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Lightcurves of many classical novae deviate from the canonical "fast rise - smooth decline" pattern and display complex variability behavior. We present the first TESS-space-photometry-based investigation of this phenomenon. We use TESS Sector 41 full-frame images to extract a lightcurve of the slow Galactic nova V606 Vul that erupted nine days prior to the start of the TESS observations. The lightcurve covers the first of two major peaks of V606 Vul that was reached 19 days after the start of the eruption. The nova reached its brightest visual magnitude V=9.9 in its second peak 64 days after the eruption onset, following the completion of Sector 41 observations. To increase the confidence level of the extracted lightcurve, we performed the analysis using four different codes implementing the aperture photometry (Lightkurve, VaST) and image subtraction (TESSreduce, tequila_shots) and find good agreement between them. We performed ground-based photometric and spectroscopic monitoring to complement the TESS data. The TESS lightcurve reveals two features: periodic variations (0.12771d, 0.01mag average peak-to-peak amplitude) that disappeared when the source was within 1mag of peak optical brightness and a series of isolated mini-flares (with peak-to-peak amplitudes of up to 0.5mag) appearing at seemingly random times. We interpret the periodic variations as the result of azimuthal asymmetry of the photosphere engulfing the nova-hosting binary that was distorted by and rotating with the binary. Whereas we use spectra to associate the two major peaks in the nova lightcurve with distinct episodes of mass ejection, the origin of mini-flares remains elusive., Comment: 31 pages, 10 figures, submitted to ApJ; comments welcome

The impact of the DART spacecraft into Dimorphos, moon of the asteroid Didymos, changed Dimorphos' orbit substantially, largely from the ejection of material. We present results from twelve Earth-based facilities involved in a world-wide campaign to monitor the brightness and morphology of the ejecta in the first 35 days after impact. After an initial brightening of ~1.4 magnitudes, we find consistent dimming rates of 0.11-0.12 magnitudes/day in the first week, and 0.08-0.09 magnitudes/day over the entire study period. The system returned to its pre-impact brightness 24.3-25.3 days after impact through the primary ejecta tail remained. The dimming paused briefly eight days after impact, near in time to the appearance of the second tail. This was likely due to a secondary release of material after re-impact of a boulder released in the initial impact, through movement of the primary ejecta through the aperture likely played a role., Comment: 16 pages, 5 Figures, accepted in the Astrophysical Journal Letters (ApJL) on October 16, 2023

We present results from the first 22 GHz space very-long-baseline interferometric (VLBI) imaging observations of M87 by RadioAstron. As a part of the Nearby AGN Key Science Program, the source was observed in Feb 2014 at 22 GHz with 21 ground stations, reaching projected $(u,v)$-spacings up to $\sim11\,$G$\lambda$. The imaging experiment was complemented by snapshot RadioAstron data of M87 obtained during 2013--2016 from the AGN Survey Key Science Program. Their longest baselines extend up to $\sim25\,$G$\lambda$. For all these measurements, fringes are detected only up to $\sim$2.8 Earth Diameter or $\sim$3 G$\lambda$ baseline lengths, resulting in a new image with angular resolution of $\sim150\,\mu$as or $\sim20$ Schwarzschild radii spatial resolution. The new image not only shows edge-brightened jet and counterjet structures down to submilliarcsecond scales but also clearly resolves the VLBI core region. While the overall size of the core is comparable to those reported in the literature, the ground-space fringe detection and slightly super-resolved RadioAstron image suggest the presence of substructures in the nucleus, whose minimum brightness temperature exceeds $T_{\rm B, min}\sim10^{12}\,$K. It is challenging to explain the origin of this record-high $T_{\rm B, min}$ value for M87 by pure Doppler boosting effect with a simple conical jet geometry and known jet speed. Therefore, this can be evidence for more extreme Doppler boosting due to a blazar-like small jet viewing angle or highly efficient particle acceleration processes occurring already at the base of the outflow., Comment: 27 pages, 13 figures, accepted for publication in ApJ

