Flares in Time-Domain Surveys

A COOL STARS 19 splinter session

07 June 2016 14:30 - 17:30
Abstracts due 09 May 2016 29 April 2016

Videos of Splinter Talks

• (14:30-15:30)
• (15:50-16:50)

Session organizers

• Adam Kowalski (University of Maryland)
• Arek Berlicki (University of Wroclaw)
• Gianna Cauzzi (INAF)
• James Davenport (Western Washington University)
• Lyndsay Fletcher (University of Glasgow)
• Suzanne Hawley (Univeristy of Washington)
• Petr Heinzel (Academy of Sciences of the Czech Republic)

Scientific Motivation

The goal of this splinter session is to identify outstanding questions in solar and stellar flare physics that can be addressed in novel ways using the large volume of flare observations in past, current, and future surveys.

Stellar flares are often the dominant source of transient variability in time-domain surveys. Critical aspects of the flare paradigm and dynamo mechanism have been revealed by serendipitous observations in recent large scale surveys (e.g. Kepler, SDSS). Discoveries such as flare rates from inactive stars and “superflares” from field G dwarfs have been enabled by surveys with long monitoring baselines and/or large spatial extents that reveal very rare events. Several existing surveys (e.g., Galex, MEarth, PTF, PanSTARRS, WASP) also offer rich samples of flaring sources. Gaia, TESS, ZTF, and LSST, will open additional new avenues for stellar flare research in the future, such as determining flare rates over cosmic time, the maximum energy of flares, and flare impacts on planet habitability and biomarker detection.

It is also an exciting time for solar flare research, with enormous databases from full-disk monitoring satellites such as SOHO and SDO, a new emphasis on the importance of solar chromospheric emission during flares (e.g. F-CHROMA), and the upcoming capabilities of DKIST. Solar physicists have been at the forefront of analyzing large volumes of high time resolution data for decades, which can inform stellar flare investigations, and improve the standard solar flare model. The connection between flare activity and the underlying magnetic field seen on the Sun can also constrain stellar flare models. However, many important aspects of solar flares are not yet well-characterized observationally, in particular the spectral energy distribution of white-light emission that constitutes the majority of radiated energy. The wealth of data from these surveys will better determine the role of solar flares in generating our own space weather, and have implications for the severity and conditions that characterize stellar space weather.

Our proposed splinter will consist of both invited and contributed talks, and a round table discussion moderated by members of the SOC. The aim of this splinter session is to identify problems common to both solar and stellar flares that can be solved by a collaborative effort combining survey data from both communities. This splinter session is timely and should be convened at Cool Stars 19 because many existing solar and stellar surveys have not been fully utilized, and new projects such as Gaia and DKIST will soon provide enormous amounts of public data on the transient sky and the Sun that can be mined for flare studies.

Program (click for abstracts)

