Рост массивных черных дыр во Вселенной

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
Chandra, exposure ~50 ksec
At GC distance of 8 kpc, 1’’ = 0.04 pc
Baganoff et al. 2003
Central X-ray source
Spectrum (~250 counts) consistent with
optically thermal emission, kT = 1-4 keV
Source is extended, σ = 0.6’’
Baganoff et al. 2003
Hot diffuse gas?
Prevailing idea (e.g., Quataert 2002; Baganoff et al. 2003; Yuan et al. 2003)
Observed size ≈ Bondi radius: RB= GMBH/cs2 ~ 0.05 pc
Gas possibly supplied by winds of hot stars (e.g. Quataert 2004; Cuadra 2006)
Possibly ADAF/RIAF type flow within RB
Alternative possibility:
Superposition of weak point sources
Nuclear stellar cluster
SINFONI,NACO/VLT
Genzel et al. 2010
Nuclear stellar cluster
Detection limit
Genzel et al. 2010
We are not yet probing late-type main-sequence stars!
Nuclear stellar cluster
Space density profile
ρ* ~ r-γ
Total counts of K<17 stars:
γ~1.2 (r<0.25 pc)
γ~1.75 (r>0.25 pc)
Early type K<15.5 stars: γ~2.5
Late-type K<15.5 stars: γ ≤ 1
- collisional destruction of red giants?
(e.g., Genzel et al. 1996; Dale et al. 2009)
Bartko et al. 2010 (also Buchholz et al. 2009; Do et al. 2009)
Nuclear stellar cluster
Stellar luminosity function
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)
Bartko et al. 2010
Nuclear stellar cluster
Diffuse K-light (summed light of K>17 stars)
VLT/adaptive optics
HST/NICMOS
Schoedel et al. 2007
γ~1 at r<0.2 pc
several x 104 Sun-like stars within 0.1 pc
Yusef-Zadeh et al. 2011
Nuclear stellar cluster
Dynamical constraints
There are dynamical constraints on the
total enclosed mass at r < 0.5-1 pc
(Trippe et al. 2008; Schoedel et al. 2009)
and at r <0.005-0.1 pc
(Gillessen et al. 2009)
Genzel et al. 2010
Nuclear stellar cluster
Summary
 r 

* (r)  A 
 0.25 pc 
1.3
MSun pc
3
at r  0.25pc
A≈1.5 x 106, 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 (0.4-1.5 Msun) stars
within 0.1 pc < 6 x 104
Tidal spin-up
SPH simulation
of a close collision
of two MS stars
Alexander & Kumar 2001
Tidal spin-up
ΔE=ΔLΩp
ΔL=IΔΩ
Efficiency falls rapidly
with periseparation
Alexander & Kumar 2001
Tidal spin-up
Spin ΔΩ reaches
0.1-0.3 of Ωb, star’s
breakup speed
(corresponds to
440 km/s, or a
period of ~3 hours
for the Sun)
Alexander & Kumar 2001
Stellar rotation => Coronal activity
Coronal activity reaches saturation at Rossby number R0~0.1,
i.e., at Ω ~ 0.1 Ωb
Guedel 2004
Magnetic braking
Guedel 2004
Saturation regime lasts for tmb~ 100 Myr
Afterwards Ω ~ t-1/2 (Skumanich 1972)
Model
 r 

*  370
 0.05 pc 
1 / 2
km / s
s bRSun / vp  if Rp  Rs
  
if Rp  Rs
0
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
R=1.5’’
Chandra, 2-8 keV, ~ 620 ksec
Flares are filtered out
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 A1.6
X-ray spectrum: data
6.4 keV line?
X-ray spectrum: produced by a few 103 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. 2007
oDuration: ~ 2 hours
oPeak luminosity: 1.3 1033 erg/s in 0.8-10 keV and 8 1032 erg/s in 10-200 keV
This is ~40% of the binary’s bolometric luminosity (~5 1033 erg/s)
oHard spectrum and significant 6.4 keV line
Superflare of dMe star EV Lac on Apr. 25, 2008
Osten et al. 2010
oDuration: < 1 hour
oPeak luminosity: 1.6 1032 erg/s in 0.3-100 keV
This is 3.1 times the star’s bolometric luminosity (~5 1031 erg/s)
oHard 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 200-300 eV, as during
the superflare of dMe EV Lac
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 1st year of operation (Tsuboi et al. 2011)
-
Lx =
5+4-2 ×1033 erg/s
1.5-10 keV
Lx = 4±1 ×1033 erg/s
Выводы
Скоплением закрученных маломассивных звезд можно объяснить
основные свойства ”спокойного” рентгеновского излучения Sgr A*
Требуется несколько 104 звезд, из которых несколько 103 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