We present the discovery of a new optical/X-ray source likely associated with the Fermi $\gamma$-ray source 4FGL J1408.6-2917. Its high-amplitude periodic optical variability, large spectroscopic radial velocity semi-amplitude, evidence for optical emission lines and flaring, and X-ray properties together imply the source is probably a new black widow millisecond pulsar binary. We compile the properties of the 41 confirmed and suspected field black widows, finding a median secondary mass of $0.027\pm0.003\,M_{\odot}$. Considered jointly with the more massive redback millisecond pulsar binaries, we find that the "spider" companion mass distribution remains strongly bimodal, with essentially zero systems having companion masses between $\sim0.07-0.1\,M_{\odot}$. X-ray emission from black widows is typically softer and less luminous than in redbacks, consistent with less efficient particle acceleration in the intrabinary shock in black widows, excepting a few systems that appear to have more efficient "redback-like" shocks. Together black widows and redbacks dominate the census of the fastest-spinning field millisecond pulsars in binaries with known companion types, making up $\gtrsim$80% of systems with $P_{\rm{spin}}<2\,\rm{ms}$. Similar to redbacks, the neutron star masses in black widows appear on average significantly larger than the canonical $1.4\,M_{\odot}$, and many of the highest-mass neutron stars claimed to date are black widows with $M_{\rm{NS}}\gtrsim2.1\,M_{\odot}$. Both of these observations are consistent with an evolutionary picture where spider millisecond pulsars emerge from short orbital period progenitors that had a lengthy period of mass transfer initiated while the companion was on the main sequence, leading to fast spins and high masses., Comment: 18 pages text, 9 figures, 5 tables; Accepted to ApJ

We report the discovery of 1RXH J082623.6-505741, a 10.4 hr orbital period compact binary. Modeling extensive optical photometry and spectroscopy reveals a $\sim 0.4 M_{\odot}$ K-type secondary transferring mass through a low-state accretion disk to a non-magnetic $\sim 0.8 M_{\odot}$ white dwarf. The secondary is overluminous for its mass and dominates the optical spectra at all epochs, and must be evolved to fill its Roche Lobe at this orbital period. The X-ray luminosity $L_X \sim 1$-$2 \times 10^{32}$ erg s$^{-1}$ derived from both new XMM-Newton and archival observations, although high compared to most CVs, still only requires a modest accretion rate onto the white dwarf of $\dot{M} \sim 3 \times 10^{-11}$ to $3 \times 10^{-10} M_{\odot}$ yr$^{-1}$, lower than expected for a cataclysmic variable with an evolved secondary. No dwarf nova outbursts have yet been observed from the system, consistent with the low derived mass transfer rate. Several other cataclysmic variables with similar orbital periods also show unexpectedly low mass transfer rates, even though selection effects disfavor the discovery of binaries with these properties. This suggests the abundance and evolutionary state of long-period, low mass transfer rate cataclysmic variables is worthy of additional attention., Comment: 21 pages, 8 figures, 1 table, accepted to ApJ

We have discovered a new X-ray emitting compact binary that is the likely counterpart to the unassociated Fermi-LAT GeV $\gamma$-ray source 4FGL J1120.0-2204, the second brightest Fermi source that still remains formally unidentified. Using optical spectroscopy with the SOAR telescope, we have identified a warm ($T_{\textrm{eff}}\sim8500$ K) companion in a 15.1-hr orbit around an unseen primary, which is likely a yet-undiscovered millisecond pulsar. A precise Gaia parallax shows the binary is nearby, at a distance of only $\sim 820$ pc. Unlike the typical "spider" or white dwarf secondaries in short-period millisecond pulsar binaries, our observations suggest the $\sim 0.17\,M_{\odot}$ companion is in an intermediate stage, contracting on the way to becoming an extremely low-mass helium white dwarf (a "pre-ELM" white dwarf). Although the companion is apparently unique among confirmed or candidate millisecond pulsar binaries, we use binary evolution models to show that in $\sim 2$ Gyr, the properties of the binary will match those of several millisecond pulsar-white dwarf binaries with very short ($< 1$ d) orbital periods. This makes 4FGL J1120.0-2204 the first system discovered in the penultimate phase of the millisecond pulsar recycling process., Comment: Accepted to ApJ; 15 pages, 8 figures, 2 tables