• 14:30 - 15:30 | Invited & Contributed Talks
  Session Chair: Adam Kowalski
• Invited | James Davenport The Kepler Catalog of Stellar Flares
  I will present results from our automated survey of stellar flares using the entire Kepler dataset. This program has produced over 200,000 flare candidates, the largest census of such events to date. Flares in our sample exhibit a wide range of energies and morphologies, including complex multi-peaked events, and possible quasi-periodic oscillations. Matching our flare stars to Kepler rotation periods, we find a decline in the energy emitted in flares as stars spin down. For a subset of Kepler M dwarfs with low resolution follow-up spectra, we also find a correlation between Halpha luminosity and the energy emitted in flares. These two results give the first definitive evidence of flare rates declining over stellar time, indicating flares are intimately connected to the age-rotation-activity evolution of the global stellar dynamo.
• Contributed | Yuta Notsu Superflares on solar-type stars found from Kepler data
  We searched superflares on solar-type stars (G-type main sequence stars) using the Kepler 1-min and 30-min cadence data. Superflares release total energy 10-10⁴ times greater than that of the biggest solar flares with energy of ~10³² erg. We found 187 superflares on 23 stars from 1-min cadence data (Q0-17) and more than 1500 superflares on 279 stars from 30-min cadence data (Q0-6) (Maehara+2012, Nature; Shibayama+2013, ApJS; Maehara+2015, EPS). Using these data, we found that the occurrence frequency (dN/dE) of superflares is expressed as a power-law function of flare energy (E) with the index of -1.5 for 10³³ < E < 10³⁶ erg. Most of superflare stars show quasi-periodic light variations with the amplitude of a few percent, which can be explained by the rotation of the star with large starspots (Notsu+2013, ApJ). This assumption is supported by the spectroscopic results with the measurement of vsini and chromospheric acrtivities using Subaru/HDS (Notsu+2015a&b, PASJ). The bolometric energy released by flares is consistent with the magnetic energy stored around such large starspots. Furthermore, our analyses indicate that the occurrence frequency of superflares depends on the rotation period, and that the flare frequency increases as the rotation period decreases. However, the energy of the largest flares observed in a given period bin does not show any clear correlation with the rotation period (Notsu+2013, ApJ).
• Contributed | Parke Loyd FUV Emission Line Flares on M and K Dwarfs in the MUSCLES Survey
  The MUSCLES HST Treasury Survey recently acquired spectrophotometric FUV and X-ray data for 4 K dwarfs and 7 M dwarfs hosting planets. None of these stars are presently labeled flare stars, though ε Eri is known to have high chromospheric activity. To characterize the volatility of the stellar high-energy radiation, we searched these data for evidence of flares, focusing on the bright FUV emission lines of C II, Si III, Si IV, and N V that were simultaneously observed for ~4 hours per target with HST COS. The search revealed clear flares on 5/7 of the M dwarfs and 1/4 of the K dwarfs including two strong flares (peak flux >10x quiescent levels) on GJ 876 and one on GJ 832. Tracing the evolution of the three strong flares in C II, Si III, Si IV, and N V emission reveals roughly coincident rises in C II, Si III, and Si IV trailed by a muted response in N V. Taken together, the MUSCLES flares follow a consistent power law relationship in frequency versus absolute radiated energy. This talk will share and explore this new body of spectrally and temporally resolved flare data.
• Invited | Juan Carlos Martinez Oliveros SDO/HMI White-light flares and their associated manifestations
  The white-light continuum of a solar flare was the first manifestation of a solar flare ever detected. Nevertheless, its mechanisms remain unknown, even today. Improved observations confirm the identification of white-light continuum emission and hard X-rays during the impulsive phase of a solar flare, both in space and in time, to within the observational limits. We describe previous results (Martinez Oliveros 2011) and place them in the context of the three extreme-limb events (within about 1°) reported by Krucker et al. (2015). The electrons responsible for hard X-ray bremsstrahlung coincide with the most intense flare energy release, but we do not presently understand the physics of energy transport nor the nature of particle acceleration apparently taking place at heights below the preflare temperature minimum. Also, we will show at least at two distinct kinds of sources appeared (chromospheric and coronal), in the early and later phases of flare development, in addition to the white-light footpoint sources commonly observed in the lower atmosphere. , which might imply the participation of cooler sources that can produce free-bound continua and possibly line emission detectable by HMI.
