On the connection between ULX and IMBHs

Overview of ULX:➢ Ultraluminous X-ray sources (ULX) are extra-galactic,off-center X-ray sources with X-ray luminosities inexcess of 2⋅10 39erg/sec.➢ If the radiation is emitted isotropically, they wouldexceed the Eddington luminosity of stellar-mass blackholes by many orders of magnitude.➢ They are therefore candidates for intermediate-massblack holes (IMBHs).➢ In addition, there is an apparent connection betweenULX and massive star formation regions and (possibly)young star clusters.

Formation of IMBHs in Young Clusters➢ Observations indicate that theremight be a connection between ULXand star clusters.➢ Matsumoto et al. (2001) for examplefound a bright X-ray source at thecenter of the starburst galaxy M82with an Eddington luminositycorresponding to a black hole ofseveral hundred solar masses.➢ Optical follow-up observationsshowed that the position of thissource coincides with that of ayoung luminous star cluster.

Formation of IMBHs in Young Clusters➢ McCrady et al. (2003) used the HSTand Keck telescopes to determine thedensity profile and total masses for anumber of young star clusters in M82.➢ MGG-11 was the most concentrated(half-light radius 1.2 pc) and secondheaviest cluster in their sample (M=3.5⋅10 5 M Sun).➢ In addition it appears to have a top-heavy IMF.

Formation of IMBHs in Young Clusters➢ The runaway merging leads tothe formation of a single objectof more than1000 M Sunwithin afew Myrs.➢ This object could form an inter-mediate-mass black hole whichcreates the X-ray radiation.➢ Simulations of other clustersshow that only MGG-11 isconcentrated enough to undergorunaway merging, in agreementwith the fact that only MGG-11contains a ULX.(from Portegies Zwart et al. 2004)

One question remaining is how to create the ULX ?

How to create a ULX out of an IMBH ?Madhusudhan et al. (2005):➢ Modeled the accretion of gas onto anIMBH from a captured donor star.➢ They found that in order to explainULX luminosities, one needs massivestars in orbits with 6-30 R Sunaroundthe IMBH.

How to create a ULX out of an IMBH ?Hopman et al. (2003):➢ Performed semi-analytic calculations toinvestigate the tidal capture of stars byan IMBH.➢ Found that 10% of IMBHs in MGG-11type clusters tidally capture a star.Blecha et al. (2005):➢ Studied encounters of low-mass IMBHswith binaries in a cluster core.➢ Found a high probability for the captureof stars but only a very lowprobability that these stars will becomeULX sources.

Tidal heating:➢A star passing inside=r M BHtM S1 /3R Stidally disrupted by the black hole.of a black hole is➢Outside this radius, the star is tidally deformed by theblack hole. The energy needed for the deformation istaken out of the orbital energy.➢As a result, the orbit shrinks and the star circularisesnear the black hole.➢In a star cluster, the inspiraling star has encounters withother stars which can scatter it away from the inspiralorbit.➢In addition, the inspiral must be slow since otherwise thestar is heated and disrupted during the inspiral.

Our runs (Baumgardt et al. 2005, astro-ph/0511752):➢ We have performed N-body simulations of the formation ofan IMBH in the center of MGG-11 and the later evolution ofthe cluster up to an age of 12 Myrs.➢ Our simulations included stellar evolution, tidal disruptionof stars and tidal heating of stars which pass near to anIMBH.➢ Our simulations also contained perturbations tocircularised stars from passing perturbers.Results:1) We found significant black hole growth. Depending on themass-radius relation adopted for the runaway star, IMBHswith masses in the range 1000 to 4000 were formed.M Sun

2) A stellar cusp forms around the IMBH and due to masssegregation,massive stars are enhanced in this cusp:

3) Within the 9 Myrs of our simulations, every IMBH disruptedon average 25 stars and experienced 3 successful inspirals.4) Due to perturbations from cusp stars, successful inspiralsoriginate predominantly from low-a orbits.

5) For about 40% all inspiraled stars, the maximum energyrate pumped into them was less than their Eddingtonluminosity, i.e. the stars are likely to survive the inspiral.

6) The mass transfer from the tidally circularised stars ontothe IMBH is enough to power a ULX for several Myrs onaverage. Our model therefore provides an explanation forthe ULX in M82 and strengthens the connection betweenULX and IMBHs.