reference to the other objects in the corresponding partition. Our catalogue provides two flags for each source, both indicating the significance level of the H{\alpha}-excess. By analysing their intensity in the H{\alpha} narrow band, 28,496 objects out of 7,474,835 are identified as H{\alpha}-excess candidates with a significance higher than 3. The completeness fraction of the H{\alpha} outliers selection is between 3% and 5%. The suggested 5{\sigma} conservative cut yields a purity fraction of 81.9%., Comment: 20 pages, 23 figures

A Ru-catalyzed conjugative isomerization of remote alkenes that affords branched 1,3-dienes is described. These kinetic products are obtained in high yields, good levels of regiocontrol, and high stereoselectivity. A broad range of functional groups and heterocycles are compatible with the mild reaction conditions, and isomerization is sustained over long distances. Control experiments support a metal-hydride mechanism consisting of iterative migratory insertions/β-H eliminations, which is initiated preferentially at the terminal olefinic site. Two one-pot multimetallic selective catalytic sequences using [Ru/Cu] and [Ru/Rh] combinations have been developed to illustrate the synthetic potential of the process.

Using \emph{TESS} we are doing a systematic study of outbursting AM~CVn systems to place some limits on the current outbursts models. We present the \emph{TESS} light curve (LC) for 9 AM~CVns showing both superoutbursts (SOs) and normal outbursts (NOs). The continuous coverage of the outbursts with \emph{TESS} allows us to place stringent limits on the duration and structures of the SOs and the NOs. We present evidence that in at least some of the systems enhanced mass transfer (EMT) has to be taken into account to explain the observed LC of the SOs and rebrighthening phase after the SOs. For others, the colour evolution from simultaneous observations in $g$ and $r$ with ZTF differs from previously reported colour evolution of longer period AM~CVns where EMT is responsible for the SO. We also find that due to the lack of sufficiently high cadence coverage the duration of many systems might have been overestimated in previous ground-based surveys. We report the SO duration for 6 AM~CVns. We also found that precursors are a common feature of SOs in AM~CVns and are seen in the LC of 5 of the 6 reported SOs. Finally, the 10-minute and 2-minute cadence LCs from \emph{TESS} also allowed us to find two new candidate orbital periods of AM~CVns, both of which are in reasonably good agreement with the predictions for their periods based on their past outburst histories., Comment: Accepted for publication to MNRAS

One of the fundamental properties of an intermediate polar is the dynamical nature of the accretion flow as it encounters the white dwarf's magnetosphere. Many works have presumed a dichotomy between disk-fed accretion, in which the WD accretes from a Keplerian disk, and stream-fed accretion, in which the matter stream from the donor star directly impacts the WD's magnetosphere without forming a disk. However, there is also a third, poorly understood regime in which the accretion flow consists of a torus of diamagnetic blobs that encircles the WD. This mode of accretion is expected to exist at mass-transfer rates below those observed during disk-fed accretion, but above those observed during pure stream-fed accretion. We invoke the diamagnetic-blob regime to explain the exceptional TESS light curve of the intermediate polar TX Col, which transitioned into and out of states of enhanced accretion during Cycles 1 and 3. Power-spectral analysis reveals that the accretion was principally stream-fed. However, when the mass-transfer rate spiked, large-amplitude quasi-periodic oscillations (QPOs) abruptly appeared and dominated the light curve for weeks. The QPOs have two striking properties: they appear in a stream-fed geometry at elevated accretion rates, and they occur preferentially within a well-defined range of frequencies (~10-25 cycles per day). We propose that during episodes of enhanced accretion, a torus of diamagnetic blobs forms near the binary's circularization radius and that the QPOs are beats between the white dwarf's spin frequency and unstable blob orbits within the WD's magnetosphere. We discuss how such a torus could be a critical step in producing an accretion disk in a formerly diskless system., Comment: Accepted for publication in AJ