Peaking at 3.7 mag on 2020 July 11, YZ Ret was the second-brightest nova of the decade. The nova's moderate proximity (2.7 kpc from Gaia) provided an opportunity to explore its multi-wavelength properties in great detail. Here we report on YZ Ret as part of a long-term project to identify the physical mechanisms responsible for high-energy emission in classical novae. We use simultaneous Fermi/LAT and NuSTAR observations complemented by XMM-Newton X-ray grating spectroscopy to probe the physical parameters of the shocked ejecta and the nova-hosting white dwarf. The XMM-Newton observations revealed a super-soft X-ray emission which is dominated by emission lines of CV, CVI, NVI, NVII, and OVIII rather than a blackbody-like continuum, suggesting CO-composition of the white dwarf in a high-inclination binary system. Fermi/LAT detected YZ Ret for 15 days with the gamma-ray spectrum best described by a power law with an exponential cut-off at 1.9 +/-0.6 GeV. In stark contrast with theoretical predictions and in keeping with previous NuSTAR observations of Fermi-detected classical novae (V5855 Sgr and V906 Car), the 3.5-78 keV X-ray emission is found to be two orders of magnitude fainter than the GeV emission. The X-ray emission observed by NuSTAR is consistent with a single-temperature thermal plasma. We detect no non-thermal tail of the GeV emission expected to extend down to the NuSTAR band. NuSTAR observations continue to challenge theories of high-energy emission from shocks in novae., Comment: 22 pages, 5 figures, 4 tables; accepted to MNRAS

We present radio observations (1--40 GHz) for 36 classical novae, representing data from over five decades compiled from the literature, telescope archives, and our own programs. Our targets display a striking diversity in their optical parameters (e.g., spanning optical fading timescales, t_2 = 1--263 days), and we find a similar diversity in the radio light curves. Using a brightness temperature analysis, we find that radio emission from novae is a mixture of thermal and synchrotron emission, with non-thermal emission observed at earlier times. We identify high brightness temperature emission (T_B > 5x10^4 K) as an indication of synchrotron emission in at least 9 (25%) of the novae. We find a class of synchrotron-dominated novae with mildly evolved companions, exemplified by V5589 Sgr and V392 Per, that appear to be a bridge between classical novae with dwarf companions and symbiotic binaries with giant companions. Four of the novae in our sample have two distinct radio maxima (the first dominated by synchrotron and the later by thermal emission), and in four cases the early synchrotron peak is temporally coincident with a dramatic dip in the optical light curve, hinting at a common site for particle acceleration and dust formation. We publish the light curves as tables and encourage use of these data by the broader community in multi-wavelength studies and modeling efforts., Comment: Submitted to AAS Journals

The discovery that many classical novae produce detectable GeV $\gamma$-ray emission has raised the question of the role of shocks in nova eruptions. Here we use radio observations of nova V809 Cep (Nova Cep 2013) with the Jansky Very Large Array to show that it produced non-thermal emission indicative of particle acceleration in strong shocks for more than a month starting about six weeks into the eruption, quasi-simultaneous with the production of dust. Broadly speaking, the radio emission at late times -- more than a six months or so into the eruption -- is consistent with thermal emission from $10^{-4} M_\odot$ of freely expanding, $10^4$~K ejecta. At 4.6 and 7.4 GHz, however, the radio light-curves display an initial early-time peak 76 days after the discovery of the eruption in the optical ($t_0$). The brightness temperature at 4.6 GHz on day 76 was greater than $10^5 K$, an order of magnitude above what is expected for thermal emission. We argue that the brightness temperature is the result of synchrotron emission due to internal shocks within the ejecta. The evolution of the radio spectrum was consistent with synchrotron emission that peaked at high frequencies before low frequencies, suggesting that the synchrotron from the shock was initially subject to free-free absorption by optically thick ionized material in front of the shock. Dust formation began around day 37, and we suggest that internal shocks in the ejecta were established prior to dust formation and caused the nucleation of dust.

