Mag Amplitude Research Articles

Following the Pulsations in the Long-term Cooling of GW Librae and V386 Serpentis

GW Lib and V386 Ser are dwarf novae systems containing pulsating white dwarfs that underwent large 8–9 mag amplitude outbursts in 2007 and 2019, respectively. Following the pulsation periods in these systems after the outburst provides a means to view the heating and cooling effects of mass accretion on the instability region of the white dwarf. Follow-up optical observations during 2021–2023 for these two systems are reported, resulting in a time span coverage of 16 yr for GW Lib and 4 yr for V386 Ser after their outbursts that reveal large differences in behavior as their white dwarfs returned to quiescence. GW Lib showed intermittent longer periods at 19 minutes, 1.4, 2, and 4 hr during the first 16 yr after the outburst, before finally showing, at 14 yr post-outburst, two of the three short-period modes apparent during preoutburst quiescence. In contrast, V386 Ser appeared to follow theoretical expectations, showing a shorter-period pulsation soon after the outburst, and progressively longer periods as it cooled to its quiescent state. While the optical light returns to quiescent value within 2 yr, it is apparent that the white dwarf takes much longer to recover to its quiescent state. Theoretical work is needed to explain the large differences in otherwise similar systems.

Outburst behavior of the dwarf nova CG Draconis

AbstractDuring the British Astronomical Association (BAA) 2022 campaign, 27,436 photometric observations of the dwarf nova (DN) CG Draconis were made, with 106 eclipses recorded. This work summarizes the new data available and provides updated ephemeris and commentary on the observed eclipse profiles. The orbital period found is . Two types of quasi‐periodic outbursts are identified: normal outbursts, of mag amplitude, and bright, of mag. The pattern resembles superoutbursts of SU UMa‐type DNe, however, no presence of superhumps characterizing these DNe was found. Given CG Dra is located above the period gap, it may represent a new intermediary subtype between SS Cyg and SU UMa‐type stars, or provide support to superoutburst models that do not rely on eccentric accretion disks.

Discovery of extreme, roughly daily superflares on the recurrent nova V2487 Oph

ABSTRACTV2487 Oph is a recurrent nova with detected eruptions in 1900 and 1998. Startlingly, V2487 Oph shows flares, called ‘Superflares’, with up to 1.10 mag amplitude, fast rises of under one-minute, always with an initial impulsive spike followed by a roughly exponential tail, typically one-hour durations, and with random event times averaging once-per-day. The typical flare energy E is over 1038 erg, while the yearly energy budget is 1041 erg. V2487 Oph Superflares obey three relations; the number distribution of flare energies scales as E−2.34 ± 0.35, the waiting time from one flare to the next is proportional to E of the first event, and flare durations scale as E0.44 ± 0.03. Scenarios involving gravitational energy and nuclear energy fail to satisfy the three relations. The magnetic energy scenario, however, can explain all three relations. This scenario has magnetic field lines above the disc being twisted and amplified by the motions of their footprints, with magnetic reconnection releasing energy that comes out as Superflare light. This exact mechanism is already well known to occur in white light solar flares, in ordinary M-type flare stars, and in the many Superflare stars observed all across the H-R diagram. Superflares on Superflare stars have rise times, light-curve shapes, and durations that are very similar to those on V2487 Oph. So we conclude that the V2487 Oph Superflares are caused by large-scale magnetic reconnection. V2487 Oph is now the most extreme Superflare star, exhibiting the largest known flare energy (1.6 × 1039 erg) and the fastest occurrence rate.