• Contributed | Chloe Pugh Quasi-Periodic Pulsations in Stellar Flares
  Quasi-periodic pulsations (QPPs) in solar flares have been widely observed, and can be used as a coronal plasma diagnostics tool. More recently, QPPs have been observed in flares of other stars, many of which are thousands of times more powerful than anything observed on the Sun. Stellar flares have been identified in the short-cadence light curves of over 200 Kepler stars, and examined for evidence of QPPs. Those showing evidence of pulsations have been analysed using the autocorrelation and wavelet techniques. QPP-like signatures have been detected in 56 flares, and of these 11 have clear damped oscillations which could be modelled. A statistical analysis suggests that the QPPs found in these stellar flares are consistent with those found in solar flares.
• Contributed | Petr Heinzel On the behavior of light curves of solar and stellar flares
  Recently a large effort has been devoted to systematic modeling of solar and stellar chromospheric flares which are typically manifested by bright ribbons embedded in the lower atmospheric layers. However, apart from ribbons detectable in a broad range of the electromagnetic spectrum, overlying flare loops are also frequently observed on the Sun. They appear in various spectral lines and can be well resolved in the so-called eruptive flares. We will show how their appearance depends on the loop plasma conditions and parameters. We will also briefly review the efforts to model the radiation properties of such loops and namely of those which cooled down to chromospheric temperatures. Light curves of the flaring ribbons may differ from time evolution of the loop brightness and this can be easily detected on the Sun. However, on cool flare stars the measured light curves may contain an unresolved information about the temporal evolution of both ribbons and cool loops and we will demonstrate how this can be modeled. Based on that we will discuss the behavior of light curves of solar and stellar flares in various spectral lines of different species.
• 15:30 - 15:50 | Coffee Break
• 15:50 - 16:50 | Invited & Contributed Talks
  Session Chair: James Davenport
• Invited | Sarah Jane Schmidt Finding the Largest Flares on Ultracool Dwarfs with ASAS-SN
  Quiescent chromospheric activity, as measured through Halpha emission, is ubiquitous on ultracool (late-M and early-L) dwarfs, but the rate of white-light flares on these objects is still under investigation. Recent work with Kepler and K2 has revealed that flares occur less frequently than on more massive M dwarfs, but the strongest flares are sufficiently rare that they are unlikely to be observed in the 90 day observational windows. The All Sky Automated Search for Supernovae (ASAS-SN) survey scans the entire sky once every two days in V band down to V> 17. In addition to discovering hundreds of Supernovae, the ASAS-SN survey has also observed hundreds of stellar flares, including two particularly dramatic flares in the ultracool regime; a deltaV ~ -9 on an M8 dwarf, and a deltaV ~ -10 flare on an L1 dwarf. Both flares radiated over 10³⁴ ergs in the V-band, placing them among the strongest observed white-light flares. While flares this strong are expected to occur less than once per year on individual ultracool dwarfs, the all-sky coverage of ASAS-SN presents a unique opportunity to detect strong flares (deltaV < -5) on all ultracool dwarfs within ~100pc. We discuss the two most dramatic ASAS-SN flares and present our initial constraints on the rate of large flares on ultracool dwarfs.
• Contributed | Subhajeet Karmakar X-ray Superflares on CC Eri
  We present an in-depth study of two large stellar flares detected on active binary system CC Eridani by Swift observatory. The first flare (F1) triggered the Burst Alert Telescope (BAT) in the hard X-ray band on 2008 October 16. The rise-phase was only observed only with BAT, whereas the decay-phase was also observed simultaneously with X-Ray Telescope (XRT). The second flare (F2) was observed only in decay phase with both BAT and XRT on 2012 February 24. The e-folding decay time indicates a faster decay in hard X-ray band than in soft X-ray. The peak X-ray luminosity in 0.3-50.0 keV reached up to 3x10³² erg/s and 5×10³¹ erg/s, which is ~3500 and ~600 times more than the quiescent value and larger than any other previously observed flares on CC Eri. Spectral analysis indicates a presence of three temperature corona with first two plasma temperatures remain constant during the flares at ~3MK and ~10MK. The flare-temperature peaked at 139 MK and 58 MK for F1 and F2, which is ~4 and ~2 times more than the minimum value. The abundances peaked at 2.0 and 1.2 solar abundances, which is larger than a factor 11 and 7 than quiescent values. Using hydrodynamic modeling we derive loop-lengths for both flares to be 5.6+/-0.7x10¹⁰ cm and 5.5+/-0.7x10¹⁰ cm, respectively. We model the K-alpha emission feature as fluorescence from the hot flare source irradiating the photospheric iron. Our preliminary estimations indicate the flare location on the stellar surface around an astrocentric angle of 70 degree.
