We construct a complete, hard X-ray flux-limited sample of intermediate polars (IPs) from the Swift-BAT 70-month survey, by imposing selection cuts in flux and Galactic latitude ($F_X > 2.5 times 10^{-11},mathrm{erg,cm^{-2}s^{-1}}$ at 14--195~keV, an
d $|b|>5^circ$). We then use it to estimate the space density ($rho$) of IPs. Assuming that this sample of 15 long-period systems is representative of the intrinsic IP population, the space density of long-period IPs is $1^{+1}_{-0.5} times 10^{-7},mathrm{pc^{-3}}$. The Swift-BAT data also allow us to place upper limits on the size of a hypothetical population of faint IPs that is not included in the flux-limited sample. While most IPs detected by BAT have 14--195~keV luminosities of $sim 10^{33} {rm erg s^{-1}}$, there is evidence of a fainter population at $L_X sim 10^{31} {rm erg s^{-1}}$. We find that a population of IPs with this luminosity may have a space density as large as $5times 10^{-6},mathrm{pc^{-3}}$. Furthermore, these low-luminosity IPs, despite appearing rare in observed samples, are probably at least as intrinsically common as the brighter systems that are better represented in the known IP sample.
The recurrent nova T Pyx underwent its sixth historical outburst in 2011, and became the subject of an intensive multi-wavelength observational campaign. We analyze data from the Swift and Suzaku satellites to produce a detailed X-ray light curve aug
mented by epochs of spectral information. X-ray observations yield mostly non-detections in the first four months of outburst, but both a super-soft and hard X-ray component rise rapidly after Day 115. The super-soft X-ray component, attributable to the photosphere of the nuclear-burning white dwarf, is relatively cool (~45 eV) and implies that the white dwarf in T Pyx is significantly below the Chandrasekhar mass (~1 M_sun). The late turn-on time of the super-soft component yields a large nova ejecta mass (>~10^-5 M_sun), consistent with estimates at other wavelengths. The hard X-ray component is well fit by a ~1 keV thermal plasma, and is attributed to shocks internal to the 2011 nova ejecta. The presence of a strong oxygen line in this thermal plasma on Day 194 requires a significantly super-solar abundance of oxygen and implies that the ejecta are polluted by white dwarf material. The X-ray light curve can be explained by a dual-phase ejection, with a significant delay between the first and second ejection phases, and the second ejection finally released two months after outburst. A delayed ejection is consistent with optical and radio observations of T Pyx, but the physical mechanism producing such a delay remains a mystery.
The short interval between successive outbursts of RS Oph strongly suggests that it has a high mass white dwarf accreting at a high rate. This, in turn, suggests the possibility of prominent X-ray emission from RS Oph in quiescence. However, archival
quiescent X-ray observations of RS Oph show it to be a modest soft X-ray source but not a strong 2-10 keV X-ray source. In this aspect, RS Oph differs markedly from T CrB. We speculate on the possible mechanisms that could significantly suppress the 2-10 keV X-ray emission in RS Oph.
We present optical and X-ray time-series photometry of EI UMa that reveal modulation at 746 and 770 s, which we interpret as the white dwarf spin and spin-orbit sidebands. These detections, combined with previous X-ray studies, establish EI UMa as an
intermediate polar. We estimate the mass accretion rate to be ~ 3.6 x 10^{17} g s^{-1}, which is close to, and likely greater than, the critical rate above which dwarf nova instabilities are suppressed. We also estimate the white dwarf to have a large magnetic moment mu > (3.4 +/- 0.2) x 10^{33} G cm^3. The high mass accretion rate and magnetic moment imply the existence of an accretion ring rather than a disk, and along with the relatively long orbital period, these suggest that EI UMa is a rare example of a pre-polar cataclysmic variable.
We present Suzaku X-ray observations of the recurrent nova T CrB in quiescence. T CrB is the first recurrent nova to be detected in the hard-X-ray band (E ~ 40.0 keV) during quiescence. The X-ray spectrum is consistent with cooling-flow emission eman
ating from an optically thin region in the boundary layer of an accretion disk around the white dwarf. The detection of strong stochastic flux variations in the light curve supports the interpretation of the hard X-ray emission as emanating from a boundary layer.
We observed with Suzaku the symbiotic star SS73 17, motivated by the discovery by the INTEGRAL satellite and the Swift BAT survey that it emits hard X-rays. Our observations showed a highly-absorbed X-ray spectrum with NH > 10^23 cm-2, equivalent to
A_V > 26, although the source has B magnitude 11.3 and is also bright in UV. The source also shows strong, narrow iron lines including fluorescent Fe K as well as Fe xxv and Fe xxvi. The X-ray spectrum can be fit with a thermal model including an absorption component that partially covers the source. Most of the equivalent width of the iron fluorescent line in this model can be explained as a combination of reprocessing in a dense absorber plus reflection off a white dwarf surface, but it is likely that the continuum is partially seen in reflection as well. Unlike other symbiotic systems that show hard X-ray emission (CH Cyg, RT Cru, T CrB, GX1+4), SS73 17 is not known to have shown nova-like optical variability, X-ray flashes, or pulsations, and has always shown faint soft X-ray emission. As a result, although it is likely a white dwarf, the nature of the compact object in SS73 17 is still uncertain. SS73 17 is probably an extreme example of the recently discovered and relatively small class of hard X-ray emitting symbiotic systems.
