Alon Retter

MACHO Alert 95-30: First Real-Time Observation of Extended Source Effects in Gravitational Microlensing

We present analysis of MACHO Alert 95-30, a dramatic gravitational microlensing event toward the Galactic bulge whose peak magnification departs significantly from the standard point-source microlensing model. Alert 95-30 was observed in real time by the Global Microlensing Alert Network (GMAN), which obtained densely sampled photometric and spectroscopic data throughout the event. We interpret the light-curve fine structure as indicating transit of the lens across the extended face of the source star. This signifies resolution of a star several kiloparsecs distant. We find a lens angular impact parameter θmin/θsource = 0.715 ± 0.003. This information, along with the radius and distance of the source, provides an additional constraint on the lensing system. Spectroscopic and photometric data indicate the source is a M4 III star of radius 61 ± 12 R☉, located on the far side of the bulge at ~9 kpc. We derive a lens angular velocity, relative to the source, of 21.5 ± 2.9 km s-1 kpc-1, where the error is dominated by uncertainty in the angular size of the source star. Likelihood analysis yields a median lens mass of 0.67 -->+ 2.53−0.46 M☉, located with 80% probability in the Galactic bulge at a distance of 6.93 -->+ 1.56−2.25 kpc. If the lens is a main-sequence star, we can include constraints on the lens luminosity. This modifies our estimates to Mlens=0.53 -->+ 0.52−0.35 M☉ and Dlens=6.57 -->+ 0.99−2.25 kpc. Spectra taken during the event show that the absorption-line equivalent widths of Hα and the TiO bands near 6700 A vary, as predicted for microlensing of an extended source. This is most likely due to center-to-limb variation in the stellar spectral lines. The observed spectral changes further support our microlensing interpretation. These data demonstrate the feasibility of using microlensing limb crossings as a tool to probe stellar atmospheres directly.

Swift X-Ray Observations of Classical Novae

The new γ-ray burst (GRB) mission Swift has obtained pointed observations of several classical novae in outburst. We analyzed all the observations of classical novae from the Swift archive up to 2006 June 30. We analyzed usable observations of 12 classical novae and found 4 nondetections, 3 weak sources, and 5 strong sources. This includes detections of two novae exhibiting spectra resembling those of supersoft X-ray binary source spectra (SSS), implying ongoing nuclear burning on the white dwarf surface. With these new Swift data, we add to the growing statistics of the X-ray duration and characteristics of classical novae.

A model of an expanding giant that swallowed planets for the eruption of V838 Monocerotis

ABSTRACT In early 2002 V838 Monocerotis had an extraordinary outburst whose nature isstill unclear. The optical light curve showed at least three peaks and imaging revealeda light echo around the object – evidence for a dust shell which was emitted severalthousand years ago and now reflecting light from the eruption. Spectral analysis sug-gests that the object was relatively cold throughout the event, which was characterizedby an expansion to extremely large radii. We show that the three peaks in the lightcurve have a similar shape and thus it seems likely that a certain phenomenon wasthree times repeated. Our suggestion that the outburst was caused by the expansionof a red giant, followed by the successive swallowing of three relatively massive plan-ets in close orbits, supplies a simple explanation to all observed peculiarities of thisintriguing object.Key words:stars: evolution – stars: individuals: V838 Mon – planetary systems 1 INTRODUCTIONThe outbust of V838 Mon was discovered in 2002 January(Brown 2002). The object was about 6 mag brighter than itspresumable quiescent brightness level (Munari et el. 2002a).After a short phase of slow decline it had a second episodeof fast brightening by nearly the same factor. V838 Mongradually faded again in February,but in March a third peakwas observed. Figs. 1 & 2 display the visual light curve of theoutburst compiled from various sources. To our knowledgesuch a light curve has never been observed before.Spectra of V838 Mon taken by several groups (Munariet el. 2002a; Goranskii et el. 2002; Banerjee & Ashok 2002;Wisniewski et al. 2003) suggested a very cool and extendedphotosphere throughout the eruption. V838 Mon becameredder during the outburst and the following decline. Thespectrum near the peak of the outbust was fitted with aK5 giant while several months later an M10 giant can beconcluded (Munari et el. 2002b; Munari & Desidera 2002).Later infrared observations in 2002 October led to the sug-gestion that this star is the first L-supergiant (Evans et al.2003). This evolution in time corresponds to a gradual de-crease in the effective temperature from ∼6000 K to less

