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Extensive Serendipitous X-Ray Coverage of a Flare Star with ROSAT

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Extensive Serendipitous X-Ray Coverage of a Flare Star with ROSAT Mon. Not. R. Astron. Soc. 323, 577±583 2001) Extensive serendipitous X-ray coverage of a flare star with ROSAT J. D. Silverman,w K. A. Eriksen, P. J. Green and S. H. Saar Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA Accepted 2000 October 18. Received 2000 October 9; in original form 1999 November 17 ABSTRACT We report the serendipitous discovery of a flare star observed with the ROSAT X-ray observatory. From optical spectra, which show strong and variable emission lines of the hydrogen Balmer series and neutral helium, we classify this object as a M3.0Ve star, and estimate a distance of 52 pc from published photometry. Owing to the close proximity of the star 13.6 arcmin) to the calibration source and RS CVn binary AR Lacertae, long-term X-ray coverage is available in the ROSAT archive ,50 h spanning 6.5 yr). Two large flare events occurred early in the mission 1990 June±July), and the end of a third flare was detected in 1996 June. One flare, observed with the Position Sensitive Proportional Counter 30 21 PSPC), had a peak luminosity LX 1:1 Â 10 erg s ; an e-folding rise time of 2.2 h and a decay time of 7 h. This decay time is one of the longest detected on a dMe star, providing evidence for the possibility of additional heating during the decay phase. A large High 30 21 Resolution Imager HRI) flare peak LX 2:9 Â 10 erg s is also studied. The `back- ground' X-ray emission is also variable ± evidence for low-level flaring or microflaring. We find that >59 per cent of the HRI counts and >68 per cent of the PSPC counts are caused by flares. At least 41 per cent of the HRI exposure time and 47 per cent of the PSPC are affected by detectable flare enhancement. Key words: stars: flare ± stars: late-type ± X-rays: stars. a variable, high-temperature component owing to compact flaring 1 INTRODUCTION regions Giampapa et al. 1996). X-ray emission from late-type M dwarfs has been studied Schmitt 1994) has shown conclusively the existence of long- extensively to investigate the structure and emission mechanisms duration flares on M stars using the ROSAT all-sky survey. In some of stellar coronae. Coronal heating to X-ray emitting temperatures flaring stars, the long decay can be attributed to continual heating is attributed to either impulsive flares or quiescent energy release of the flaring region Schmitt & Favata 1999; Ottmann & Schmitt in magnetic structures. The corona of these stars are thought to be 1996). New flare models have been developed Reale & Micela similar to the Sun, but often with luminosities that are orders of 1998) in which the additional heating determines the character- magnitude higher. The high magnetic activity of these flare stars is istics of the decay. Using this model, an analysis of long-duration also seen in their optical spectra. Continuum enhancement and flares on AD Leo Favata, Micela & Reale 2000b) and EV Lac strong emission lines of the H Balmer series, Ca ii and neutral He Favata et al. 2000a) have shown the emitting regions to be are often evident Montes et al. 1999). compact with length-scales of less than the stellar radius and An EXOSAT study Pallavicini, Tagliaferri & Stella 1990) similar in size to solar flares, thereby providing evidence that showed that flares have a wide range of energies and time-scales. long-duration flares are produced in high-pressure structures. Most outbursts can be described as either impulsive decay time During an analysis of observations with the ROSAT High ,1 h) or long-decay flares decay time .1 h) and have thermal Resolution Imager HRI) of the RS CVn binary AR Lac, we X-ray spectra with temperatures similar to solar X-ray flares. The noticed an X-ray source within the field of view to be highly impulsive stellar flares have time-scales similar to the compact variable and undetected in many fields. The first X-ray detection solar flares. The long-duration flares have greater total energy and of this source was with the Einstein observatory hence the are more similar to two-ribbon flare events. catalogue name, 2E 2206.614517; Harris et al. 1993). Coaddition From ROSAT observations, coronal emission from dMe stars of six IPC observations spanning 26.6 ks yield a 4s detection with has been shown to have two distinct spectral components, a low- 115 net counts. The source is undetected in