GALEX UV Light-curves of M-Dwarf Flare Stars: “ THE FLARING UV SKY”

1. GALEX UV Light-curves of M-Dwarf Flare Stars: “THE FLARING UV SKY” Barry Welsh, Jonathan Wheatley & Stanley Browne (UC Berkeley) Richard Robinson (Catholic University of America) Mark Siebert, Michael Rich & the GALEX Science team(JPL/Caltech/UCLA)

2. THE FLARING UV SKY • The NASA Galactic Evolution Explorer (GALEX) satellite is finding that the ultraviolet sky is far from being quiescent, in that we are serendipitously detecting significant numbers of highly variable and transient sources. • We report on observations of the largest stellar flare event yet seen, in which the overall brightness of the nearby red dwarf star (GJ 3685A) increased by more than a factor of 10,000 in less than 200 seconds!

3. THE NASA GALEX MISSION • The Galactic Evolution Explorer (GALEX) satellite was launched in April 2003 and is carrying out wide-field (1.5°) imaging and spectroscopic observations of the sky in two near and far ultraviolet pass bands • Its main scientific goal is to explore the history of star formation processes in nearby galaxies • However, during exposures of its prime targets (galaxies) the telescope’s large field of view also serendipitously records data from the surrounding star fields • We report on the variable & transient sources found in these fields, that include flare stars, RR Lyrae stars, soft X-ray transients & foreground objects such as satellites & space junk. 84 of these stellar sources have been reported in the GALEX UV variability (GUVV) catalog (Welsh et al, AJ, 2005)

4. A GIANT FLARE ON A RED DWARF • The star GJ 3685A is a red dwarf (dM4e) star with a distance of 45 light-years from the Sun • Red dwarfs are only ~ 30% the size of the Sun and are about half as hot. They are the most numerous type of object in our Galaxy and account for ~ 75% of all known stars • Normally GJ 3685A is a very dim 13th magnitude star in the constellation of Virgo ------ but on April 20th 2004 it suddenly flared and increased its brightness in the ultraviolet by a factor of more than 10,000 • At the peak of its eruption, the UV flare outshone the entire red dwarf star by more than a factor of 10 in the UV!!

5. THE FORMATION OF FLARES • The magnetic field-lines of a star stretch up into their outer atmosphere - the corona - in loop-like structures • When adjacent magnetic field lines “re-connect” (probably due to disturbances within the stellar interior) they release energy in the form of electrons that gyrate down the field lines towards the star’s surface • When the electrons encounter the denser material in the stellar chromosphere they release impulsive energy (in the form of hard X-rays) and the collisions ionize and heat the chromospheric gas • Large pressure gradients are produced which drive the heated & ionized material up into the corona where it cools by conduction and radiation (which we observe as the UV flare)

6. FLARE LIGHT CURVES • The GALEX UV detectors were built at the Space Sciences Laboratory in Berkeley and, unlike CCD detectors, have the ability to count individual photons with very high time precision • This means that we can re-construct the emitted photon flux from a flare event as a function of time. • We can show this in 2 ways: a) as a movie b) as a light-curve

8. LIGHT CURVES • We show the increase in FUV and NUV magnitudes as a function of time:

9. OTHER FLARES • GALEX is detecting very large flare events about every 2 months • Typically we observe an increase in brightness of 4 - 6 UV magnitudes ( > 100 increase in flux) in ~ 100 seconds • Re-construction of their light-curves shows 2 types of eruptive event - a rapid rise in flux followed by a gradual decay - a rapid rise in flux followed by further, sometimes larger, eruptions with long decay times • At the unprecedented time-resolution of 0.05 seconds, we can determine whether the flare emission at NUV wavelengths occurs at the same time as that at FUV wavelengths. We can also observe extremely short-lived “flare impulse” structures in the light curve.

10. OTHER M-star FLARES

11. Satellites, Asteroids and cosmic streakers • During observations of distant galaxies, GALEX has detected the trails of several very nearby space objects as they fly across the telescope’s field of view • These ‘cosmic streakers’ are asteroids, orbiting satellites and possibly large chunks of space debris • The trails of reflected light from these objects are removed during the scientific processing of the astronomical data, but here we show some interesting artifacts

12. OTHER FLARES

13. A family of cosmic streakers!

14. Kallisto, a distant asteroid

15. A very bright satellite (?)

16. CONCLUSIONS • The largest ultraviolet flare on a red dwarf star has been recorded throughout its entirety by the GALEX mission • Emission structure on timescales of 0.05 seconds has been revealed for the first time • Galex is serendipitously detecting other large flare events such that we are amassing a growing database of a variety of flare light curves • The GALEX photon counting detectors are regularly capturing the flight paths of satellites, asteroids and space junk.