We present optical light curves from the Transiting Exoplanet Survey Satellite (TESS) for the archetypical dwarf active galactic nucleus (AGN) in the nearby galaxy NGC 4395 hosting a $\sim 10^5\,M_\odot$ supermassive black hole (SMBH). Significant variability is detected on timescales from weeks to hours before reaching the background noise level. The $\sim$month-long, 30 minute-cadence, high-precision TESS light curve can be well fit by a simple damped random walk (DRW) model, with the damping timescale $\tau_{\rm DRW}$ constrained to be $2.3_{-0.7}^{+1.8}$~days ($1\sigma$). NGC 4395 lies almost exactly on the extrapolation of the $\tau_{\rm DRW}-M_{\rm BH}$ relation measured for AGNs with BH masses that are more than three orders of magnitude larger. The optical variability periodogram can be well fit by a broken power law with the high-frequency slope ($-1.88\pm0.15$) and the characteristic timescale ($\tau_{\rm br}\equiv 1/(2\pi f_{\rm br})=1.4_{-0.5}^{+1.9}\,$days) consistent with the DRW model within 1$\sigma$. This work demonstrates the power of TESS light curves in identifying low-mass accreting SMBHs with optical variability, and a potential global $\tau_{\rm DRW}-M_{\rm BH}$ relation that can be used to estimate SMBH masses with optical variability measurements., Comment: 12 pages, 5 figures, 1 table. Accepted for publication in ApJ

We present a study of the eclipses in the accreting white dwarf EX Dra during TESS Cycles 14 and 15. During both of the two outbursts present in this dataset, the eclipses undergo a hysteretic loop in eclipse-depth/out-of-eclipse-flux space. In each case, the direction in which the loops are executed strongly suggests an outburst which is triggered near the inner edge of the accretion disk and propagates outwards. This in turn suggests that the outbursts in EX Dra are 'Inside Out' outbursts; events predicted by previous hydrodynamic studies of dwarf nova accretion disks and confirmed spectroscopically in a number of other accreting white dwarf systems. We therefore propose that the direction of the loop executed in eclipse-depth/out-of-eclipse flux space be used as a test to phenomenologically distinguish between 'inside out' and 'outside in' outbursts in other eclipsing dwarf novae; a reliable and purely photometric test to differentiate between these phenomena., Comment: 10 pages, 6 figures

Magnetically gated accretion has emerged as a proposed mechanism for producing extremely short, repetitive bursts of accretion onto magnetized white dwarfs in intermediate polars (IPs), but this phenomenon has not been detected previously in a confirmed IP. We report the 27-day TESS light curve of V1025 Cen, an IP that shows a remarkable series of twelve bursts of accretion, each lasting for less than six hours. The extreme brevity of the bursts and their short recurrence times (~1-3 days) are incompatible with the dwarf-nova instability, but they are natural consequences of the magnetic gating mechanism developed by Spruit & Taam to explain the Type II bursts of the accreting neutron star known as the Rapid Burster. In this model, the accretion flow piles up at the magnetospheric boundary and presses inward until it couples with the star's magnetic field, producing an abrupt burst of accretion. After each burst, the reservoir of matter at the edge of the magnetosphere is replenished, leading to cyclical bursts of accretion. A pair of recent studies applied this instability to the suspected IPs MV Lyr and TW Pic, but the magnetic nature of these two systems has not been independently confirmed. In contrast, previous studies have unambiguously established the white dwarf in V1025 Cen to be significantly magnetized. The detection of magnetically gated bursts in a confirmed IP therefore validates the extension of the Spruit & Taam instability to magnetized white dwarfs., Accepted for publication in ApJL

We present the first short time-scale ($\sim$hours to days) optical variability study of a large sample of Active Galactic Nuclei (AGN) observed with the Kepler/K2 mission. The sample contains 252 AGN observed over four campaigns with $\sim 30$ minute cadence selected from the Million Quasar Catalogue with R magnitude $< 19$. We performed time series analysis to determine their variability properties by means of the power spectral densities (PSDs) and applied Monte Carlo techniques to find the best model parameters that fit the observed power spectra. A power-law model is sufficient to describe all the PSDs of our sample. A variety of power-law slopes were found indicating that there is not a universal slope for all AGN. We find that the rest-frame amplitude variability in the frequency range of $6\times10^{-6}-10^{-4}$ Hz varies from $1-10$ % with an average of 1.7 %. We explore correlations between the variability amplitude and key parameters of the AGN, finding a significant correlation of rest-frame short-term variability amplitude with redshift. We attribute this effect to the known "bluer when brighter" variability of quasars combined with the fixed bandpass of Kepler data. This study also enables us to distinguish between Seyferts and Blazars and confirm AGN candidates. For our study we have compared results obtained from light curves extracted using different aperture sizes and with and without de-trending. We find that limited de-trending of the optimal photometric precision light curve is the best approach, although some systematic effects still remain present., Published in MNRAS 16 February 2018