Solar-like oscillations in the metal-poor subgiant nu Indi: constraining the mass and age using asteroseismology

Asteroseismology is a powerful method for determining fundamental properties of stars. We report the first application to a metal-poor object, namely the subgiant star ν Ind. We measured precise velocities from two sites, allowing us to detect oscillations and infer a large frequency separation of Δν = 24.25 ± 0.25 μHz. Combining this value with the location of the star in the H-R diagram and comparing with standard evolutionary models, we were able to place constraints on the stellar parameters. In particular, our results indicate that ν Ind has a low mass (0.85 ± 0.04 M☉) and is at least 9 Gyr old.

A 6.3-h superhump in the cataclysmic variable TV Columbae: the longest yet seen

We present results from a two-week multi-longitude photometric campaign on TV Col held in 2001 January. The data confirm the presence of a permanent positive superhump found in re-examination of extensive archive photometric data of TV Col. The 6.3-h period is 15 per cent longer than the orbital period and obeys the well-known relation between superhump period excess and binary period. At 5.5 h, TV Col has an orbital period longer than any known superhumping cataclysmic variable and, therefore, a mass ratio which might be outside the range at which superhumps can occur according to the current theory. We suggest several solutions for this problem.

Detection of negative superhumps in a low-mass X-ray binary – an end to the long debate on the nature of V1405 Aql (X1916−053)

The detection of two similar periodicities (3001 and 3028 s) in the light curve of V 1405 Aql, a low mass X-ray binary (LMXRB), has attracted the attention of many observers. Two basic competing models have been offered for this system. According to the first, V1405 Aql is a triple system. The second model invokes the presence of an accretion disc that precesses in the apsidal plane, suggesting that the shorter period is the orbital period while the longer is a positive superhump. The debate on the nature of V1405 Aql has been continued until very recently. Re-examination of previously published X-ray data reveals an additional periodicity of 2979 s, which is naturally interpreted as a negative superhump. The recently found 4.8-d period is consequently understood as the nodal precession of the disc. This is the first firm detection of negative superhumps and nodal precession in a LMXRB. Our results thus confirm the classification of V1405 Aql as a permanent superhump system. The 14-yr argument on the nature of this intriguing object has thus finally come to an end. We find that the ratio between the negative superhump deficit (over the orbital period) and the positive superhump excess is a function of orbital period in systems that show both types of superhumps. This relation presents some challenge to theory as it fits binaries with different components. We propose that a thickening in the disc rim, which causes increased occultation of the X-ray source, is the mechanism responsible for both types of superhumps in LMXRBs. However, the positive signal is related only to the pronounced dips in the light curve, where the point-like central source is covered up, whereas the morphology of the negative superhump signal appears quite smooth, implying obscuration of a larger X-ray emitting region, possibly the inner accretion disc or a corona. According to our model, superhumps (both in the X-ray and optical regimes) are permitted in high-inclination LMXRBs contrary to the Haswell et al. prediction.

Superhumps and Accretion Disk Precession in TT Arietis

We have been conducting a long-term (1988-1998) photometric study of the nova-like variable TT Arietis. The main periodic signal in the stars light curve normally occurs at a period that varies but averages ~0.1329 days, which is about 3.5% shorter than the orbital period of the binary. In 1997, this signal disappeared and was replaced by a stronger signal 8.5% longer than the orbital period. This new wave strongly resembles thesuperhumps commonly seen in SU UMa-type dwarf novae during superoutburst. In superhump parlance, we could say that a negative superhump was replaced by a positive superhump (P > Porb

Altair: The Brightest delta Scuti Star

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The light curve of the semiregular variable L2 Puppis - I. A recent dimming event from dust

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The Swift X-Ray Flaring Afterglow of GRB 050607

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