The Complex Rotational Light Curve of (385446) Manwë–Thorondor, a Multicomponent Eclipsing System in the Kuiper Belt

Abstract Kuiper Belt Object (385446) Manwë–Thorondor is a multiobject system with mutual events predicted to occur from 2014 to 2019. To detect the events, we observed the system at 4 epochs (UT 2016 August 25 and 26, 2017 July 22 and 25, 2017 November 9, and 2018 October 6) in g, r, and VR bands using the 4 m SOAR and the 8.1 m Gemini South telescopes at Cerro Pachón, Chile, and Lowell Observatory’s 4.3 m Discovery Channel Telescope at Happy Jack, Arizona. These dates overlap the uncertainty range (±0.5 day) for four inferior events (Thorondor eclipsing Manwë). We clearly observe variability for the unresolved system with a double-peaked period 11.88190 ± 0.00005 hr and ∼0.5 mag amplitude together with much longer-term variability. Using a multicomponent model, we simultaneously fit our observations and earlier photometry measured separately for Manwë and Thorondor with the Hubble Space Telescope. Our fit suggests Manwë is bilobed, close to the “barbell” shape expected for a strengthless body with density ∼0.8 g cm−3 in hydrostatic equilibrium. For Manwë, we thereby derive maximum width to length ratio ∼0.30, surface area equivalent to a sphere of diameter 190 km, geometric albedo 0.06, mass 1.4 × 1018 kg, and spin axis oriented ∼75° from Earth’s line of sight. Changes in Thorondor’s brightness by ∼0.6 mag with a ∼300 day period may account for the system’s long-term variability. Mutual events with unexpectedly shallow depth and short duration may account for residuals to the fit. The system is complex, providing a challenging puzzle for future modeling efforts.

Double cyclic variations in orbital period of the eclipsing cataclysmic variable EX Dra

EX Dra is a long-period eclipsing dwarf nova with $\sim2-3$ mag amplitude outbursts. This star has been monitored photometrically from November, 2009 to March, 2016 and 29 new mid-eclipse times were obtained. By using new data together with the published data, the best fit to the $O-C$ curve indicate that the orbital period of EX Dra have an upward parabolic change while undergoing double-cyclic variations with the periods of 21.4 and 3.99 years, respectively. The upward parabolic change reveals a long-term increase at a rate of $\dot{P}={+7.46}\times10^{-11}{s} {s^{-1}}$. The evolutionary theory of cataclysmic variables (CVs) predicts that, as a CV evolves, the orbital period should be decreasing rather than increasing. Secular increase can be explained as the mass transfer between the secondary and primary or may be just an observed part of a longer cyclic change. Most plausible explanation for the double-cyclic variations is a pair of light travel-time effect via the presence of two companions. Their masses are determined to be $M_{A}sini'_{A}=29.3(\pm0.6) M_{Jup}$ and $M_{B}sini'_{B}=50.8(\pm0.2) M_{Jup}$. When the two companions are coplanar to the orbital plane of the central eclipsing pair, their masses would match to brown dwarfs.

T Pyxidis: death by a thousand novae

We report a 20-year campaign to track the 1.8 hour photometric wave in the recurrent nova T Pyxidis, using the global telescope network of the Center for Backyard Astrophysics. During 1996-2011, that wave was highly stable in amplitude and waveform, resembling the orbital wave commonly seen in supersoft binaries. The period, however, was found to increase on a timescale P/P-dot=3x10^5 years. This suggests a mass transfer rate in quiescence of ~10^-7 M_sol/yr, in substantial agreement with the accretion rate based on the star's luminosity. This is ~2000x greater than is typical for cataclysmic variables of that orbital period. During the post-eruption quiescence (2012-2016), the star continued on its merry but mysterious way - similar luminosity, similar P/P-dot (2.4x10^5 years). The orbital signal became vanishingly weak (<0.003 mag) near maximum light of the 2011 eruption. By day 170 of the eruption, near V=11, the orbital signal reappeared with an amplitude of 0.005 mag. It then gradually strengthened to its normal 0.08 mag amplitude, as the star declined to its "quiescent" magnitude of 15.7. During the ~1 year of invisibility and low amplitude, the orbital signal had increased in period by 0.0054(7)%. This is probably a measure of the mass ejected in the nova outburst. For a plausible choice of binary parameters, that mass is at least 3x10^-5 M_sol, and probably more. This represents >300 years of accretion at the pre-outburst rate, but the time between outbursts was only 45 years. Thus the erupting white dwarf seems to have ejected at least 6x more mass than it accreted. If this eruption is typical, the white dwarf must be eroding, rather than growing, in mass. Unless the present series of eruptions is a short-lived episode, the binary dynamics appear to be a mutual suicide pact between the eroding white dwarf and the low-mass secondary, excited ... . (etc., abstract continues)

