Accreting flows at (sub) millimeter wavelengths P. Ivanov P.N. Lebedev Physical Institute

Radiatively inefficient accreting flows onto supermassive black holes  Perhaps the most studied example is the source in our own Galaxy - Sagittarius A*  Basic parameters: distance D ~ 8kpc, mass M ~ 4*10 6 M ☼, bolometric luminosity ~ 3*10 36 ers/s, gravitational radius r g =2GM/c 2 ~ cm,  angular size ~ r g /D ~10μas (observed structures are of order of this size)  Exhibits variability at time scales of minutes to hours in NIR, X-rays and submillimeter bands

Spectral energy distribution of the emission from Sgr A*. This plot shows the extinction and absorption corrected luminosities. All error bars are ±1 sigma and include statistical and systematic errors. Black triangles denote the radio spectrum of Sgr A*. Open grey circles mark various infrared upper limits from the literature. The three X-ray data ranges are (from bottom to top) the quiescent state as determined with Chandra (black; Baganoff et al., 2003), the autumn 2000 Chandra flare (red; Baganoff et al., 2000), and the autumn 2002 flare observed by XMM (light blue; Porquet et al., 2003). Open red squares with crosses mark the de-reddened peak emission (minus quiescent emission) of the four NIR flares. Open blue circles mark the de-reddened H, KS, and L' luminosities of the quiescent state, derived from the local background subtracted flux density of the point source at the position at Sgr A*, thus eliminating the contribution from extended, diffuse light due to the stellar cusp around Sgr A*.

Variability

Physical Conditions  The small value of luminosity of Sgr A * implies that either the accretion rate in the innermost region of the system is rather small (dM/dt ~ M ☼ /yr) or efficiency of conversion of gravitational energy to radiation is quite small ~ The former case is preferred by numerical modeling. In this case the accreting flow is geometrically  thick with h/r ~ 0.5, hot (T p ~ K, T e ~ K), optically thin, with ratio of magnetic field energy to the thermal energy of order of The density profile is rather “flat” n ~ r -3/2+p, with p= Close to black hole n ~ cm -3. The energy conversion factor is of order of for this case.

Modeling of spectra

 Possible sources of variability in the sub mm range could be: 1) intrinsic variability due to MHD turbulence  2) reconnection events/hot spots in the disc 3) excitation  of different modes of disc’s pulsations (e.g. so-called “corrugation” or “twisted” modes). These possibilities are  exploited in recent numerical models of Sgr A *. However, to disentangle them more observations in different wavebands, longer sets of data and more resolution are required. Polarization measurements are also important.  The last three possibilities are provided by  Millimetron.

Numerical models

Dexter et al, 2010

Recent simulations

Other galaxies

Sensitivity and resolution requirements  In the interferometer mode Millimetron will have sensitivity of order of Jy at λ~ 0.3mm. The corresponding minimal flux F min =νF ν ~ ergs/cm 2. From the constraint that the received flux should be larger than F min we get L > D 1 2 ergs/s, where L is a typical source luminosity in the submillimeter waveband, D is the distance from the source and D 1 =D/1Mpc.  As a typical interferometer base I take B=1.5*10 6 km. The corresponding resolution limit θ crit ~ 2* Rad at λ~ 0.3mm. In order to get something really interesting scales smaller than or of the order of R g should be resolved. Accordingly, we should have θ g = R g /D > θ crit. From this condition  one obtains: M 8 > D 1, where M 8 =M/(10 8 M ☼ ).  It turns out that assuming that the submillimeter luminosity is of the order  of a typical X-ray luminosity both conditions are fulfilled for  almost all nearby supermassive black holes. 

R= θ crit /θ g Jy, squares – Jy, and diamonds Jy Circles correspond to detection threshold Jy, squares – Jy, and diamonds Jy Additionally, potential intermediate mass black holes within our Galaxy may have ~ 1. For example, for GC M15 (D ~ 10kpc and M ~ 4*10 3 M ☼ ), IMBH may have R ~ 1. For example, for GC M15 (D ~ 10kpc and M ~ 4*10 3 M ☼ ), IMBH may have R ~ 2.

 The record breakers : Sgr A *, R=2*10 -3, M87, R=5*10 -3, NGC 4649, R=8*10 -3, NGC 4594 (Sombrero), R=10 -2, IC 1459, R=1.16*10 -2, NGC 5128 (Cen A), R=1.75*10 -2, NGC 4472 (M49), R=2*10 -2.

Non-active galaxies exhibiting x-ray flares on time-scale of a few years (tidal disruption event candidates) potential candidates: NGC 5905 (eg. Komossa and Bade 1999), D~ 40 Mpc, M ~ M ☼ and Bade 1999), D~ 40 Mpc, M ~ M ☼ R ~ 0.4-4, RXJ A (eg. Komossa et al, 2004),D~200 Mpc, M ~ 10 8 M ☼ and, accordingly, R ~ 2. It would be VERY interesting to look for sub-mm radiation from such galaxies using Millimetron.

Swift J /GRB A  It was as an extra long GRB coming from a distance of order of 3.8Gpc. It is interpreted as emission of a jet formed after a tidal disruption event. The source emits in radio and microwave bands, see the Fig. The MILLIMETRON could probe scales order of pc at such distances! This could help to confirm or refute the tidal disruption hypothesis for sources of such type on a quite solid basis.

CONCLUSIONS 1) Millimetron is able to resolve scales of order of gravitational radius for almost all nearby SMBH (D < 50Mpc). Also, its sensitivity is sufficient for this task. For many extragalactic objects Millimetron is able to resolve structures several order of magnitude smaller than r g. 2) Practically all nearby SMBH are underluminous (L << L edd ). In this regime the radiatively inefficient accretion is likely to occur. If so, the flow may be expected to be optically thin in the (sub) mm waveband, close to black hole. Thus, Millimetron may be able to see black holes themselves, and probe the structure of the flow in the very vicinity of BH. This may enable to determine both mass and angular momentum of BH’s and parameters of the flow: its geometrical structure (including the possibility of jet/outflow), orientation, physical conditions in the flow as well as to clarify the origin of time variability of such accreting flows. 3) It will be quite interesting to have a possibility to measure flux variability from the sources at short time scales, from minutes to hours. 4) Millimetron may be quite useful for observing many other disc-like structures within our own Galaxy, such as e.g. the debris discs. within our own Galaxy, such as e.g. the debris discs.