We report the discovery of a new low-mass X-ray binary near the center of the unassociated Fermi GeV gamma-ray source 4FGL J0540.0-7552. The source shows the persistent presence of an optical accretion disk and exhibits extreme X-ray and optical variability. It also has an X-ray spectrum well-fit by a hard power law with a Gamma = 1.8 and a high ratio of X-ray to gamma-ray flux. Together, these properties are consistent with the classification of the binary as a transitional millisecond pulsar (tMSP) in the sub-luminous disk state. Uniquely among the candidate tMSPs, 4FGL J0540.0-7552 shows consistent optical, X-ray, and gamma-ray evidence for having undergone a state change, becoming substantially brighter in the optical and X-rays and fainter in GeV gamma-rays sometime in mid-2013. In its current sub-luminous disk state, and like one other candidate tMSP in the Galactic field, 4FGL J0540.0-7552 appears to always be in an X-ray "flare mode", indicating that this could be common phenomenology for tMSPs., Comment: ApJ in press

The unprecedented sky coverage and observing cadence of the All-Sky Automated Survey for SuperNovae (ASAS-SN) has resulted in the discovery and continued monitoring of a large sample of Galactic transients. The vast majority of these are accretion-powered dwarf nova outbursts in cataclysmic variable systems, but a small subset are thermonuclear-powered classical novae. Despite improved monitoring of the Galaxy for novae from ASAS-SN and other surveys, the observed Galactic nova rate is still lower than predictions. One way classical novae could be missed is if they are confused with the much larger population of dwarf novae. Here, we examine the properties of 1617 dwarf nova outbursts detected by ASAS-SN and compare them to classical novae. We find that the mean classical nova brightens by ~11 magnitudes during outburst, while the mean dwarf nova brightens by only ~5 magnitudes, with the outburst amplitude distributions overlapping by roughly 15%. For the first time, we show that the amplitude of an outburst and the time it takes to decline by two magnitudes from maximum are positively correlated for dwarf nova outbursts. For classical novae, we find that these quantities are negatively correlated, but only weakly, compared to the strong anti-correlation of these quantities found in some previous work. We show that, even if located at large distances, only a small number of putative dwarf novae could be mis-classified classical novae suggesting that there is minimal confusion between these populations. Future spectroscopic follow-up of these candidates can show whether any are indeed classical novae., Comment: 24 pages, 11 Figures

The detection of GeV $\gamma$-ray emission from Galactic novae by $Fermi$-LAT has become routine since 2010, and is generally associated with shocks internal to the nova ejecta. These shocks are also expected to heat plasma to $\sim 10^7$ K, resulting in detectable X-ray emission. In this paper, we investigate 13 $\gamma$-ray emitting novae observed with the Neil Gehrels $Swift$ Observatory, searching for 1-10 keV X-ray emission concurrent with $\gamma$-ray detections. We also analyze $\gamma$-ray observations of novae V407 Lup (2016) and V357 Mus (2018). We find that most novae do eventually show X-ray evidence of hot shocked plasma, but not until the $\gamma$-rays have faded below detectability. We suggest that the delayed rise of the X-ray emission is due to large absorbing columns and/or X-ray suppression by corrugated shock fronts. The only nova in our sample with a concurrent X-ray/$\gamma$-ray detection is also the only embedded nova (V407 Cyg). This exception supports a scenario where novae with giant companions produce shocks with external circumbinary material and are characterized by lower density environments, in comparison with novae with dwarf companions where shocks occur internal to the dense ejecta., Comment: 23 pages, 19 figures, submitted to ApJ