ROTATIONAL PROPERTIES OF THE HAUMEA FAMILY MEMBERS AND CANDIDATES: SHORT-TERM VARIABILITY

ABSTRACT Haumea is one of the most interesting and intriguing trans-Neptunian objects (TNOs). It is a large, bright, fast rotator, and its spectrum indicates nearly pure water ice on the surface. It has at least two satellites and a dynamically related family of more than 10 TNOs with very similar proper orbital parameters and similar surface properties. The Haumean family is the only one currently known in the trans-Neptunian belt. Various models have been proposed, but the formation of the family remains poorly understood. In this work, we have investigated the rotational properties of the family members and unconfirmed family candidates with short-term variability studies, and report the most complete review to date. We present results based on five years of observations and report the short-term variability of five family members and seven candidates. The mean rotational periods, from Maxwellian fits to the frequency distributions, are 6.27 ± 1.19 hr for the confirmed family members, 6.44 ± 1.16 hr for the candidates, and 7.65 ± 0.54 hr for other TNOs (without relation to the family). According to our study, there is a possibility that Haumea family members rotate faster than other TNOs; however, the sample of family members is still too limited for a secure conclusion. We also highlight the fast rotation of 2002 GH32. This object has a 0.36 ± 0.02 mag amplitude lightcurve and a rotational period of about 3.98 hr. Assuming 2002 GH32 is a triaxial object in hydrostatic equilibrium, we derive a lower limit to the density of 2.56 g cm−3. This density is similar to Haumea’s and much more dense than other small TNO densities.

The past photometric history of the FU Ori-type young eruptive star 2MASS J06593158-0405277 = V960 Mon

The known FU Ori-type young eruptive stars are exceedingly rare (a dozen or so confirmed objects) and 2MASS J06593158-0405277, with its 2014 outburst, is likely the latest addition to the family. All members have displayed just one such eruption in their recorded history, an event lasting for decades. To test the FU Ori nature of 2MASS J06593158-0405277, we have reconstructed its photometric history by measuring its brightness on Harvard photographic plates spanning the time interval 1899–1989. No previous large amplitude eruption similar to that initiated in 2014 has been found, as in bona fide FU Ori-type objects. The median value of the brightness in quiescence of 2MASS J06593158-0405277 is B = 15.5, with the time interval 1935–1950 characterized by a large variability ( ∼ 1 mag amplitude) that contrasts with the remarkable photometric stability displayed at later epochs. The variability during 1935–1950 can either be ascribed to some T Tau like activity of 2MASS J06593158-0405277 itself or to the also young and fainter star 2MASS J06593168-0405224 that lies 5 arcsec to the North and forms an unresolved pair at the astrometric scale of Harvard photographic plates.

The young low-mass star ISO-Oph-50: extreme variability induced by a clumpy, evolving circumstellar disc