We present results from a study of TESS observations of the eclipsing dwarf nova system Z Cha, covering both an outburst and a superoutburst. We discover that Z Cha undergoes hysteretic loops in eclipse depth - out-of-eclipse flux space in both the outburst and the superoutburst. The direction that these loops are executed in indicates that the disk size increases during an outburst before the mass transfer rate through the disk increases, placing constraints on the physics behind the triggering of outbursts and superoutbursts. By fitting the signature of the superhump period in a flux-phase diagram, we find the rate at which this period decreases in this system during a superoutburst for the first time. We find that the superhumps in this source skip evolutionary stage "A" seen during most dwarf nova superoutbursts, even though this evolutionary stage has been seen during previous superoutbursts of the same object. Finally, O-C values of eclipses in our sample are used to calculate new ephemerides for the system, strengthening the case for a third body in Z Cha and placing new constraints on its orbit., Comment: Accepted by MNRAS July 2019. 13 pages, 16 figures, 4 tables

We recently reported the discovery of a candidate jet-driving microquasar (S10) in the nearby spiral galaxy NGC 300. However, in the absence of kinematic information, we could not reliably determine the jet power or the dynamical age of the jet cavity. Here, we present optical MUSE integral field unit (IFU) observations of S10, which reveal a bipolar line-emitting jet structure surrounding a continuum-emitting central source. The optical jet lobes of S10 have a total extent of $\sim$ 40 pc and a shock velocity of $\sim$ 150 km s$^{-1}$. Together with the jet kinematics, we exploit the MUSE coverage of the Balmer H$\beta$ line to estimate the density of the surrounding matter and therefore compute the jet power to be $P_{jet}\approx$ 6.3 $\times$ 10$^{38}$ erg s$^{-1}$. An optical analysis of a microquasar jet bubble and a consequent robust derivation of the jet power have been possible only in a handful of similar sources. This study therefore adds valuable insight into microquasar jets, and demonstrates the power of optical integral field spectroscopy in identifying and analysing these objects., Comment: Accepted for publication in MNRAS

We present the discovery of a powerful candidate microquasar in NGC 300, associated with the S 10 optical nebula (previously classified as a supernova remnant). Chandra images show four discrete X-ray knots aligned in the plane of the sky over a length of $\approx$150 pc. The X-ray emission from the knots is well fitted with a thermal plasma model at a temperature of $\approx$0.6 keV and a combined 0.3-8 keV luminosity of $\approx$10$^{37}$ erg s$^{-1}$. The X-ray core, if present at all, does not stand out above the thermal emission of the knots: this suggests that the accreting compact object is either currently in a dim state or occulted from our view. We interpret the emission from the knots as the result of shocks from the interaction of a jet with the interstellar medium (possibly over different epochs of enhanced activity). Cooler shock-heated gas is likely the origin also of the optical bubble and lobes near the X-ray structure, detected in images from the Hubble Space Telescope and the Very Large Telescope. In the radio bands, we observed the region with the Australia Telescope Compact Array, and discovered an elongated radio nebula (about 170 $\times$ 55 pc in size) with its major axis aligned with the chain of Chandra sources. The radio nebula has an integrated 5.5 GHz radio luminosity of $\approx$10$^{34}$ erg s$^{-1}$ for a distance of 1.88 Mpc. The morphology, size and luminosity of the extended X-ray, optical and radio structure suggest that NGC 300-S 10 belongs to the same class of powerful ($P_{\rm jet} > 10^{39}$ erg s$^{-1}$) microquasars as SS 433, Ho II X-1 and NGC 7793-S 26., 19 pages, 8 figures, 8 tables. Accepted for publication in MNRAS

