A&A 589, A48 (2016) Astronomy DOI: 10.1051/0004-6361/201628199 & c ESO 2016 Astrophysics Optical microflaring on the nearby flare star binary UV Ceti J. H. M. M. Schmitt1, G. Kanbach2, A. Rau2, and H. Steinle2 1 Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany e-mail: [email protected] 2 Max-Planck-Institut für extraterrestrische Physik, 85748 Garching bei München, Gießenbachstraße 1, Germany Received 27 January 2016 / Accepted 11 March 2016 ABSTRACT We present extremely high time resolution observations of the visual flare star binary UV Cet obtained with the Optical Pulsar Timing Analyzer (OPTIMA) at the 1.3 m telescope at Skinakas Observatory (SKO) in Crete, Greece. OPTIMA is a fiber-fed optical instrument that uses Single Photon Avalanche Diodes to measure the arrival times of individual optical photons. The time resolution of the observations presented here was 4 µs, allowing to resolve the typical millisecond variability time scales associated with stellar flares. We report the detection of very short impulsive bursts in the blue band with well resolved rise and decay time scales of about 2 s. The overall energetics put these flares at the lower end of the known flare distribution of UV Cet. Key words. stars: activity – stars: flare – stars: low-mass 1. Introduction conservative estimate of τth can be derived in the diffusion ap- proximation (which of course need not apply in the photosphere Solar and stellar flares are known to occur on vastly different and chromosphere) and one finds – apart from geometry factors time scales, ranging from a few seconds (Vilmer et al. 1994; CV (cf., discussion by Sánchez Almeida 2001)– tth ≈ 3 , where Schmitt et al. 1993) and possibly below (Beskin et al. 1982) to 16κRσT CV denotes the specific heat at constant volume, κR the Rosse- more than a week (Kürster & Schmitt 1996). The primary energy land mean opacity, σ the Boltzmann constant and T the tempera- release process of solar and stellar flares is – at least according ture. With typical parameters computed for a stellar photosphere to the prevailing paradigm – coronal, but a substantial amount of one finds cooling times on the order of a few seconds. If a sce- energy is (often) emitted at optical wavelengths. The response of nario of non-thermal heating and thermal cooling is assumed, the plasma, heated by particle beams and shock waves, depends one would thus expect cooling times on the order of seconds and sensitively on the mode of energy input and is in fact radiated heating time scales which in principle could be very short, i.e., over many regions of the electromagnetic spectrum. similar to the time scales of non-thermal solar X-ray or γ-ray Because of their low contrast white light flares are rather dif- bursts (Vilmer et al. 1994). ficult to observe on the Sun. However, on stars with sufficiently A thorough study of the photospheric heating (and cooling) low effective temperatures the photospheric response of a coro- processes during flares thus requires observations with very high nal energy release produces emissions that dominate the over- time resolution. Yet only few investigations of flare stars have all stellar output, even when confined to quite small portions of been carried out at extremely high time resolution at the level the stellar surface. Most researchers agree that in the flare pro- of, say, milliseconds or less. Since our OPTIMA instrument (for cess some particle acceleration does take place in the corona, a description see Sect.2) does provide this temporal resolution, causing the accelerated particles and/or plasma waves to move we decided to carry out some preliminary flare star observations along the magnetic field lines before finally reaching and heat- in the msec range and below. ing cooler atmospheric layers (Syratovskii & Shmeleva 1972; Of course, other flares star studies with high time resolution Brown 1973). The heated chromospheric and photospheric lay- have been carried out with various instruments. Robinson et al. ers are quite dense and start radiating essentially instantaneously, (1995) used the High Speed Photometer aboard the Hubble thus serving as a proxy indicator for non-thermal particles. On Space Telescope to study CN Leo with high time resolution somewhat longer time scales the heated chromospheric layers (0.01 s) for 2 h in the UV (around 2400 Å) and detected a num- “evaporate”, leading to a soft X-ray flare (Neupert 1978; Peres ber of events with “substantial variations sometimes occurring 2000). on time scales of less than 1 s”. Tovmassian et al.