ISO-Oph-50 is a young low-mass object in the ~Myr old Ophiuchus star forming region undergoing dramatic changes in its optical/near/mid-infrared brightness by 2-4 mag. We present new multi-band photometry and near-infrared spectra, combined with a synopsis of the existing literature data. Based on the spectroscopy, the source is confirmed as a mid M dwarf, with evidence for ongoing accretion. The near-infrared lightcurves show large-scale variations, with 2-4 mag amplitude in the bands IJHK, with the object generally being bluer when faint. Near its brightest state, the object shows colour changes consistent with variable extinction of dAV~7 mag. High-cadence monitoring at 3.6mu reveals quasi-periodic variations with a typical timescale of 1-2 weeks. The best explanation for these characteristics is a low-mass star seen through circumstellar matter, whose complex variability is caused by changing inhomogeneities in the inner parts of the disk. When faint, the direct stellar emission is blocked; the near-infrared radiation is dominated by scattered light. When bright, the emission is consistent with a photosphere strongly reddened by circumstellar dust. Based on the available constraints, the inhomogeneities have to be located at or beyond ~0.1 AU distance from the star. If this scenario turns out to be correct, a major portion of the inner disk has to be clumpy, structured, and/or in turmoil. In its observational characteristics, this object resembles other types of YSOs with variability caused in the inner disk. Compared to other objects, however, ISO-Oph-50 is clearly an extreme case, given the large amplitude of the brightness and colour changes combined with the erratic behaviour. ISO-Oph-50 has been near its brightest state since 2013; further monitoring is highly encouraged.

Variable optical/infrared counterpart to the transient gamma-ray source J0109+6134

Context. We investigate the optical/infrared counterpart to the flaring gamma-ray source J0109+6134, which is believed to be a blazar seen through the Galactic plane.Aims. The original aim of this work was to confirm the previously proposed optical counterpart by means of studying its temporal behaviour. The study was later extended to infrared wavelengths as new data became available.Methods. We conducted a long-term differential CCD photometry campaign using the robotic Liverpool telescope. In addition, we used infrared satellite observations to also explore the source variability at these longer wavelengths.Results. Evidence of variability well correlated with gamma-ray flares has been observed so far only in the infrared domain. This fact strongly supports that the proposed optical/infrared counterpart identification is correct. Moreover, our optical photometric campaign revealed an intense optical flare with 1.7 mag amplitude that occurs on time-scales of weeks. This optical event was observed to evolve without a counterpart in the nearly simultaneous gamma-ray monitoring by the Fermi satellite. Gamma-ray orphan optical flares have rarely been observed in other blazars, and J0109+6134 appears to be an interesting additional example for future studies.

MULTIBAND STUDIES OF THE OPTICAL PERIODIC MODULATION IN THE X-RAY BINARY SAX J1808.4–3658 DURING ITS QUIESCENCE AND 2008 OUTBURST

We report on time-resolved optical imaging of the X-ray binary SAX J1808.4-3658 during its quiescent state and 2008 outburst. The binary, containing an accretion-powered millisecond pulsar, has a large sinusoidal-like modulation in its quiescent optical emission. We employ a Markov chain Monte Carlo technique to fit our multi-band light curve data in quiescence with an irradiated star model, and derive a tight constraint of $50^{+6}_{-5}$ deg on the inclination angle $i$ of the binary system. The pulsar and its companion are constrained to have masses of $0.97^{+0.31}_{-0.22} M_{\sun}$ and $0.04^{+0.02}_{-0.01} M_{\sun}$ (both 1$\sigma$ ranges), respectively. The dependence of these results on the measurements of the companion's projected radial velocity is discussed. We also find that the accretion disk had nearly constant optical fluxes over a $\sim$500 day period in the quiescent state our data covered, but started brightening 1.5 months before the 2008 outburst. Variations in modulation during the outburst were detected in our four observations made 7-12 days after the start of the outburst, and a sinusoidal-like modulation with 0.2 mag amplitude changed to have a smaller amplitude of 0.1 mag. The modulation variations are discussed. We estimate the albedo of the companion during its quiescence and the outburst, which was approximately 0 and 0.8 (for isotropic emission), respectively. This large difference probably provides additional evidence that the neutron star in the binary turns on as a radio pulsar in quiescence.