We report the discovery of a variable optical and X-ray source within the error ellipse of the previously unassociated Fermi Large Area Telescope $\gamma$-ray source 4FGL J0407.7--5702. A 22 ksec observation from XMM-Newton/EPIC shows an X-ray light curve with rapid variability and flaring. The X-ray spectrum is well-fit by a hard power law with $\Gamma = 1.7$. Optical photometry taken over several epochs is dominated by aperiodic variations of moderate amplitude. Optical spectroscopy with SOAR and Gemini reveals a blue continuum with broad and double-peaked H and He emission, as expected for an accretion disk around a compact binary. Overall, the optical, X-ray, and $\gamma$-ray properties of 4FGL J0407.7--5702 are consistent with a classification as a transitional millisecond pulsar in the sub-luminous disk state. We also present evidence that this source is more distant than other confirmed or candidate transitional millisecond pulsar binaries, and that the ratio of X-ray to $\gamma$-ray flux is a promising tool to help identify such binaries, indicating that a more complete census for these rare systems is becoming possible., Comment: Accepted to ApJ; 11 pages, 8 figures

Shocks in gamma-ray emitting classical novae are expected to produce bright thermal and non-thermal X-rays. We test this prediction with simultaneous NuSTAR and Fermi/LAT observations of nova V906 Car, which exhibited the brightest GeV gamma-ray emission to date. The nova is detected in hard X-rays while it is still gamma-ray bright, but contrary to simple theoretical expectations, the detected 3.5-78 keV emission of V906 Car is much weaker than the simultaneously observed >100 MeV emission. No non-thermal X-ray emission is detected, and our deep limits imply that the gamma-rays are likely hadronic. After correcting for substantial absorption (N_H ~ 2 x 10^23 cm^-2), the thermal X-ray luminosity (from a 9 keV optically-thin plasma) is just ~2% of the gamma-ray luminosity. We consider possible explanations for the low thermal X-ray luminosity, including the X-rays being suppressed by corrugated, radiative shock fronts or the X-rays from the gamma-ray producing shock are hidden behind an even larger absorbing column (N_H >10^25 cm^-2). Adding XMM-Newton and Swift/XRT observations to our analysis, we find that the evolution of the intrinsic X-ray absorption requires the nova shell to be expelled 24 days after the outburst onset. The X-ray spectra show that the ejecta are enhanced in nitrogen and oxygen, and the nova occurred on the surface of a CO-type white dwarf. We see no indication of a distinct super-soft phase in the X-ray lightcurve, which, after considering the absorption effects, may point to a low mass of the white dwarf hosting the nova., Comment: 19 pages, 6 figures, 7 tables; accepted to MNRAS

Classical novae are thermonuclear explosions that occur on the surfaces of white dwarf stars in interacting binary systems (Bode & Evans 2008). It has long been thought that the luminosity of classical novae is powered by continued nuclear burning on the surface of the white dwarf after the initial runaway (Gallaher & Starrfield 1978). However, recent observations of GeV $\gamma$-rays from classical novae have hinted that shocks internal to the nova ejecta may dominate the nova emission. Shocks have also been suggested to power the luminosity of events as diverse as stellar mergers (Metzger & Pejcha 2017), supernovae (Moriya et al. 2018), and tidal disruption events (Roth et al. 2016), but observational confirmation has been lacking. Here we report simultaneous space-based optical and $\gamma$-ray observations of the 2018 nova V906 Carinae (ASASSN-18fv), revealing a remarkable series of distinct correlated flares in both bands. The optical and $\gamma$-ray flares occur simultaneously, implying a common origin in shocks. During the flares, the nova luminosity doubles, implying that the bulk of the luminosity is shock-powered. Furthermore, we detect concurrent but weak X-ray emission from deeply embedded shocks, confirming that the shock power does not appear in the X-ray band and supporting its emergence at longer wavelengths. Our data, spanning the spectrum from radio to $\gamma$-ray, provide direct evidence that shocks can power substantial luminosity in classical novae and other optical transients., Comment: 49 pages including supplementary information. Accepted and published in Nature Astronomy

