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Aims. We present near ultraviolet (NUV: 1750-2800 Å) and far ultraviolet (FUV: 1350-1750 Å) light-curves for flares on 4 nearby dMe-type stars (GJ 3685A, CR Dra, AF Psc and SDSS J084425.9+513830.5) observed with the GALEX satellite. Methods. Taking advantage of the time-tagged events recorded with the GALEX photon counting detectors, we present high temporal resolution (<0.01 s) analysis of these UV flare data. Results. A statistical analysis of 700 s of pre-flare quiescence data for both CR Dra and SDSS J084425.9+513830.5 failed to reveal the presence of significant micro-flare activity in time bins of 0.2, 1 and 10 s intervals. Using an appropriate differential emission measure for both the quiescent and flaring state, it is possible to reproduce the observed FUV:NUV flux ratios. A major determinant in reproducing this flux ratio is found to be the value of plasma electron density during the flare. We also searched the count rate data recorded during each of the four flare events for periodicity associated with magneto-hydrodynamic oscillations in the active region coronal loops. Significant oscillations were detected during the flare events observed on all 4 stars, with periodicities found in the 30 to 40 s range. Flare oscillations with this periodicity can be explained as acoustic waves in a coronal loop of length of ≈10 9 cm for an assumed plasma temperature of 5−20 × 10 6 K. This suggests a loop length for these M-dwarf flares of less than 1/10th of the stellar radii. We believe that this is the first detection of non-solar coronal loop flare oscillations observed at ultraviolet wavelengths.

We present the preliminary results from implementing a new software tool that enables inspection of time-tagged photon data for the astronomical sources contained within individual GALEX ultraviolet (UV) images of the sky. We have inspected the photon data contained within 1802 GALEX images to reveal rapid, short-term ( 500 sec) UV source variability in the form of stellar 'flares'. The mean associated change in near UV (NUV) magnitude due to this flaring activity is 2.7 ±0.3 mag. A list of 49 new UV variable-star candidates is presented, together with their associated Sloan Digital Sky Survey (SDSS) photometric magnitudes. From these data we can associate the main source of these UV flare events with magnetic activity on M-dwarf stars. Photometric parallaxes have been determined for 32 of these sources, placing them at distances ranging from approximately 25 to 1000pc. The average UV flare energy for these flare events is 2.5 x 10 30 ergs, which is of a similar energy to that of U-band, X-ray and EUV flares observed on many local M-dwarf stars. We have found that stars of classes M0 to M5 flare with energies spanning a far larger range and with an energy approximately 5 times greater than those of later (M6 to M8) spectral type.

We present the second Galaxy Evolution Explorer (GALEX ) Ultraviolet Variability (GUVV-2) Catalog that contains information on 410 newly discovered time-variable sources gained through simultaneous near (NUV 1750 -2750Å) and far (FUV 1350 -1750Å) ultraviolet photometric observations. Source variability was determined by comparing the NUV and/or FUV fluxes derived from orbital exposures recorded during a series of multiple observational visits to 169 GALEX fields on the sky. These sources, which were contained within a sky-area of 161 deg 2 , varied on average by ∆NUV = 0.6 mag and ∆FUV = 0.9 mag during these observations. Of the 114 variable sources in the catalog with previously known identifications, ∼ 67% can be categorized as being active galaxies (QSO's, Seyfert 1 or BL Lac objects). The next largest groups of UV variables are RR Lyrae stars, X-ray sources and novae.By using a combination of UV and visible color-color plots we have been able to tentatively identify 36 possible RR Lyrae and/or δ Scuti type stars, as well as 35 probable AGN's, many of which may be previously unidentified QSO's or blazars. Finally, we show data for 3 particular variable objects: the contact binary system of SDSS J141818.97+525006.7, the eclipsing dwarf nova system of IY UMa and the highly variable unidentified source SDSS J104325.06+563258.1.

We present version 1.0 of the NASA Galaxy Evolution Explorer (GALEX ) ultraviolet variability (GUVV) catalog, which contains information on 84 time-variable and transient sources gained with simultaneous near-ultraviolet (NUV) and far-ultraviolet (FUV) photometric observations. These time-variable sources were serendipitously revealed in the various 1N2 diameter star fields currently being surveyed by the GALEX satellite in two ultraviolet bands (NUV 1750-2750 8, FUV 1350-1750 8) with limiting AB magnitudes of 23-25. The largest amplitude variable objects currently detected by GALEX are M dwarf flare stars, which can brighten by 5-10 mag in both the NUV and FUV bands during short-duration (<500 s) outbursts. Other types of large-amplitude ultraviolet variable objects include ab-type RR Lyrae stars, which can vary periodically by 2-5 mag in the GALEX FUV band. This first GUVV catalog lists galactic positions and possible source identifications in order to provide the astronomical community with a list of time-variable objects that can now be repeatedly observed at other wavelengths. We expect the total number of time-variable source detections to increase as the GALEX mission progresses, such that later version numbers of the GUVV catalog will contain substantially more variable sources.

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serendipitously obtained with the NASA GALEX satellite that show changes of up to AB ¼ 6:5 mag in the far-ultraviolet (FUV; 1350-1800 Å) wavelength band. For the test case SW Aqr, we combine the GALEX FUV and NUV (1800-2800 Å) observations with visible U, B, and V photometry to determine the variation of physical parameters such as temperature, surface gravity, and radius during the RR Lyrae pulsation cycle of 0.459299 days. Assuming that SWAqr changes temperature from 6100 K to 8400 K over this cycle, we find that the FUV and NUV light curves can be well matched using the 2004 stellar atmosphere models of Castelli and Kurucz (CK) and a best-fit metallicity of ½Fe=H ¼ À1:5. Pulsation periods are also found for the remaining six stars from their UV light curves, and we have shown that the UV data are consistent with the predictions of CK models using an assumed temperature variation of 6100 K to 8000 K and a log g variation of 2.15 to 3.75 over the pulsation cycle for assumed metallicity values of À2:0 < ½Fe=H < 0:0. Our main finding from the present analysis is that CK models can be successfully used to predict the light-curve behavior of RR Lyrae stars at UV wavelengths if the minimum and maximum temperatures are known from (preferably) visible light-curve observations, such that the stellar metallicity values can be derived. Finally, we note that the model fits to the GALEX data are quite sensitive to the light curves around minimum light, such that the best-fit metallicity is well constrained over this region of the pulsation cycle.

We have used high-precision differential astrometry from the Palomar High-precision Astrometric Search for Exoplanet Systems (PHASES) project and radial velocity measurements covering a time span of 20 years to determine the orbital parameters of the 88 Tau A system. 88 Tau is a complex hierarchical multiple system comprising a total of six stars; we have studied the brightest four, consisting of two short-period pairs orbiting each other with an $18 yr period. We present the first orbital solution for one of the short-period pairs, and determine the masses of the components and distance to the system to the level of a few percent. In addition, our astrometric measurements allow us to make the first determination of the mutual inclinations of the orbits. We find that the subsystems are not coplanar.