VARIABILITY AT THE EDGE: OPTICAL NEAR/IR RAPID-CADENCE MONITORING OF NEWLY OUTBURSTING FU ORIONIS OBJECT HBC 722

We present the detection of day-timescale periodic variability in the r-band lightcurve of newly outbursting FU Orionis-type object HBC 722, taken from > 42 nights of observation with the CQUEAN instrument on the McDonald Observatory 2.1m telescope. The optical/near-IR lightcurve of HBC 722 shows a complex array of periodic variability, clustering around 5.8 day (0.044 mag amplitude) and 1.28 day (0.016 mag amplitude) periods, after removal of overall baseline variation. We attribute the unusual number of comparable strength signals to a phenomenon related to the temporary increase in accretion rate associated with FUors. We consider semi-random "flickering", magnetic braking/field compression and rotational asymmetries in the disk instability region as potential sources of variability. Assuming the 5.8 day period is due to stellar rotation and the 1.28 day period is indicative of Keplerian rotation at the inner radius of the accretion disk (at 2 R(star)), we derive a B-field strength of 2.2-2.7 kG, slightly larger than typical T Tauri stars. If instead the 5.8 day signal is from a disk asymmetry, the instability region has an outer radius of 5.4 R(star), consistent with models of FUor disks. Further exploration of the time domain in this complicated source and related objects will be key to understanding accretion processes.

The accretion disk in the post period-minimum cataclysmic variable SDSS J080434.20 + 510349.2

This study of SDSS0804 is primarily concerned with the double-hump shape in the light curve and its connection with the accretion disk in this bounce-back system. Time-resolved photometric and spectroscopic observations were obtained to analyze the behavior of the system between superoutbursts. A geometric model of a binary system containing a disk with two outer annuli spiral density waves was applied to explain the light curve and the Doppler tomography. Observations were carried out during 2008-2009, after the object's magnitude decreased to V~17.7(0.1) from the March 2006 eruption. The light curve clearly shows a sinusoid-like variability with a 0.07 mag amplitude and a 42.48 min periodicity, which is half of the orbital period of the system. In Sept. 2010, the system underwent yet another superoutburst and returned to its quiescent level by the beginning of 2012. This light curve once again showed a double-humps, but with a significantly smaller ~0.01mag amplitude. Other types of variability like a "mini-outburst" or SDSS1238-like features were not detected. Doppler tomograms, obtained from spectroscopic data during the same period of time, show a large accretion disk with uneven brightness, implying the presence of spiral waves. We constructed a geometric model of a bounce-back system containing two spiral density waves in the outer annuli of the disk to reproduce the observed light curves. The Doppler tomograms and the double-hump-shape light curves in quiescence can be explained by a model system containing a massive >0.7Msun white dwarf with a surface temperature of ~12000K, a late-type brown dwarf, and an accretion disk with two outer annuli spirals. According to this model, the accretion disk should be large, extending to the 2:1 resonance radius, and cool (~2500K). The inner parts of the disk should be optically thin in the continuum or totally void.

MEASURING MICROLENSING USING SPECTRA OF MULTIPLY LENSED QUASARS

We report on a program of spectroscopic observations of gravitationally-lensed QSOs with multiple images. We seek to establish whether microlensing is occurring in each QSO image using only single-epoch observations. We calculate flux ratios for the cores of emission lines in image pairs to set a baseline for no microlensing. The offset of the continuum flux ratios relative to this baseline yields the microlensing magnification free from extinction, as extinction affects the continuum and the lines equally. When we find chromatic microlensing, we attempt to constrain the size of the QSO accretion disk. SDSSJ1004+4112 and HE1104-1805 show chromatic microlensing with amplitudes $0.2< |\Delta m| < 0.6$ and $0.2< |\Delta m| < 0.4$ mag, respectively. Modeling the accretion disk with a Gaussian source ($I\propto \exp(-R^2/2r_s^2)$) of size $r_s\propto \lambda^p$ and using magnification maps to simulate microlensing we find $r_s(\lambda 3363)=7\pm3 light-days (18.1\pm7.8 \times 10^{15} cm$) and $p=1.1\pm 0.4$ for SDSS1004+4112, and $r_s(\lambda 3363)=6\pm2 light-days (15.5\pm5.2 \times 10^{15} cm$) and $p=0.7\pm0.1$ for HE1104-1805. For SDSSJ1029+2623 we find strong chromaticity of $\sim 0.4$ mag in the continuum flux ratio, which probably arises from microlensing although not all the available data fit within this explanation. For Q0957+561 we measure B-A magnitude differences of 0.4 mag, much greater than the $\sim$0.05 mag amplitude usually inferred from lightcurve variability. It may substantially modify the current interpretations of microlensing in this system, likely favoring the hypothesis of smaller sources and/or larger microdeflectors. For HS0818+1227, our data yield posible evidence of microlensing.

