Lighting up Stars: New X-ray Diagnostics of Stellar and Young Stellar Atmospheres Rachel Osten June 2, 2010

GRBs in disguise Swift’s capabilities were designed for GRB study: large FOV HXT (BAT) sr ability to reorient toward transient HXR source in < 2 minutes, arcminute positional discrimination narrow FOV instruments to catch afterglow (XRT, UVOT), arcsecond positional discrimination These characteristics also make it ideal for the study of large stellar flares

Basic flare scenario on the Sun flares: are a consequence of magnetic reconnection occurring high in the corona involve the entire atmosphere produce emissions across the EM spectrum

Outline “New” discoveries of “old” spectral diagnostics Iron Kα 6.4 keV emission line and its use in nearby active stellar coronae Iron Kα 6.4 keV emission line in young stellar objects, and what we can learn

Stellar X-ray Spectra generally well-described by plasma in collisional ionization equilibrium

flare emissions plasma heating: X-ray, EUV, FUV line and continuum emission from upper chromosphere through corona flare enhancements 10s-1000s white light stellar flare: blackbody of T ∼ 10 4 K fits data, but could also be Balmer jump flare enhancements 10s-100s accelerated particles: radio gyrosynchrotron, coherent emission (type III-like bursts, noise storms), nonthermal hard X-ray flare enhancements 10s-100s

Stellar Flares are Multi- Wavelength HR 1099 (G5IV+K1V); Osten et al. 2004

And Have Fast Rise Times EV Lac (dM3.5e): Favata et al RAO et al. 2005

Origin of the white light flare is still under debate RHD models including nonthermal energy input from e- beam; Allred et al AD Leo (dM3); Hawley et al. 2003

Solar Flares involve particle acceleration Emslie et al energy estimates from solar flares indicate that a large fraction of energy in a flare goes into accelerating particles, and exceeds the energy in soft X-ray emitting plasma Aschwanden 2002 T ∝ EM in solar, stellar flares (Feldman et al. 1996), need large stellar flares to see nonthermal HXR emission, but contribution from thermal emission complicates analysis see NT stellar radio emission, know particle acceleration happens, but difficult to extract parameters of accelerated particles from this emission

Stellar Flares involve particle acceleration Neupert effect = observational relationship between signatures of particle acceleration and plasma heating -White light solar flares correlated with nonthermal HXR emission (Neidig & Kane 1978) -modelling of white light solar & stellar flares involves energy input from acc. ptcls (Allred et al. 2005,6) -gyrosynchrotron radio flares detail transient particle accel. episodes (Gudel 2002) L x (t) ∝ ∫L nt (t’)dt’ Gudel et al RAO et al. 2004

Previous Superflares & Nonthermal Emission previous HXR detections inconclusive as to presence of NT emission HXR spectra could be explained by thermal tail of superthermal plasma detections out to <100 keV Franciosini et al. (2001); large long-duration flare on UX Ari (G5V+K0IV; Porb=6.44d) seen by BeppoSAX

The gamma-ray burst that wasn’t RAO et al peak X-ray flux keV: erg cm -2 s -1 L x ~10 33 erg/s ( keV) L x /L bol ( keV) at peak 38% XRT keV BAT keV II Peg (K2IV+dM, P orb =6.7d) d=42 pc

First evidence for nonthermal HXR emission from a stellar flare during Orbits 1 and 2, XRT+BAT spectral analysis requires more than 2 thermal components: excess continuum emission E>30 keV can be fit by high T bremsstrahlung (300 MK) or NT thick-target bremsstrahlung with δ~3 and F 0 ~10 36 erg s -1 in accelerated e - reject thermal explanation for hard X-ray emission:τ relax /τ cond = 200 T 8 4 /(n 10 2 L 9 2 ) requires high densities and/or large length scales Neupert effect behavior relating hardest X-rays/soft X-rays is seen E NT ~E thermal rough equipartition around erg (9 orders of magnitude larger than typical large solar flares!) would indicate lack of cooling plasma in flare decay (T 3 in orbit 2 ~T 3 in orbit 1)

energy arguments: E rad, hot plasma ~10 37 erg in solar flares, total radiated energy over all wavelengths E rad,tot is ~10 E rad,hot plasma (Woods et al. 2004) no constraint on kinetic energy in directed, random motions conductive energy losses E cond ~ erg/(L 9 2 n 10 2 ) Drake et al. (2008) estimate n 10 ~400, L 9 ~31 E NT ~ erg Neupert effect seen between hardest X-rays and soft X-rays

