Корональное излучение закрученных звезд и рентгеновская светимость Sgr A* С. Сазонов, Р. Сюняев, М. Ревнивцев Институт космических исследований РАН MNRAS, 2011 Центр Галактики, Chandra Солнце, TRACE

X-rays from the Galactic Center Baganoff et al At GC distance of 8 kpc, 1 = 0.04 pc Chandra, exposure ~50 ksec

Central X-ray source Baganoff et al Source is extended, σ = 0.6 Spectrum (~250 counts) consistent with optically thermal emission, kT = 1-4 keV

Hot diffuse gas? Prevailing idea (e.g., Quataert 2002; Baganoff et al. 2003; Yuan et al. 2003) Observed size Bondi radius: R B = GM BH /c s 2 ~ 0.05 pc Gas possibly supplied by winds of hot stars (e.g. Quataert 2004; Cuadra 2006) Possibly ADAF/RIAF type flow within R B Alternative possibility: Superposition of weak point sources

Nuclear stellar cluster Genzel et al SINFONI,NACO/VLT

Nuclear stellar cluster Detection limit Genzel et al We are not yet probing late-type main-sequence stars!

Nuclear stellar cluster Space density profile Bartko et al (also Buchholz et al. 2009; Do et al. 2009) ρ * ~ r -γ Total counts of K

Nuclear stellar cluster Stellar luminosity function Bartko et al Except for the young stellar disk(s) at 0.8 < r < 12 (Paumard et al. 2006), the luminosity function in the GC is consistent with continuous star formation with usual IMF (Loeckmann et al. 2010)

Nuclear stellar cluster Diffuse K-light (summed light of K>17 stars) Yusef-Zadeh et al γ~1 at r

Nuclear stellar cluster Dynamical constraints There are dynamical constraints on the total enclosed mass at r < pc (Trippe et al. 2008; Schoedel et al. 2009) and at r

Nuclear stellar cluster Summary A1.5 x 10 6, with uncertainty ~ a factor of 2 (Genzel et al. 2010) The slope changes to Γ~1.8 at r > 0.25 pc Total number of low-mass ( M sun ) stars within 0.1 pc < 6 x 10 4

Tidal spin-up Alexander & Kumar 2001 SPH simulation of a close collision of two MS stars

Tidal spin-up Alexander & Kumar 2001 ΔE=ΔLΩ p ΔL=IΔΩ Efficiency falls rapidly with periseparation

Tidal spin-up Alexander & Kumar 2001 Spin ΔΩ reaches of Ω b, stars breakup speed (corresponds to 440 km/s, or a period of ~3 hours for the Sun)

Stellar rotation => Coronal activity Guedel 2004 Coronal activity reaches saturation at Rossby number R 0 ~0.1, i.e., at Ω ~ 0.1 Ω b

Magnetic braking Guedel 2004 Saturation regime lasts for t mb ~ 100 Myr Afterwards Ω ~ t -1/2 (Skumanich 1972)

Model

Active binaries (RS CVn systems)

X-ray light curves

Average stellar properties 50% of total X-ray luminosity produced by faster rotators

Surface brightness profile: data Chandra, 2-8 keV, ~ 620 ksec Flares are filtered out R=1.5 σ=0.6

Surface brightness profile: model There is ~20% uncertainty in conversion from unabsorbed flux to counts

Surface brightness profile: model X-ray luminosity grows with density approximately as A 1.6

X-ray spectrum: data 6.4 keV line?

X-ray spectrum: produced by a few 10 3 stars? kT = 3.2±0.3 keV kT = 3.7±0.1 keV kT = 2.4±0.3 keV

Stellar flares Superflare of RS CVn binary II Peg on Dec. 16, 2005 Swift, Osten et al o Duration: ~ 2 hours o Peak luminosity: erg/s in keV and erg/s in keV This is ~40% of the binarys bolometric luminosity (~ erg/s) o Hard spectrum and significant 6.4 keV line

Superflare of dMe star EV Lac on Apr. 25, 2008 Osten et al o Duration: < 1 hour o Peak luminosity: erg/s in keV This is 3.1 times the stars bolometric luminosity (~ erg/s) o Hard spectrum and strong 6.4 keV line

6.4 keV –fluorescent emission? Has been predicted (Bai 1979; Basko 1979) and observed (Culhane 1981) during solar flares Recently, has been observed in a few giant stellar flares Typically expected to be 50 eV, but can reach eV, as during the superflare of dMe EV Lac

L x = ×10 33 erg/s L x = 4±1 ×10 33 erg/s - Superflares: frequency few flares per object per year (e.g., Pye & McHardy 1983; Ostin et al. 2007) ~1 flare per 1000 objects at any given moment MAXI on ISM seems to be confirming this: 14 superflares from RS CVns in the 1 st year of operation (Tsuboi et al. 2011) keV

- Выводы Скоплением закрученных маломассивных звезд можно объяснить основные свойства спокойного рентгеновского излучения Sgr A* Требуется несколько 10 4 звезд, из которых несколько 10 3 быстро вращаются, внутри ~0.1 пк – не противоречит инфракрасным наблюдениям и динамическим ограничениям Можно обойтись и меньшим количеством звезд, если 1) вблизи Sgr A* есть много черных дыр – более эффективная закрутка, и/или 2) гигантские корональные вспышки доминируют в суммарном потоке излучения Данные Чандры накладывают верхний предел на количество звезд около Sgr A* Нельзя объяснить центральный радиоисточник и мощные вспышки Sgr A*. Однако более слабые вспышки могут быть связаны со звездами