Black Hole Mass of the Ultraluminous X-ray source M82 X-1

We report the first clear evidence for the simultaneous presence of a low frequency break and a QPO in the fluctuation power spectrum of a well known ultraluminous X-ray source (ULX) in M82 using long XMM-Newton observations. The break occurs at a frequency of 34.2_{-3}^{+6}mHz. The QPO has a centroid at 114.3\pm1.5mHz, a coherence Q~3.5 and an amplitude (rms) of 19% in the 2-10keV band. The power spectrum is approximately flat below the break frequency and then falls off above the break frequency as a power law with the QPO superimposed. This form of the power spectrum is characteristic of the Galactic X-ray binaries (XRBs) in their high or intermediate states. M82 X-1 was likely in an intermediate state during the observation. The EPIC PN spectrum is well described by a model comprising an absorbed power-law (Gamma~2) and an iron line at ~6.6keV with a width sigma~0.2keV and an equivalent width of ~180eV. Using the well established correlations between the power and energy spectral parameters for XRBs, we estimate a black hole mass for M82 X-1 in the range of 25-520Msun including systematic errors that arise due to the uncertainty in the calibration of the photon spectral index versus QPO frequency relation.Comment: Final version, accepted for publication in ApJ Letter

On the origin of the featureless soft X-ray excess emission from the Seyfert 1 galaxy ESO~198--G24

We present medium and high resolution X-ray spectral study of a Seyfert 1 galaxy ESO~198--G24 using a long (122 ks) XMM-Newton observation performed in February 2006. The source has a prominent featureless soft X-ray excess below 2\kev. This makes the source well suited to investigate the origin of the soft excess. Two physical models -- blurred reflection, and optically thick thermal Comptonization in a warm plasma, describe the soft-excess equally well resulting in similar fits in the 0.3-10\kev band. These models also yield similar fits to the broad-band UV (Optical Monitor) and X-ray data. XMM-Newton observations performed in 2000, 2001 and 2006 on this source show flux variability. From 2001 to 2006, the UV flux increased by $\sim23\%$ while the 2-10\kev X-ray flux as well as the soft-excess flux decreased by ~ 20. This observation can be described in the blurred reflection scenario by a truncated accretion disk whose inner-most radius had come closer to the blackhole. We find that the best-fit inner radius of the accretion disk decreases from R_{in}=4.93_{-1.10}^{+1.12}R_G to R_{in}<2.5R_G from 2001 to 2006. This leads to an increase in the UV flux and compressing the corona, leading to reduction of the powerlaw flux and therefore the soft-excess. The blurred reflection model seems to better describe the soft-excess for this source.Comment: Accepted for publication in the MNRA