Iron Kα line seen in a variety of astrophysical objects fluorescence in a solar flare; Parmar et al K shell ionization edge is at 7.11 keV

Iron Kα line seen in a variety of astrophysical objects galactic microquasar; Miller et al relativistic broadening

Utility of this Line in Stellar Coronae strength of line depends on height of source above photosphere: larger solid angle for smaller h Bai (1979) pointed out that this line could give information on the height of the X-ray source and photospheric iron abundance Testa et al. 2008

Osten et al Testa et al Recent detections of Iron Kα emission line in flares from nearby active stars confirm the relatively compact nature of flares deduced from hydrodynamic modelling II Peg (Osten et al. 2007): h/R ★ ≤ 0.5 HR 9024 (Testa et al. 2008): h/R ★ ≤ 0.3 II Peg: K2IV +dM HR 9024: G1 III

EV Lac (dM4, d=5 pc) Swift trigger April 25, 2008 ➜ F( keV) = 5.3x10 -8 ergs/cm 2 /s ➜ factor of 7000 increase over quiescent value ➜ peak estimated L X /L bol ∼ 3.1 RAO et al. submitted a “GRB” at 5 pc!

(1)UVOT observed in v filter, then white (2) instrument “safed” during white filter observation (3) Δmag in white filter is >4.7

RAO et al. submitted no strong evidence for superthermal (>140 MK) or suprathermal component to HXR emission Temp ∼ 80 MK

RAO et al., submitted Typical evolution of flare parameters, with exception of magnitude E rad ( keV) ∼ ergs

count rate limit is 300,000 cps over A (white filter) assume flare is a BB: F λ =(xR 2 /d 2 ) π B λ (T BB ) for T BB 10 4 K, d=5 pc, R=0.3 R star, 3471 A, x=0.039 compare to x=1e-4 for flares on AD Leo (Hawley et al. 2003) lower limit to optical brightness constrains flare size

first few minutes of EV Lac (M3.5V) flare decay Osten et al. 2010, ApJ submitted

production mechanism may be more complicated some impulsive solar flares show additional Iron Kα flux beyond that produced by thermal photoionization: collisional ionization of the K shell electron Osten et al. (2010) show that variability of Iron Kα line flux in two flares produces an excess Kα flux above thermal photoionization, find plausible parameters for collisional ionization from a beam of electrons to produce this excess

coronal length scale from hydrodynamic modelling from Reale et al (1997), coronal loop length can be estimated from light curve decay, slope of points in T-n e, calibrated to a particular instrument l/R ★ =0.37±0.07 h/R ★ =0.24±0.04

Fe Kα 6.4 keV line is seen... XRT spectrum Kα flux and fluorescence model using Drake et al. (2008) F Kα =f(θ) Γ N πd 2

then not seen...

and seen again

Fe Kα seen when kT > 4 keV error bars are 3 σ

and strange behavior late in the flare decay

The Fe Kα emission line at 6.4 keV is a relatively newly used diagnostic for stellar flares Testa et al geometry for “classic” X-ray fluorescence for stars without a disk EV Lac is only the 3rd nearby stellar flare to exhibit Kα

effect of nonthermal electrons in soft X-ray spectrum? consistent parameter set can explain both excess Kα emission and the lack of detection in hard X-ray spectrum consistent parameter set can explain both excess Kα emission and the lack of detection in hard X-ray spectrum have constraints on hard X-ray flux have constraints on hard X-ray flux Osten et al. 2010, ApJ submitted

consistent flare picture a simultaneously observed white light flare gives optical area constraints A>2x10 19 cm 2, footpoint radius of 10 9 cm implies beam fluxes of erg cm -2 s -1 aspect ratio α= r/2l of 0.1 Osten et al. 2010, ApJ submitted

