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"The scaling of X-ray variability with luminosity in ULX sources"

Dr. Omaira Gonzalez-Martin
Physics department, University of Crete, Greece
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When: Wednesday, 12th of May of 2010 at 2:00 pm
Where: 2nd Floor Seminar Room

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Abstract:
  
ULXs could be the missed population of accreting objects with black-hole (BH) 
masses between Galactic BHs (GBHs) and supermassive BH (SMBHs): the intermediate 
mass BHs (IMBHs). However, their true nature is still unclear. We have investigated 
the relationship between X-ray variability amplitude and luminosity for a sample of 
14 bright ULXs with XMM-Newton/EPIC data (Gonzalez-Martin et al.2010 to be submitted). 
We determined the normalized excess variance in the 2-10keV light curves together 
with the intrinsic luminosity L(2-10keV). We have compared this relationship to that 
found for AGN. Our main results are: (1) A significant evidence that the variability 
amplitude in ULXs decreases with increasing the L(2-10keV) and (2) for a given 
luminosity, the variability amplitude of ULXs is significantly smaller than that 
expected from a simple extrapolation of the AGN correlation to lower luminosities. 
We tested in which conditions ULXs could be`scale-down' version of AGN (i.e. same 
shape on the power density spectra (PSD), same efficiency of the mass-to-energy 
conversion, and same X-ray-to-bolometric luminosity conversion). We concluded that 
they could be consistent with AGN if its PSD show a second break and BH masses and 
accretion rates range M(BH)=5000-10000M(Solar) and m(Edd)=0.05-0.2, respectively.
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