• Contributed | John Pye The frequency of dM-star X-ray flares from a large-scale XMM-Newton sample
  We present preliminary results from a uniform, large-scale survey of X-ray flare emission from dM stars, and provide estimates of flare occurrence rates. The 3XMM Serendipitous Source Catalogue together with several dM-star catalogues (primarily SDSS and LAMOST) have been used as the basis for this survey of X-ray flares from dM stars. The 3XMM catalogue and its associated data products provide an excellent basis for a ~decade-long comprehensive and sensitive survey of stellar flares – both from targeted active stars and from those observed serendipitously in the half-degree diameter field-of-view of each observation. The standard 3XMM results and products have been augmented by the output of the EXTraS project (Exploring the X-ray transient and variable sky). We present results from both target and serendipitous observations. The latter provide an unbiased (with respect to stellar activity) study of flare energetics and occurrence rates. We compare the dM-star X-ray flare rates with those for our previously published XMM/Tycho survey (Pye et al, 2015, A&A, 581, A28; which was weighted towards earlier-type, i.e. F – K, stars) and published values from optical-band surveys.
• Invited | Arkadiusz Berlicki F-CHROMA project: Observations and modelling of solar flare chromospheres
  F-CHROMA is a collaborative project funded under the EU-Framework Programe 7, involving seven different European research Institutes and Universities. The main goal of F-CHROMA is the investigation of the physics of solar flares, with a particular focus on the physical conditions and emission of their chromospheric parts. During the project realization we will perform joint observations and analysis of space-based and ground-based observations of flares. In addition, physical parameters of flaring plasma will be determined using forward and semiempirical NLTE modelling techniques. A crucial component of F-CHROMA will be the dissemination effort to make results of these activities available to and usable by the community, including non-professional astronomers. We already produced a catalogue of ground-based solar flare observations, linked to available space-based counterparts, and make it available to the community. An important task in our dissemination plan is to involve amateur solar observers in our flare observing campaigns. Amateur astronomers can provide the observational data, especially time series of images, which we believe to have scientific values. We hope that this activity might also start a closer cooperation between amateur and professional solar physicists during the project realization and beyond. The first pro-am solar flare "F-HUNTERS" observing campaign was organized on September 19-27th, 2015 and there were many amateur astronomers involved in these observations. The second campaign will be organized in July 2016. Here we will present the main output of the F-CHROMA project and some results of the F-HUNTERS campaign.
• Contributed | Ettore Flaccomio A multi-wavelength view of magnetic flaring from PMS stars
  I will present results of a study of flaring from the young pre-main-sequence stars in the NGC2264 star forming region. Simultaneous observations with Chandra (X-rays), CoRoT (optical), and Spitzer (mIR) were obtained as part of the “Coordinated Synoptic Investigation of NGC2264” (CSI-NGC2264), providing an unprecedented view of stellar flaring. Tens of observed X-ray flares have counterparts in the optical and/or nIR bands. This allowed us to derive relations between the emitted energies, peak fluxes, and durations in the three bands. The picture that emerges is a complex one. Although flares are more prominent and easily detected in the X-ray band, much of the radiated energy is emitted at optical/UV wavelengths. The inner circumstellar disks, when present, appears to play a significant role in the observed nIR emission.
• Contributed | Suzanne Hawley Recovering Flares in LSST
  We have run some simulations of recovering flares in the sparsely sampled LSST data as part of the recent NOAO/LSST followup capabilities workshop, and will report on the results.
• 16:50 - 17:00 | Break
• 17:00 - 17:30 | Round Table Discusison
  A lively discussion

Participation

Submissions for Contributed Talks are currently being accepted via this webform.
Deadline: 09 May 2016 29 April 2016

Contact

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