X-rays from young stars coronal emission from magnetic reconnection, but possible contribution from an accretion disk studies have shown the existence of large flaring loops which may connect the star to the disk

Favata et al. (2005) performed hydrodynamic modelling of large flares seen on young stars in ONC during COUP, derived loop semi-lengths 3/4 of the flares had loop sizes >1 R ★ ≈ 12 YSOs show Iron Kα emission line ➡ use Iron Kα to give size constraints? Sizes of X-ray emitting loops on young stars are large

Geometry for young stars with disks (from Camenzind 1990); 6.4 keV fluorescent line seen during some X-ray flares implies a geometry due to reflection of stellar X-rays off disk material Tsujimoto et al Kα emission in young stars

high equivalent width of Iron Kα line no observed variability in X-ray spectrum (<10 keV) equivalent widths larger than can be produced by photospheric fluorescence or reflection off a centrally illuminated disk alternate formation mechanism: collisional ionization from accelerated particles (but also: obscuration?) Giardino et al keV emission from the young star Elias 29 Kα emission in young stars

in solar flares, energetics of accelerated particles >>those of heated plasma stellar X-ray astronomy is only reaching the tip of the iceberg Emslie et al importance of accelerated particles in stellar atmospheres & environments

x-rays bathe the disk, play a role in photoionizing circumstellar material collisional ionization interpretation of Kα line supports the role of nonthermal particles in young stars, impact on the disk ⇒ solar system evidence suggesting MeV particles in flash heating of chondrules from Feigelson 2003 the role of accelerated particles needs to be tested against better understood conditions, as obtained in nearby stellar atmospheres

Conclusions study of Iron Kα emission in nearby stellar flares provides constraints on size scales, complementary to hydrodynamic modelling variability on short timescales points to additional mechanism producing Iron Kα geometry is more complicated in young stellar objects, but can use results from the Sun and nearby stars (well-characterized environments) to investigate constraints on the nature of accelerated electrons

trigger on Algol (B8V+KIV) 13 Oct severe optical loading of XRT due to Algol’s brightness (V=2.8) d=28 pc

and CC Eri (K7V+M0V) d=11.5 pc 16 Oct ➜ Kα emission also seen in this flare decay ➜ HD analysis suggests h/R ★ = 0.16±0.01 possible excess Kα emission in early stages of flare decay

and CC Eri (K7V+M0V) d=11.5 pc 16 Oct. 2008

Sub-threshold Swift events on HR 1099 HR 1099 (V711 Tau), K1IV+G5IV, Porb=2.8d event 11/29/06 3 detections in ~4 min keV, SNR~7 intensity ~400 mCrab, 1/2 peak flux of II Peg event, L x 1032 erg/s XRT TOO took place 40 hours later also detected in March ’06 at ~8 times lower intensity

Occurrence rates radio surveys: HR /yr, UX Ari 12/yr flare frequency distributions from EUVE (Osten & Brown 1999): 0.08 flares/yr/star above 100x min. flare EUV luminosity or erg/s HXR II-Peg level flares 0.003/yr/star X-ray surveys: Ariel-V (Schwartz et al. 1981) 11/yr >6e-10 erg/cm2/s II Peg (1032 erg/s) Pye & McHardy (1983) all-sky 23/yr above 4e-10, 2.3/yr above 4e-9

New Insights into Large Stellar Flares 3/4 of Swift-detected flares have shown Kα emission, all show Kα variability only 1/2 with direct evidence for NT emission (analysis ongoing on CC Eri & Algol); indirect evidence in EV Lac avoids the “flare at the beginning/ending of observation” bias (point & the star will flare!) multiwavelength response impt: fast radio? HD modelling of flare loops

Implications impact of large stellar flares on habitability study Fe Kα emission in relatively well- understood stellar contexts (diskless nearby active stars), take results and apply to YSOs where Fe Kα emission is seen in ∼ 4x more objects