Hard X-ray View of Ultraluminous X-ray Sources and Cataclysmic Variables with NuSTAR

The Nuclear Spectroscopic Telescope Array (NuSTAR), with its unprecedented combination of sensitivity and, spatial and spectral resolution, turned out to be an ideal observatory to probe the hard X-ray nature of Ultraluminous X-ray sources (ULXs) and Cataclysmic Variables (CVs) as well as other X-ray binaries. A true nature of compact object in ULXs and what powers them, is subject of active debate even after almost three decades of their discovery. Prior to NuSTAR launch, the study of ULXs were confined below 10 keV. NuSTAR opened up the hard X-ray window and allowed us to look at these sources above 10 keV for the first time. Broad-band X-ray study of a sample of bright ULXs with XMM-Newton and NuSTAR revealed that all the sources in the sample show a clear cutoff at or above ~10 keV, strongly suggesting different physical origin for their X-ray emission compared to Galactic black hole X-ray binaries and supporting exotic super-Eddington modes of accretion. The data favors interpreting most of these ULXs as stellar mass BHs accreting close to or above Eddington limit. CVs, on the other hand, contain an accreting white dwarf as a compact object. NuSTAR provided a first direct detection of reflection component in three magnetic CVs, namely, V709 Cas, Ny Lup and V1223 Sgr. In the past, there was only an indirect evidence for the existence of reflection component based on presence of Fe fluorescence line. NuSTAR observations provided an important piece of information required to obtain a complete picture of magnetic CVs. During this talk, I will discuss recent interesting science results on ULXs and CVs using hard X-ray data from NuSTAR and simultaneous soft X-ray coverage with XMM-Newton and Suzaku.The Nuclear Spectroscopic Telescope Array (NuSTAR), with its unprecedented combination of sensitivity and, spatial and spectral resolution, turned out to be an ideal observatory to probe the hard X-ray nature of Ultraluminous X-ray sources (ULXs) and Cataclysmic Variables (CVs) as well as other X-ray binaries. A true nature of compact object in ULXs and what powers them, is subject of active debate even after almost three decades of their discovery. Prior to NuSTAR launch, the study of ULXs were confined below 10 keV. NuSTAR opened up the hard X-ray window and allowed us to look at these sources above 10 keV for the first time. Broad-band X-ray study of a sample of bright ULXs with XMM-Newton and NuSTAR revealed that all the sources in the sample show a clear cutoff at or above ~10 keV, strongly suggesting different physical origin for their X-ray emission compared to Galactic black hole X-ray binaries and supporting exotic super-Eddington modes of accretion. The data favors interpreting most of these ULXs as stellar mass BHs accreting close to or above Eddington limit. CVs, on the other hand, contain an accreting white dwarf as a compact object. NuSTAR provided a first direct detection of reflection component in three magnetic CVs, namely, V709 Cas, Ny Lup and V1223 Sgr. In the past, there was only an indirect evidence for the existence of reflection component based on presence of Fe fluorescence line. NuSTAR observations provided an important piece of information required to obtain a complete picture of magnetic CVs. During this talk, I will discuss recent interesting science results on ULXs and CVs using hard X-ray data from NuSTAR and simultaneous soft X-ray coverage with XMM-Newton and Suzaku.