We present a multi-wavelength study of the low-mass X-ray binary Sco X-1 using Kepler K2 optical data and Fermi GBM and MAXI X-ray data. We recover a clear sinusoidal orbital modulation from the Kepler data. Optical fluxes are distributed bimodally around the mean orbital light curve, with both high and low states showing the same modulation. The high state is broadly consistent with the flaring branch of the Z diagram and the low state with the normal branch. We see both rapid optical flares and slower dips in the high state, and slow brightenings in the low state. High state flares exhibit a narrow range of amplitudes with a striking cut-off at a maximum amplitude. Optical fluxes correlate with X-ray fluxes in the high state, but in the low state they are anti-correlated. These patterns can be seen clearly in both flux-flux diagrams and cross-correlation functions and are consistent between MAXI and GBM. The high state correlation arises promptly with at most a few minutes lag. We attribute this to thermal reprocessing of X-ray flares. The low state anti-correlation is broader, consistent with optical lags of between zero and 3 ~min, and strongest with respect to high energy X-rays. We suggest that the decreases in optical flux in the low state may reflect decreasing efficiency of disc irradiation, caused by changes in the illumination geometry. These changes could reflect the vertical extent or covering factor of obscuration or the optical depth of scattering material., Monthly Notices of the Royal Astronomical Society, In press

EPIC 204278916 has been serendipitously discovered from its K2 light curve which displays irregular dimmings of up to 65% for ~25 consecutive days out of 78.8 days of observations. For the remaining duration of the observations, the variability is highly periodic and attributed to stellar rotation. The star is a young, low-mass (M-type) pre-main-sequence star with clear evidence of a resolved tilted disk from ALMA observations. We examine the K2 light curve in detail and hypothesise that the irregular dimmings are caused by either a warped inner-disk edge or transiting cometary-like objects in either circular or eccentric orbits. The explanations discussed here are particularly relevant for other recently discovered young objects with similar absorption dips., Comment: 9 pages, 10 figures. Accepted for publication in MNRAS

We present a sub-arcsecond cross-match of Gaia DR2 against the INT Photometric H-alpha Survey of the Northern Galactic Plane Data Release 2 (IPHAS DR2) and the Kepler-INT Survey (KIS). The resulting value-added catalogues (VACs) provide additional precise photometry to the Gaia photometry (r, i and H-alpha for IPHAS, with additional U and g for KIS). In building the catalogue, proper motions given in \gaia\ DR2 are wound back to match the epochs of IPHAS DR2, thus ensuring high proper motion objects are appropriately cross-matched. The catalogues contain 7,927,224 and 791,071 sources for IPHAS and KIS, respectively. The requirement of >5-sigma parallax detection for every included source means that distances out to 1--1.5 kpc are well covered. We define two additional parameters for each catalogued object: (i) $f_c$, a magnitude-dependent tracer of the quality of the Gaia astrometric fit; (ii) $f_{FP}$, the false-positive rate for parallax measurements determined from astrometric fits of a given quality at a given magnitude. Selection cuts based on these parameters can be used to clean colour-magnitude and colour-colour diagrams in a controlled and justified manner. We provide both full and light versions of the VAC, with VAC-light containing only objects that represent our recommended trade-off between purity and completeness. Uses of the catalogues include the identification of new variable stars in the matched data sets, and more complete identification of H-alpha-excess emission objects thanks to separation of high-luminosity stars from the main sequence., Accepted for publication in MNRAS. Full catalogs are being submitted to VizieR, and are also available at http://www.phys.ttu.edu/~simo/GaiaVACs

A nova eruption in a cataclysmic variable (CV) is a thermonuclear explosion on the surface of the white-dwarf primary once it has accreted a critical mass from its late-type companion. Between these eruptions, the binary is supposed to appear as a ’normal’ CV, although for the first tens or hundreds of years, the white dwarf is still heated up which might influence the mass-transfer rate of the binary. To compare the nova-populations with the general CV-one, a large number of old novae are needed. We have conducted a program to recover old novae and study the binary within. We here present the results of this program as well as a new deep wide-field Hα + [NII] survey of cataclysmic variables to search for remnant nova shells around these objects. Such shells have been found around some cataclysmic variables that were hitherto not known as novae. They provide the unambiguous evidence that the system has experienced a nova eruption in the past and thus point to the oldest novae observable.