OPTICAL AND INFRARED PHOTOMETRY OF THE BLAZAR PKS 0537–441: LONG AND SHORT TIMESCALE VARIABILITY

We present a large collection of photometric data on the Blazar PKS 0537-441 in the VRIJHK bands taken in 2004-2009. At least three flare-like episodes with months duration, and >3 mag amplitude are apparent. The spectral energy distribution is consistent with a power law, and no indication of a thermal component is found. We searched for short time scale variability, and an interesting event was identified in the J band, with a duration of ~25 minutes.

The Apollo Group Asteroid 2008 OS9: Discovery, Orbit, Rotation and the Yarkovsky/YORP Effects

Abstract A project for astrometric and photometric observations of asteroids at the Baldone Observatory is described. One of the most important results of the project is the discovery of 2008 OS9, a 600 meter asteroid of the NEO Apollo group. The results of its astrometric and photometric observations at the Molėtai and Baldone observatories are presented. From the brightness variation with the 0.27 mag amplitude, a rotation period of 8.430 ± 0.005 h is determined. Close approaches of the asteroid to Earth and Venus during the next millenium are predicted. The mean values of secular changes in the semi-major axis, eccentricity and inclination are computed with and without the Yarkovsky and YORP effects. A negative value of the difference between the value of semi-major axis computed with the Yarkovsky and YORP effects and without them, da/dt, may indicate retrograde rotation of the asteroid.

THE CHANDRA VARIABLE GUIDE STAR CATALOG

Variable stars have been identified among the optical-wavelength light curves of guide stars used for pointing control of the Chandra X-ray Observatory. We present a catalog of these variable stars along with their light curves and ancillary data. Variability was detected to a lower limit of 0.02 mag amplitude in the 4000-10000 A range using the photometrically stable Aspect Camera on board the Chandra spacecraft. The Chandra Variable Guide Star Catalog (VGUIDE) contains 827 stars, of which 586 are classified as definitely variable and 241 are identified as possibly variable. Of the 586 definite variable stars, we believe 319 are new variable star identifications. Types of variables in the catalog include eclipsing binaries, pulsating stars, and rotating stars. The variability was detected during the course of normal verification of each Chandra pointing and results from analysis of over 75,000 guide star light curves from the Chandra mission. The VGUIDE catalog represents data from only about 9 years of the Chandra mission. Future releases of VGUIDE will include newly identified variable guide stars as the mission proceeds. An important advantage of the use of space data to identify and analyze variable stars is the relatively long observations that are available. The Chandra orbit allows for observations up to 2 days in length. Also, guide stars were often used multiple times for Chandra observations, so many of the stars in the VGUIDE catalog have multiple light curves available from various times in the mission. The catalog is presented as both online data associated with this paper and as a public Web interface. Light curves with data at the instrumental time resolution of about 2 s, overplotted with the data binned at 1 ks, can be viewed on the public Web interface and downloaded for further analysis. VGUIDE is a unique project using data collected during the mission that would otherwise be ignored. The stars available for use as Chandra guide stars are generally 6-11 mag and are commonly spectral types A and later. Due to the selection of guide stars entirely for positional convenience, this catalog avoids the possible bias of searching for variability in objects where it is to be expected. Statistics of variability compared to spectral type indicate the expected dominance of A-F stars as pulsators. Eclipsing binaries are consistently 20%-30% of the detected variables across all spectral types.