a 1.5 ks HRI obser- temperature component attributed to quiescent active regions and vation. No flaring activity is evident during these observations. We analysed 39 observations from the ROSAT public data archive 16 PSPC; 23 HRI) which included this source position w E-mail: [email protected] within the field of view. The total observing time was <50 h. The q 2001 RAS 578 J. D. Silverman et al. observations span two flares with almost complete light curves, We measured count rates using the iraf/pros data analysis the tail end of a third flare, and show variability in the low-level software and corrected for vignetting caused by the large range of X-ray emission. The flare detected with the PSPC has a long off-axis angles 2 , u , 47 arcmin: The observations were sub- decay time, possibly providing evidence for significant continual divided into multiple time bins to achieve a higher temporal heating during the flare decay. Thus, a further in-depth study of resolution while preserving a minimal 2s detection for each bin. flares and quiescent emission from 2E 2206.614517 could An annular region was centred on the source to correct for the provide a useful test of current models. background count rate for most cases. For detections near the edge of the field, a nearby circular background region was chosen. A log of the ROSAT observations is given in Table 1 which includes the exposure time and off-axis angle. 2 OBSERVATIONS AND DATA REDUCTION Spectral fitting of the PSPC data was done with the xspec We detect the X-ray source 2E 2206.614517 in 16 observations software package. Source and background counts were extracted with the ROSAT Position Sensitive Proportional Counter PSPC) using the xselect task from the ftools package, and the ancillary and 23 observations with the HRI, within the instrument bandpass response files were constructed with pcarf to account for off-axis of 0:1±2:4 keV: Extensive and continuous X-ray observations vignetting. We ignored the lowest 11 spectral energy bins to avoid <17 h) were made with the PSPC between 1990 June 18±22 scattered solar extreme ultraviolet EUV) contamination. The during the ROSAT in-orbit calibration period), 1991 December highest 56 spectral channels were also omitted because of poor 30±31; and 1993 May 29 and June 02. The HRI calibration statistics ± a result of a marked decrease in the effective area of observations of AR Lac began 13 d after the completion of the the instrument at these energies. The energy bin distribution PSPC observations. 14 HRI pointings between 1990 July 2±8; oversamples the intrinsic spectral resolution of the PSPC, so we include the source 2E 2206.614517. Eight additional HRI grouped the bins by a factor of 5 to improve the statistics. observations are available in the archive over the period of 1992 Multiple optical spectra were taken by Perry Berlind with the June through 1996 November for a total observing time of 33 h. Tillinghast 60 arcsec telescope and FAST spectrograph Fabricant et al. 1998) at the Fred Lawrence Whipple Observatory on Mount Table 1. ROSAT X-ray observations. 2 Hopkins. A slit width of 3 arcsec, a 300 lines mm 1 grating, and the Loral charge-coupled device CCD) with 15 mm pixels Seq. name Obs. date MJD Exp. time Off-axis < Ê s) angle provided a resolution of 5A. A 3.5-min exposure was taken arcmin) on ut 1998 May 16 and two 10-min exposures were acquired on ut 1998 May 30 and ut 1998 June 24. An observation of Feige 34 rp100588 18/06/90±29/06/90 480 60.13 27 843.3 16.5 rp110586 19/06/90 480 61.25 1 945.2 29.3 was used for extinction correction and flux calibration. Standard rp110591 19/06/90 480 61.53 1 919.7 4.4 bias subtraction, flat-fielding, the extraction of one-dimensional rp110599 19/06/90 480 61.85 1 986.5 19.2 spectra and wavelength calibration were performed using iraf. rp110589 19/06/90±20/06/90 480 61.85 1 826.9 37.2 rp110592 20/06/90 480 62.19 13 932.7 28.3 rp110601 19/06/90±22/06/90 480 61.98 3 330.2 47.1 rp110590 20/06/90 480 62.48 1 885.0 10.5 3 SOURCE IDENTIFICATION AND OPTICAL rp110602 20/06/90 480 62.79 2 340.6 20.8 rp110595 20/06/90±21/06/90 480 62.92 2 132.7 27.4 SPECTRA rp110598 21/06/90±22/06/90 480 63.85 2 542.1 9.0 After obtaining an X-ray source position from a nearly on-axis rp110596 20/06/90 480 62.12 1 838.5 39.3 rp110597 20/06/90 480 62.05 2 255.3 34.7 HRI observation, we identified two candidate optical counterparts rp110600 19/06/90 480 61.91 2 011.2 45.5 within 10 arcsec on the Digitized Sky Survey red plates.
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