We report on the first continuous, 80 day optical monitoring of the transitional millisecond pulsar PSR J1023+0038 carried out in mid-2017 with Kepler in the K2 configuration, when an X-ray subluminous accretion disk was present in the binary. Flares lasting from minutes to 14 hr were observed for 15.6% of the time, which is a larger fraction than previously reported on the basis of X-ray and past optical observations, and more frequently when the companion was at the superior conjunction of the orbit. A sinusoidal modulation at the binary orbital period was also present with an amplitude of ~16%, which varied by a few percent over timescales of days, and with a maximum that took place 890 +/- 85 s earlier than the superior conjunction of the donor. We interpret these phenomena in terms of reprocessing of the X-ray emission by an asymmetrically heated companion star surface and/or a non-axisymmetric outflow possibly launched close to the inner Lagrangian point. Furthermore, the non-flaring average emission varied by up to ~ 40% over a time scale of days in the absence of correspondingly large variations of the irradiating X-ray flux. The latter suggests that the observed changes in the average optical luminosity might be due to variations of the geometry, size, and/or mass accretion rate in the outer regions of the accretion disk., Comment: 6 pages, 4 figures. Published in ApJ Letters

To understand the physical processes governing accretion discs we can study active galactic nuclei (AGN), X-ray binary systems (XRBs) and cataclysmic variables (CVs). It has been shown that XRBs and CVs show similar observational properties such as recurrent outbursts and aperiodic variability. The latter has been extensively studied for XRBs, but only recently have direct phenomenological analogies been found between XRBs and CVs, including the discovery of the rms--flux relation and the optical detection of Fourier-dependent time-lags. We present a Fourier analysis of the well-known CV SS Cyg in quiescence based on data collected at the 4.2--m William Herschel Telescope using ULTRACAM. Light curves in SDSS filters $u'$, $g'$ and $r'$ were taken simultaneously with sub-second cadence. The high cadence and sensitivity of the camera allow us to study the broad-band noise component of the source in the time range $\sim 10000 - 0.5$ s ($10^{-4}-2$ Hz). Soft/negative lags with an amplitude ~ 5 s at a time-scale of ~ 250 s were observed, indicating that the emission in the redder bands lags the emission in the bluer bands. This effect could be explained by thermal reprocessing of hard photons in the innermost region of the accretion disc, assuming a high viscosity parameter $\alpha > 0.3$, and high irradiation of the disc. Alternatively, it could be associated with the recombination time-scale on the upper layer of the accretions disc., Comment: Accepted in MNRAS 20 Aug 2018

The cool white dwarf SDSS J124231.07+522626.6 exhibits photospheric absorption lines of 8 distinct heavy elements in medium resolution optical spectra, notably including oxygen. The Teff = 13000 K atmosphere is helium-dominated, but the convection zone contains significant amounts of hydrogen and oxygen. The four most common rock-forming elements (O, Mg, Si, and Fe) account for almost all the accreted mass, totalling at least 1.2e+24 g, similar to the mass of Ceres. The time-averaged accretion rate is 2e+10 g/s, one of the highest rates inferred among all known metal-polluted white dwarfs. We note a large oxygen excess, with respect to the most common metal oxides, suggesting that the white dwarf accreted planetary debris with a water content of ~38 per cent by mass. This star, together with GD 61, GD 16, and GD 362, form a small group of outliers from the known population of evolved planetary systems accreting predominantly dry, rocky debris. This result strengthens the hypothesis that, integrated over the cooling ages of white dwarfs, accretion of water-rich debris from disrupted planetesimals may significantly contribute to the build-up of trace hydrogen observed in a large fraction of helium-dominated white dwarf atmospheres., Comment: 12 pages, 5 figures, 5 tables. Accepted for publication in MNRAS

We present the first fully simultaneous fits to the NIR and X-ray spectral slope (and its evolution) during a very bright flare from Sgr A*, the supermassive black hole at the Milky Way's center. Our study arises from ambitious multi-wavelength monitoring campaigns with XMM-Newton, NuSTAR and SINFONI. The average multi-wavelength spectrum is well reproduced by a broken power-law with $\Gamma_{NIR}=1.7\pm0.1$ and $\Gamma_X=2.27\pm0.12$. The difference in spectral slopes ($\Delta\Gamma=0.57\pm0.09$) strongly supports synchrotron emission with a cooling break. The flare starts first in the NIR with a flat and bright NIR spectrum, while X-ray radiation is detected only after about $10^3$ s, when a very steep X-ray spectrum ($\Delta\Gamma=1.8\pm0.4$) is observed. These measurements are consistent with synchrotron emission with a cooling break and they suggest that the high energy cut-off in the electron distribution ($\gamma_{max}$) induces an initial cut-off in the optical-UV band that evolves slowly into the X-ray band. The temporal and spectral evolution observed in all bright X-ray flares are also in line with a slow evolution of $\gamma_{max}$. We also observe hints for a variation of the cooling break that might be induced by an evolution of the magnetic field (from $B\sim30\pm8$ G to $B\sim4.8\pm1.7$ G at the X-ray peak). Such drop of the magnetic field at the flare peak would be expected if the acceleration mechanism is tapping energy from the magnetic field, such as in magnetic reconnection. We conclude that synchrotron emission with a cooling break is a viable process for Sgr A*'s flaring emission., Comment: Accepted for publication in MNRAS

