L105 The Astrophysical Journal, 593:L105–L108, 2003 August 20  2003. The American Astronomical Society. All rights reserved. Printed in U.S.A. THE EXTREME-ULTRAVIOLET CONTINUUM OF A STRONG STELLAR FLARE D. J. Christian, M. Mathioudakis, and D. Jevremovic ´ 1 Department of Pure and Applied Physics, Queens University Belfast, Belfast, BT7 1NN, Northern Ireland; d.christian@qub.ac.uk, m.mathioudakis@qub.ac.uk, d.jevremovic@qub.ac.uk J. Dupuis Johns Hopkins University, Department of Physics and Astronomy and S. Vennes and A. Kawka Department of Mathematics, Australian National University, Canberra, ACT 0200, Australia Received 2003 June 4; accepted 2003 July 7; published 2003 July 18 ABSTRACT We present the serendipitous detection of an extreme-ultraviolet flare on EUVE J0613-23.9B. The flare showed over a 200-fold increase above the quiescent emission in the DS/Lexan 60–200 A ˚ wavelength band. Optical spectroscopy revealed that the event was associated with an active dM3.5e star. The EUVE spectra are dominated by emission lines formed at temperatures in excess of 10 7 K. The observation is unique as we have detected, for the first time, a strong Lyman continuum in the EUVE long-wavelength range (320–650 A ˚ ). The flare in the continuum ( K) was extremely short, lasting for less than 500 s, while in the DS ( K) 7 T ≈ 20,000–30,000 T ≈ 10 its duration was ≈28 ks. The total energy of the flare in the DS is ∼ ergs. We have made a fit to the 34 3 # 10 continuum using semiempirical model atmospheres and derived the time-averaged temperature and density structures. Subject headings: stars: flare — stars: late-type — ultraviolet: stars 1. INTRODUCTION The extensive surveys of the X-ray sky over the past 30 years revealed that in almost one-third of the sources discovered the emission is of coronal origin (Schmitt 1997). This fraction is considerably higher in the extreme ultraviolet, where 50% of the sources are late-type stars. EUVE observations allowed detailed spectroscopic studies of flares on several cool stars including AU Mic, AD Leo (Cully et al. 1994, 1997), and EQ Peg (Monsignori-Fossi et al. 1995). Despite the extensive wavelength coverage of EUVE (70–760 A ˚ ), the flares have been detected primarily in the 70–300 A ˚ range. These flares are best pronounced in the coronal lines of highly ionized iron (Fe xxi–Fe xxiv) and in the short-wavelength continuum (≈100 A ˚ ). The extreme-ultraviolet (EUV) continuum (300–912 A ˚ ) in late-type stars is generated in the chromosphere and transition region. Semiempirical models of active stellar atmospheres show that the radiated energy is a strong function of the tran- sition region pressure and can provide information of the plasma properties in this region. The importance of the EUV Lyman continuum of late-type stars in the atomic equilibrium (especially during stellar flares) has been emphasized; however, no observations of stellar flares for EUV wavelengths longer than ≈300 A ˚ have been reported in the literature (Houdebine et al. 1996). This is primarily due to the low sensitivity of the instruments, weak fluxes, and strong interstellar medium ab- sorption in wavelengths below the Lyman limit. In this Letter, we present the serendipitous detection of a strong flare on an active M star during an EUVE observation of the G star HD 43162. Our observations show the detection of continuum emission extended to wavelengths as long as 650 A ˚ . In § 2 we present the EUVE observations and follow- up optical spectroscopy. In § 3 we examine the temporal be- 1 Astronomical Observatory, Volgina 7, 11160 Belgrade, Serbia and Montenegro. havior of the flare, derive the flare energy, and use the contin- uum to derive the time-averaged temperature and density structure of the atmosphere. Concluding remarks are given in § 4. 2. EUVE AND OPTICAL OBSERVATIONS The Extreme Ultraviolet Explorer (EUVE) obtained 14 ob- servations of HD 43162 between 2000 October 20 and De- cember 8 as part of a Guest Observer program (PI: E. Gaidos). The observations were carried out with the Deep Survey/Spec- trometer telescope (DS/S). Three of the telescope segments provided light for the main imager with a central Lexan/B filter (60–200 A ˚ ), while the remaining three provided light for the short-wavelength (SW, 70–190 A ˚ ), medium-wavelength (MW, 140–380 A ˚ ), and long-wavelength (LW, 280–760 A ˚ ) spectrom- eters (Bowyer & Malina 1991). Following aspect verification and exposure time corrections, a total of 180 ks of data were accumulated. We obtained the EUVE data files of the HD 43162 field from the Multimission Archive (MAST). An inspection of the DS images revealed a second source (R.A. p 06 h 13 m 47 s .2, decl. p -23°54'26; EUVE J0613-23.9B) lo- cated 2' .5 southeast of the Guest Observer target. At this po- sition off-axis, the point-spread function of the DS is still op- timal, and there is no source confusion for EUVE J0613-23.9B and HD 43162. The source is not close to the edge of the detector and remains unaffected by vignetting (see Figs. 3, 6, and 7 of Sirk et al. 1997). We converted these data products into QPOE (quick position oriented event) files for the Deep Survey and spectrometer data. DS QPOE files were used to derive light curves for both sources (as described in Christian 2002 and references therein). Spectrometer QPOE files were used to perform spectral extractions as a function of time. A search of SIMBAD revealed no cataloged sources within 60 of EUVE J0613-23.9B. We have obtained optical spectra for all the likely candidates located within the DS positional uncertainty circle. The optical observations were conducted on