Abstract
We present the results of the first large program of broadband ULX observations with NuSTAR , XMM –Newton and Suzaku , yielding high-quality spectra and timing measurements from 0.3–30 keV in 6 ULXs, providing powerful information for understanding the accretion modes and nature of the central BHs. In particular, we find that all ULXs in our sample have a clear cutoff above 10 keV. This cutoff is less pronounced than expected by Comptonization from a cold, thick corona. We confirm the presence of a soft excess at low energies in the brightest ULXs, with temperatures below ~ 0.5 keV. We make an estimates on the masses of several ULXs based on spectral variability and model fitting.
Highlights
Without constraint on the high-energy part of the spectrum, this disk-like component is in turn hard to constrain
Since the launch of the satellite, we have observed a sample of luminous (Lx ∼ 1040 erg s−1), close-by (d 10 Mpc) and hard Ultraluminous X-ray Sources (ULXs) simultaneously with NuSTAR and Suzaku or XMM–Newton, producing the first ULX spectra extending over the range 0.3 – 30 keV
Our campaign of observations with NuSTAR plus a satellite with coverage below 10 keV (XMM– Newton or Suzaku) of the high-luminosity ULXs NGC 1313 X-1 and X-2, IC 342 X-1 and X-2, Holmberg IX X-1 and a serendipitous source in the outskirts of the Circinus Galaxy, Circinus ULX5 [15] in the 0.3–30 keV have revealed a number of interesting results, that we briefly summarize here
Summary
Without constraint on the high-energy part of the spectrum, this disk-like component is in turn hard to constrain. Since the launch of the satellite, we have observed a sample of luminous (Lx ∼ 1040 erg s−1), close-by (d 10 Mpc) and hard (showing X-ray power law photon index Γ 2 below 10 keV) ULXs simultaneously with NuSTAR and Suzaku or XMM–Newton, producing the first ULX spectra extending over the range 0.3 – 30 keV.
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