White dwarfs are often found in binary systems with orbital periods ranging from tens of minutes to hours in which they can accrete gas from their companion stars. In about 15% of these binaries, the magnetic field of the white dwarf is strong enough ($\geq 10^6$ Gauss) to channel the accreted matter along field lines onto the magnetic poles. The remaining systems are referred to as "non-magnetic", since to date there has been no evidence that they have a dynamically significant magnetic field. Here we report an analysis of archival optical observations of the "non-magnetic" accreting white dwarf in the binary system MV Lyrae (hereafter MV Lyr), whose lightcurve displayed quasi-periodic bursts of $\approx 30$ minutes duration every $\approx 2$ hours. The observations indicate the presence of an unstable magnetically-regulated accretion mode, revealing the existence of magnetically gated accretion, where disk material builds up around the magnetospheric boundary (at the co-rotation radius) and then accretes onto the white dwarf, producing bursts powered by the release of gravitational potential energy. We infer a surface magnetic field strength for the white dwarf in MV Lyr between $2 \times 10^4 \leq B \leq 10^5$ Gauss, too low to be detectable by other current methods. Our discovery provides a new way of studying the strength and evolution of magnetic fields in accreting white dwarfs and extends the connections between accretion onto white dwarfs, young stellar objects and neutron stars, for which similar magnetically gated accretion cysles have been identified., Comment: 7 pages, 4 figures. Accepted in Nature

We report the first study of Fourier-frequency-dependent coherence and phase/time lags at optical wavelengths of cataclysmic variables (MV Lyr and LU Cam) displaying typical flickering variability in white light. Observations were performed on the William Herschel Telescope using ULTRACAM. Lightcurves for both systems have been obtained with the SDSS filters $u'$, $g'$ and $r'$ simultaneously with cadences between $\approx0.5-2$ seconds, and allow us to probe temporal frequencies between ~10^{-3} Hz and ~1 Hz. We find high levels of coherence between the u', g' and r' lightcurves up to at least ~10^{-2} Hz. Furthermore we detect red/negative lags where the redder bands lag the bluer ones at the lowest observed frequencies. For MV Lyr time lags up to ~3 seconds are observed, whilst LU Cam displays larger time lags of ~10 seconds. Mechanisms which seek to explain red/negative lags observed in X-ray binaries and Active Galactic Nuclei involve reflection of photons generated close to the compact object onto the surface layers of the accretion disk, where the lag delay is simply the light-travel time from the emitting source to the reflecting accretion disk area. Although this could be a viable explanation for the lags observed in MV Lyr, the lags observed in LU Cam are too large to be explained by reflection from the disk and/or the donor star. We suggest reprocessing on the thermal timescale of boundary layer photons onto the accretion disk as a possible mechanism to explain the lags observed in accreting white dwarfs, or reverse (inside-out) shocks within the disk travelling through cooler disk regions as they move outwards., 8 pages, 3 figures, 1 table. Accepted for publication in MNRAS