Aims: We use new deep XMM-Newton observations of the northern disk of M 31 to trace the hot interstellar medium (ISM) in unprecedented detail and to characterise the physical properties of the X-ray emitting plasmas. Methods: We used all XMM-Newton data up to and including our new observations to produce the most detailed image yet of the hot ISM plasma in a grand design spiral galaxy such as our own. We compared the X-ray morphology to multi-wavelength studies in the literature to set it in the context of the multi-phase ISM. We performed spectral analyses on the extended emission using our new observations as they offer sufficient depth and count statistics to constrain the plasma properties. Data from the Panchromatic Hubble Andromeda Treasury were used to estimate the energy injected by massive stars and their supernovae. We compared these results to the hot gas properties. Results: The brightest emission regions were found to be correlated with populations of massive stars, notably in the 10 kpc star-forming ring. The plasma temperatures in the ring regions are ~0.2 keV up to ~0.6 keV. We suggest this emission is hot ISM heated in massive stellar clusters and superbubbles. We derived X-ray luminosities, densities, and pressures for the gas in each region. We also found large extended emission filling low density gaps in the dust morphology of the northern disk, notably between the 5 kpc and 10 kpc star-forming rings. We propose that the hot gas was heated and expelled into the gaps by the populations of massive stars in the rings. Conclusions. It is clear that the massive stellar populations are responsible for heating the ISM to X-ray emitting temperatures, filling their surroundings, and possibly driving the hot gas into the low density regions. Overall, the morphology and spectra of the hot gas in the northern disk of M 31 is similar to other galaxy disks., Comment: 23 pages, 14 figures, accepted for publication in Astronomy & Astrophysics

The RadioAstron space radio telescope provides a unique opportunity to study the extreme brightness temperatures ($\mathrm{T_B }$) in AGNs with unprecedented long baselines of up to 28 Earth diameters. Since interstellar scintillation (ISS) may affect the visibilities observed with space VLBI (sVLBI), a complementary ground based flux density monitoring of the RadioAstron targets, which is performed near in time to the VLBI observation, could be beneficial. The combination/comparison with the sVLBI data can help to unravel the relative influence of source intrinsic and ISS induced effects, which in the end may alter the conclusions on the $\mathrm{T_B }$ measurements from sVLBI. Since 2013, a dedicated monitoring program has been ongoing to observe the ISS of RadioAstron AGN targets with a number of radio telescopes. Here we briefly introduce the program and present results from the statistical analysis of the Effelsberg monitoring data. We discuss the possible effects of ISS on $\mathrm{T_B }$ measurements for the RadioAstron target B0529+483 as a case study., Comment: Proceedings for the 14th European VLBI Network Symposium & Users Meeting (EVN 2018) 8-11 October 2018, Granada, Spain

We present the first results of an ongoing intra-day variability (IDV) flux density monitoring program of 107 blazars, which were selected from a sample of RadioAstron space very long baseline interferometry (VLBI) targets. The~IDV observations were performed with the Effelsberg 100-m radio telescope at 4.8\,GHz, focusing on the statistical properties of IDV in a relatively large sample of compact active galactic nuclei (AGN). We investigated the dependence of rapid ($<$3 day) variability on various source properties through a likelihood approach. We found that the IDV amplitude depends on flux density and that fainter sources vary by about a factor of 3 more than their brighter counterparts. We also found a significant difference in the variability amplitude between inverted- and flat-spectrum radio sources, with the former exhibiting stronger variations. $\gamma$-ray loud sources were found to vary by up to a factor 4 more than $\gamma$-ray quiet ones, with 4$\sigma$ significance. However a galactic latitude dependence was barely observed, which suggests that it is predominantly the intrinsic properties (e.g., angular size, core-dominance) of the blazars that determine how they scintillate, rather than the directional dependence in the interstellar medium (ISM). We showed that the uncertainty in the VLBI brightness temperatures obtained from the space VLBI data of the RadioAstron satellite can be as high as $\sim$70\% due to the presence of the rapid flux density variations. Our statistical results support the view that IDV at centimeter wavelengths is predominantly caused by interstellar scintillation (ISS) of the emission from the most compact, core-dominant region in an AGN., Comment: 23 pages, 9 figures, published online by MDPI Galaxies