V405 Peg (RBS 1955): A Nearby, Low-Luminosity Cataclysmic Binary11Based in part on observations obtained at the MDM Observatory, operated by Dartmouth College, Columbia University, The Ohio State University, and the University of Michigan.

ABSTRACTThe cataclysmic binary V405 Peg, originally discovered as ROSAT Bright Source (RBS) 1955 (= 1 RXS J230949.6 + 213523), shows a strong contribution from a late-type secondary star in its optical spectrum. This led Schwope et al. to suggest that it is among the nearest cataclysmic binaries. Here we present extensive optical observations of V405 Peg. Time-series spectroscopy shows the orbital period, Porb, to be 0.1776469(7) days (= 4.2635 hr), or 5.629 cycles day-1. We classify the secondary star as M3–M4.5, and estimate its contribution to the light. Astrometry with the MDM 2.4 m telescope gives a parallax πabs = 7.2 ± 1.1 mas, and a relative proper motion of 58 ± 1 mas yr-1. We employ a Bayesian technique to form a best estimate of the distance from the parallax and other indicators; this yields d = 149(+26,-20) pc. The semiamplitude of the secondary stars’s radial velocity is K2 = 92 ± 3 km s-1, indicating a fairly low orbital inclination if the masses are typical. Extensive time-series observations in the I band show the system varying between a minimum brightness level of I = 14.14 mag and states of enhanced activity about 0.2 mag brighter. During the low state, the light curve is dominated by ellipsoidal modulation of the secondary star at a period consistent with the spectroscopic value. In the high state, we detect an additional photometric modulation with ∼0.1 mag amplitude, which occurs in the range 220–280 minutes. The frequency of this modulation appears to be stable for a month or so, but no single period was consistently detected from one observing season to the next. The occurrence of low states can be reconciled with a magnetic cataclysmic variable. While the additional signal could point to an asynchronously rotating white dwarf, our data do not show this signal to be truly coherent. We estimate the system luminosity by combining optical measurements with the archival X-ray spectrum. The implied mass accretion rate is orders of magnitudes below the predictions for the standard angular momentum loss above the period gap. The system may possibly belong to a largely undiscovered population of hibernating cataclysmic variable stars.

The ultraluminous X-ray source NGC 1313 X-2

NGC 1313 X-2 is one of the brightest ultraluminous X-ray sources in the sky, at both X-ray and optical wavelengths; therefore, quite a few studies of available ESO VLT and HST data have appeared in the literature. Here, we present our analysis of VLT/FORS1 and HST/ACS photometric data, confirming the identification of the B ~ 23 mag blue optical counterpart. We show that the system is part of a poor cluster with an age of 20 Myr, leading to an upper mass limit of some 12 for the mass donor. We attribute the different results with respect to earlier studies to the use of isochrones in the F435W and F555W HST/ACS photometric system that appear to be incompatible with the corresponding Johnson B and V isochrones. The counterpart exhibits significant photometric variability of about 0.2 mag amplitude, both between the two HST observations and during the one month of monitoring with the VLT. This includes variability within one night and suggests that the light is dominated by the accretion disk in the system and not by the mass donor.

Long-term V-band monitoring of the bright stars of M33 at the Wise Observatory

Abstract We have conducted a long-term V-band photometric monitoring of M33 on 95 nights during four observing seasons (2000–2003). A total number of 6418 light curves of bright objects in the range of 14–21 mag have been obtained. All measurements are publicly available. A total of 127 new variables were detected, of which 28 are periodic. 10 previously known non-periodic variables were identified as periodic, three of which are Cepheids, and another previously known periodic variable was identified as an eclipsing binary. Our derived periods range from 2.11 to almost 300 d. For 50 variables we have combined our observations with those of the DIRECT project, obtaining light curves of up to 500 measurements, with a time-span of ∼7 yr. We have detected a few interesting variables, including a 99.3 d periodic variable with a 0.04 mag amplitude, at the position of supernova remnant (SNR) 19.