We analyse the K2 short cadence data of the intermediate polar FO Aqr and provide accurate and updated orbital and spin periodicities. We additionally find small spin period changes as a function of orbital phase of ~0.02 seconds translating to velocities of ~ a few km/s. The obtained orbital-folded velocity profile displays two clear maxima and minima, and cannot be explained by the radial velocity of the orbiting white dwarf. Instead we propose that the observed velocities are the sum of the radial velocities of both the white dwarf and of the stellar surface facing the white dwarf which reprocesses the WD spin pulses. This combination can explain the observed low velocities in FO Aqr. However asymmetries in the orbital configuration are required to explain the double peaked velocity profile. One possible scenario would invoke binary eccentricity. We thus developed a simple binary model to explain and fit our observations, and find a small binary eccentricity of e=0.03. Although small, persistent eccentricity in a close interacting binary would induce enhanced mass transfer occurring preferentially at periastron passages. We thus discuss alternative scenarios where other asymmetries might explain our observations assuming circular orbits. Since FO Aqr is the first system where the combined radial velocities of both the WD and secondary surface have been measured, it is possible that other mass-transferring binaries also display similar velocity curves when observed with Kepler. These will provide additional valuable tests to either confirm or rule out small eccentricities in similar systems., Comment: This paper has been withdrawn by the authors due to a crucial error in the data analysis resulting in misinterpretation of the results. Specifically, the observed frequency changes as a function of orbital period cannot be explained by Doppler shifts as described in the paper. Although the frequency changes do persist in the analysis, we currently cannot determine their origin

We present a broad-band timing analysis of the accreting white dwarf system MV Lyrae based on data obtained with the Kepler satellite. The observations span 633 days at a cadence of 58.8 seconds and allow us to probe 4 orders of magnitude in temporal frequency. The modelling of the observed broad-band noise components is based on the superposition of multiple Lorentzian components, similar to the empirical modelling adopted for X-ray binary systems. We also present the detection of a frequency varying Lorentzian component in the lightcurve of MV Lyrae, where the Lorentzian characteristic frequency is inversely correlated with the mean source flux. Because in the literature similar broad-band noise components have been associated to either the viscous or dynamical timescale for different source types (accreting black holes or neutron stars), we here systematically explore both scenarios and place constraints on the accretion disk structure. In the viscous case we employ the fluctuating accretion disk model to infer parameters for the viscosity and disk scale height, and infer uncomfortably high parameters to be accommodated by the standard thin disk, whilst in the dynamical case we infer a large accretion disk truncation radius of ~10 white dwarf radii. More importantly however, the phenomenological properties between the broad-band variability observed here and in X-ray binaries and Active Galactic Nuclei are very similar, potentially suggesting a common origin for the broad-band variability., 11 pages, 9 figures, 1 table. Accepted for publication in MNRAS

We present the results of the first spectroscopic follow-up of 132 optically blue UV-excess sources selected from the UV-excess survey of the Northern Galactic Plane (UVEX). The UV-excess spectra are classified into different populations and grids of model spectra are fit to determine spectral types, temperatures, surface gravities and reddening. From this initial spectroscopic follow-up 95% of the UV-excess candidates turn out to be genuine UV-excess sources such as white dwarfs, white dwarf binaries, subdwarfs type O and B, emission line stars and QSOs. The remaining sources are classified as slightly reddened main-sequence stars with spectral types later than A0V. The fraction of DA white dwarfs is 47% with reddening smaller than E(B-V) 17 and Galactic latitude smaller than |b

We report the discovery of a linear relationship between the root-mean-square (rms) variability amplitude and the mean flux in the accreting white dwarf binary system MV Lyrae. Our lightcurve, obtained with the Kepler satellite, spans 633 days with quasi-continuous 58.8 second cadence resolution. We show, for the first time, how this cataclysmic variable displays linear rms-flux relations similar to those observed in many other black hole binaries, neutron star binaries and Active Galactic Nuclei. The phenomenological similarity between the rms-flux relation observed here and in other X-ray binaries suggests a common physical origin for the broad-band variability, independent of source type, mass or size of the compact accretor. Furthermore, we infer the viscosity parameter, alpha, and disk scale height, H/R, using two independent methods. In both cases, both values are found to be uncomfortably high to be accommodated by the disk instability model.

We present the first catalogue of point-source UV-excess sources selected from the UVEX survey. UVEX images the Northern Galactic Plane in the U, g, r and HeI5875 bands in the Galactic latitude range -5deg

We apply a recently developed method for classifying broad absorption line quasars (BALQSOs) to the latest QSO catalogue constructed from Data Release 5 of the Sloan Digital Sky Survey. Our new hybrid classification scheme combines the power of simple metrics, supervised neural networks and visual inspection. In our view the resulting BALQSO catalogue is both more complete and more robust than all previous BALQSO catalogues, containing 3552 sources selected from a parent sample of 28,421 QSOs in the redshift range 1.7, Comment: 10 pages, 5 figures, 1 table available through the VizieR server. To appear in MNRAS