Photometric variability detection is often considered as a hypothesis testing problem: an object is variable if the null-hypothesis that its brightness is constant can be ruled out given the measurements and their uncertainties. Uncorrected systematic errors limit the practical applicability of this approach to high-amplitude variability and well-behaving data sets. Searching for a new variability detection technique that would be applicable to a wide range of variability types while being robust to outliers and underestimated measurement uncertainties, we propose to consider variability detection as a classification problem that can be approached with machine learning. We compare several classification algorithms: Logistic Regression (LR), Support Vector Machines (SVM), k-Nearest Neighbors (kNN) Neural Nets (NN), Random Forests (RF) and Stochastic Gradient Boosting classifier (SGB) applied to 18 features (variability indices) quantifying scatter and/or correlation between points in a light curve. We use a subset of OGLE-II Large Magellanic Cloud (LMC) photometry (30265 light curves) that was searched for variability using traditional methods (168 known variable objects identified) as the training set and then apply the NN to a new test set of 31798 OGLE-II LMC light curves. Among 205 candidates selected in the test set, 178 are real variables, 13 low-amplitude variables are new discoveries. We find that the considered machine learning classifiers are more efficient (they find more variables and less false candidates) compared to traditional techniques that consider individual variability indices or their linear combination. The NN, SGB, SVM and RF show a higher efficiency compared to LR and kNN., Comment: 19 pages, 13 figures, 5 tables; accepted to MNRAS

Variability Search Toolkit (VaST) is a software package designed to find variable objects in a series of sky images. It can be run from a script or interactively using its graphical interface. VaST relies on source list matching as opposed to image subtraction. SExtractor is used to generate source lists and perform aperture or PSF-fitting photometry (with PSFEx). Variability indices that characterize scatter and smoothness of a lightcurve are computed for all objects. Candidate variables are identified as objects having high variability index values compared to other objects of similar brightness. The two distinguishing features of VaST are its ability to perform accurate aperture photometry of images obtained with non-linear detectors and handle complex image distortions. The software has been successfully applied to images obtained with telescopes ranging from 0.08 to 2.5m in diameter equipped with a variety of detectors including CCD, CMOS, MIC and photographic plates. About 1800 variable stars have been discovered with VaST. It is used as a transient detection engine in the New Milky Way (NMW) nova patrol. The code is written in C and can be easily compiled on the majority of UNIX-like systems. VaST is free software available at http://scan.sai.msu.ru/vast/, Comment: 23 pages, 9 figures, 1 table; accepted to Astronomy and Computing

Earth-space interferometry with RadioAstron provides the highest direct angular resolution ever achieved in astronomy at any wavelength. RadioAstron detections of the classic quasar 3C273 on interferometric baselines up to 171,000 km suggest brightness temperatures exceeding expected limits from the "inverse-Compton catastrophe" by two orders of magnitude. We show that at 18 cm, these estimates most probably arise from refractive substructure introduced by scattering in the interstellar medium. We use the scattering properties to estimate an intrinsic brightness temperature of 7*10^12 K, which is consistent with expected theoretical limits, but which is ~15 times lower than estimates that neglect substructure. At 6.2 cm, the substructure influences the measured values appreciably but gives an estimated brightness temperature that is comparable to models that do not account for the substructure. At 1.35 cm, the substructure does not affect the extremely high inferred brightness temperatures, in excess of 10^13 K. We also demonstrate that for a source having a Gaussian surface brightness profile, a single long-baseline estimate of refractive substructure determines an absolute minimum brightness temperature, if the scattering properties along a given line of sight are known, and that this minimum accurately approximates the apparent brightness temperature over a wide range of total flux densities., Comment: 6 Pages, 3 Figures, Accepted for publication in ApJ Letters