(1997) used If the optical emission were also non-thermal, the relevant smaller ground based telescopes to study the flare stars EV Lac time scales could be very short indeed. If, however, the emission and V 577 Mon with a time resolution of 0.1 s in the U and is thermal, as most researchers would assume, the relevant time B-band and argue that some of the reported events have durations scales are the typical propagation time of a shock τsh, driven by of less than 1 s. Similarly, Zhilyaev et al.(1998) report ground- the flare explosion into the photospheric layers, and the radiative based UBV observations again of EV Lac with a time resolution cooling time scale τth, on which a heated parcel of gas looses of down to 0.05 s, with some events having widths below 1 s. its energy by radiation. The shock propagation time can be es- The most systematic efforts we are aware of in that re- timated from the expression τsh ∼ H=vsh, with H denoting the spect are the studies undertaken at the 6 m Special Astrophys- scale height of the emission layer and vsh the shock speed. A ical Observatory (SAO) using the Multichannel Analysis of Article published by EDP Sciences A48, page 1 of6 A&A 589, A48 (2016) Nanosecond Intensity Alterations (MANIA); a detailed descrip- tion of this hardware has been presented by Beskin et al.(1982). As with our OPTIMA instrument, MANIA recorded individual photon arrival times with a time resolution of 300 nsec. Most of the MANIA observations were taken with a Johnson U filter, some with a B filter, however, due to data storage limitations at the time only the strongest flares could be recorded for subse- quent analysis. Needless to say, data storage is not an issue any more. The MANIA team published data on a number of nearby flare stars (Beskin et al. 1988), in particular on UV Cet, here we present OPTIMA data on the nearby (d = 2:62 pc) flare star binary UV Cet (=Gl 65 A+B). According to the SIM- BAD data base, both components are spectral type dM5.5e, the brighter component has a visual magnitude of 12.57 mag with a B − V color of 1.85 mag, while the B component with a vi- sual magnitude of 12.7 mag is only a little fainter. Both compo- nents are dMe stars, both are known to be active, and both are known to produce flares. Both binary components have also been 20" detected at X-ray (Audard et al. 2003) and radio wavelengths (Jackson et al. 1989), with the B component usually being the more active one. According to Geyer et al.(1988) the two com- Fig. 1. Image of the UV Cet system taken with the OPTIMA field view- ponents orbit each other in a rather eccentric orbit with a period ing CCD camera. The positions of the optical fibers are indicated rela- of 25.52 yr; at the time of our observations the two stars were tive to the location of the target. separated by a little under 2 arcsec at a position angle of 50◦. 100 2. Observations and data reduction 80 The observations reported in this paper were taken with the Op- 60 tical Pulsar Timing Analyzer (OPTIMA) mounted at the 1.3 m 40 Johnson-B OPTIMA telescope at Skinakas Observatory (SKO) in Crete, Greece in 20 2008. OPTIMA is a transportable visiting instrument that was (%) Transmission 0 350 400 450 500 550 operated at SKO for several weeks each year between 2006 and 30 2012. It records the arrival times of individual optical photons 25 using eight single photon avalanche diodes (SPADs). Each diode 20 is fed with by an optical fiber mounted in the telescopes optical 15 plane on a slanted mirror. A central (“target”, cf., Fig.1) fiber is 10 surrounded hexagonally by six outer fibers, while an eighth fiber (%) Efficiency 5 observes the sky background a few arcmin away from the target. 0 The observations discussed here have been performed with 350 400 450 500 550 Wavelength (nm) a set of 300 µm diameter fibers corresponding to 6 arcsec on the sky; thus it is clear that the two components of UV Cet cannot Fig. 2. Top: comparison of the transmission of blue filter used within be resolved by OPTIMA. The data acquisition system used in OPTIMA and the standard Johnson B-band filter. Bottom: overall effi- 2008 recorded all photons from the eight SPADs together with ciency of OPTIMA with the blue filter taking into account the quantum a time stamp accurate to 4 µs and synchronized with a GPS sig- efficiency of the single photon avalanche diodes. nal in absolute time. At this time resolution dead time effects become important at count rates above ≈105 ct/s. The data were stored in a buffer, which, once full, was written to a hard drive. During this writing process no further data were recorded lead- the observations this sequence is reversed and another set of sky ing to an additional dead time of several seconds approximately background and SPAD dark current measurements is taken.