Do flares in Sagittarius A* reflect the last stage of tidal capture? Andrej Čadež 1, Massimo Calvani 2, Andreja Gomboc 1, Uroš Kostić 1 1 University of Ljubljana, Slovenia 2 Osservatorio Astronomico di Padova, Italy

X-ray, XMM Newton, October 2002: D. Porquet et al. IR flare June 2003,VLT-NACO,: R. Genzel, R. Schodel, T. Ott et al. Hard X-ray, Chandra,October 2000: F. K. Baganoff et al. X-ray, XMM Newton, Sept. 2001: A. Goldwurm et al.

The time scale puzzle  1. The rise and switch off rates of all flares are very similar  2. The rise-switch-off time scale is about 900 s  3. The light curve is similar in all wavebands  Radiation diffusion time of a light source (t g ) : Let the source be a homogeneous sphere made of the most transparent material available – hydrogen at a high enough temperature -, so that its opacity is due only to Thomson scattering (k = 0.4cm 2 /g), and assume that photons are loosing no energy when diffusively scattering to the surface. Then: t g = (1/c) (k M/R), or Let the source be a homogeneous sphere made of the most transparent material available – hydrogen at a high enough temperature -, so that its opacity is due only to Thomson scattering (k = 0.4cm 2 /g), and assume that photons are loosing no energy when diffusively scattering to the surface. Then: t g = (1/c) (k M/R), or t g = 4400 s (M/M Moon ) 2/3 r -1/3 t g = 4400 s (M/M Moon ) 2/3 r -1/3

Infalling point particle: obseved intensity in the orbital plane as a funtion of time and longitude of observer

IR flare June 2003 model Ingredients: a small body, like a comet or asteroid, heated on the way toward complete tidal disruption on a parabolic orbit with l orbital = 4 m.M bh. c mass of the black hole: solar mass heating time scale t = 2300 s effective length of tidal tail L t =circumference of last stable orbit assumed inclination 90 0 observer longitude ~60 0

Scenario  1. stars moving close to the Galactic center black hole are gradually beeing stripped off their comets, asteroids, planets. In the process the remaining stellar system is increasing its internal angular momentum at the expence of orbital angular momentum, making the stellar system orbit more and more elliptical

 2. A stripped asteroid is likely to move on a highly eccentric orbit, reaching deep into the potential well of the black hole. Each periastron passage produces an increasing tidal wave and reduces the orbital angular momentum and the orbital energy in such a way that the orbit is becoming more and more eccentric (parabolic) with the angular momentum slowly approaching the angular momentum of tidal capture l=4 m M BH c. The last tidal kick, that occurs just before capture, releases ~10% mc 2 of tidal energy to the asteroid, which is more than enough to heat it to X-ray temperatures.

3. Time scales and energetics:  The radiation diffusion time for an asteroid, heated to X-ray tempertures is t g = 4400 s (m/M Moon ) 2/3 r -1/3 = 240 s t g = 4400 s (m/M Moon ) 2/3 r -1/3 = 240 s for m=10 21 g and r=1g/cm 3 for m=10 21 g and r=1g/cm 3  Energy release: up to ΔE~0.1mc 2 = erg  Capture rate: stellar capture rate × no. of asteroids per star: stellar capture rate × no. of asteroids per star: ~(10 -4 y -1 ) × 10 5 = 10 y -1 ~(10 -4 y -1 ) × 10 5 = 10 y -1

Conclusion  The light curve of tidal capture of an asteroid size body is modeled as the light curve of an almost point particle beeing captured and heated by tides on a critical orbit by the massive black hole.  The time scale fits the known mass of the central galactic black hole,  the expected energy release is compatible with observation and  the observed frequency of events is consistent with the expected capture rate.  This simple model fits the observed light curve of the IR flare surprisingly well.