We present the first polarimetric space VLBI imaging observations at 22 GHz. BL Lacertae was observed in 2013 November 10 with the RadioAstron space VLBI mission, including a ground array of 15 radio telescopes. The instrumental polarization of the space radio telescope is found to be within 9%, demonstrating the polarimetric imaging capabilities of RadioAstron at 22 GHz. Ground-space fringes were obtained up to a projected baseline distance of 7.9 Earth's diameters in length, allowing us to image the jet in BL Lacertae with a maximum angular resolution of 21 $\mu$as, the highest achieved to date. We find evidence for emission upstream of the radio core, which may correspond to a recollimation shock at about 40 $\mu$as from the jet apex, in a pattern that includes other recollimation shocks at approximately 100 $\mu$as and 250 $\mu$as from the jet apex. Polarized emission is detected in two components within the innermost 0.5 mas from the core, as well as in some knots 3 mas downstream. Faraday rotation analysis, obtained from combining RadioAstron 22 GHz and ground-based 15 GHz and 43 GHz images, shows a gradient in rotation measure and Faraday corrected polarization vector as a function of position angle with respect to the core, suggesting that the jet in BL Lacertae is threaded by a helical magnetic field. The intrinsic de-boosted brightness temperature in the unresolved core exceeds $3\!\times\!10^{12}$ K, suggesting at the very least departure from equipartition of energy between the magnetic field and radiating particles., Comment: Accepted for publication in ApJ

RadioAstron is a project to use the 10m antenna on board the dedicated SPEKTR-R spacecraft, launched on 2011 July 18, to perform Very Long Baseline Interferometry from space - Space-VLBI. We describe the strategy and highlight the first results of a 92/18/6/1.35cm fringe survey of some of the brighter radio-loud Active Galactic Nuclei (AGN) at baselines up to 25 Earth diameters (D_E). The survey goals include a search for extreme brightness temperatures to resolve the Doppler factor crisis and to constrain possible mechanisms of AGN radio emission, studying the observed size distribution of the most compact features in AGN radio jets (with implications for their intrinsic structure and the properties of the scattering interstellar medium in our Galaxy) and selecting promising objects for detailed follow-up observations, including Space-VLBI imaging. Our survey target selection is based on the results of correlated visibility measurements at the longest ground-ground baselines from previous VLBI surveys. The current long-baseline fringe detections with RadioAstron include OJ 287 at 10 D_E (18cm), BL Lac at 10 D_E (6cm) and B0748+126 at 4.3 D_E (1.3 cm). The 18 and 6cm-band fringe detections at 10 D_E imply brightness temperatures of T_b ~ 10^13 K, about two orders of magnitude above the equipartition inverse Compton limit. These high values of T_b might indicate that the jet flow speed is often higher than the jet pattern speed., Comment: 6 pages, 3 figures, 1 table. Proceedings of the 11th EVN Symposium. Bordeaux (France), October 2012

Continuing the project described by Kato et al. (2009, PASJ 61, S395, arXiv:0905.1757), we collected times of superhump maxima for 51 SU UMa-type dwarf novae mainly observed during the 2010-2011 season. Although most of the new data for systems with short superhump periods basically confirmed the findings by Kato et al. (2009) and Kato et al. (2010, PASJ 62, 1525, arXiv:1009.5444), the long-period system GX Cas showed an exceptionally large positive period derivative. An analysis of public Kepler data of V344 Lyr and V1504 Cyg yielded less striking stage transitions. In V344 Lyr, there was prominent secondary component growing during the late stage of superoutbursts, and the component persisted at least for two more cycles of successive normal outbursts. We also investigated the superoutbursts of two conspicuous eclipsing objects: HT Cas and the WZ Sge-type object SDSS J080434.20+510349.2. Strong beat phenomena were detected in both objects, and late-stage superhumps in the latter object had an almost constant luminosity during the repeated rebrightenings. The WZ Sge-type object SDSS J133941.11+484727.5 showed a phase reversal around the rapid fading from the superoutburst. The object showed a prominent beat phenomenon even after the end of the superoutburst. A pilot study of superhump amplitudes indicated that the amplitudes of superhumps are strongly correlated with orbital periods, and the dependence on the inclination is weak in systems with inclinations smaller than 80 deg., Comment: 87 pages, 92 figures, Accepted for publication in PASJ, correction in the author list and text