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Register a new userHard X-ray and Infrared Emission from Apparently Single White Dwarfs
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Y.-H. Chu
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The photospheric emission of a white dwarf (WD) is not expected to be detectable in hard X-rays or the mid-IR. Hard X-ray (~1 keV) emission associated with a WD is usually attributed to a binary companion; however, emission at 1 keV has been detected from three WDs without companions: KPD 0005+5106, PG 1159, and WD 2226-210. The origin of their hard X-ray emission is unknown, although it has been suggested that WD 2226-210 has a late-type companion whose coronal activity is responsible for the hard X-rays. Recent Spitzer observations of WD 2226-210 revealed mid-IR excess emission indicative of the existence of a dust disk. It now becomes much less clear whether WD 2226-210s hard X-ray emission originates from the corona of a late-type companion or from the accretion of the disk material. High-quality X-ray observations and mid-IR observations of KPD 0005+5106 and PG 1159 are needed to help us understand the origin of their hard X-ray emission.
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KPD 0005+5106, with an effective temperature of $simeq$200,000 K, is one of the hottest white dwarfs (WDs). ROSAT unexpectedly detected hard ($sim$1 keV) X-rays from this apparently single WD. We have obtained Chandra observations that confirm the spatial coincidence of this hard X-ray source with KPD 0005+5106. We have also obtained XMM-Newton observations of KPD 0005+5106, as well as PG 1159$-$035 and WD 0121$-$756, which are also apparently single and whose hard X-rays were detected by ROSAT at 3$sigma$-4$sigma$ levels. The XMM-Newton spectra of the three WDs show remarkably similar shapes that can be fitted by models including a blackbody component for the stellar photospheric emission, a thermal plasma emission component, and a power-law component. Their X-ray luminosities in the $0.6-3.0$ keV band range from $4times10^{29}$ to $4times10^{30}$ erg~s$^{-1}$. The XMM-Newton EPIC-pn soft-band ($0.3-0.5$ keV) lightcurve of KPD 0005+5106 is essentially constant, but the hard-band ($0.6-3.0$ keV) lightcurve shows periodic variations. An analysis of the generalized Lomb-Scargle periodograms for the XMM-Newton and Chandra hard-band lightcurves finds a convincing modulation (false alarm probability of 0.41%) with a period of 4.7$pm$0.3 hr. Assuming that this period corresponds to a binary orbital period, the Roche radii of three viable types of companion have been calculated: M9V star, T brown dwarf, and Jupiter-like planet. Only the planet has a size larger than its Roche radius, although the M9V star and T brown dwarf may be heated by the WD and inflate past the Roche radius. Thus, all three types of companion may be donors to fuel accretion-powered hard X-ray emission.
Interacting binaries in which a white dwarf accretes material from a companion - cataclysmic variables (CVs) in which the mass donor is a Roche-lobe filling star on or near the main sequence, and symbiotic stars in which the mass donor is a late type giant - are relatively commonplace. They display a wide range of behaviors in the optical, X-rays, and other wavelengths, which still often baffle observers and theorists alike. Here I review the existing body of research on X-ray emissions from these objects for the benefits of both experts and newcomers to the field. I provide introductions to the past and current X-ray observatories, the types of known X-ray emissions from these objects, and the data analysis techniques relevant to this field. I then summarize of our knowledge regarding the X-ray emissions from magnetic CVs, non-magnetic CVs and symbiotic stars, and novae in eruption. I also discuss space density and the X-ray luminosity functions of these binaries and their contribution to the integrated X-ray emission from the Galaxy. I then discuss open questions and future prospects.
Observations made with the Rossi X-ray Timing Explorer (RXTE) Proportional Counter Array (PCA) to constrain the hard X-ray emission in the NGC 5044 group are reported here. Modeling a combined PCA and ROSAT position sensitive proportional counter (PSPC) spectrum with a 0.5 - 15 keV energy range shows excess hard emission above 4 keV. Addition of a powerlaw component with spectral index of 2.6 - 2.8 and luminosity of 2.6 x10^42 ergs/s within 700 kpc in the observed energy band removes these residuals. Thus, there is a detection of a significant non-thermal component that is 32% of the total X-ray emission. Point source emission makes up at most 14% of the non-thermal emission from the NGC 5044 group. Therefore, the diffuse, point source subtracted, non-thermal component is 2.2 - 3.0x10^42 ergs/s . The cosmic-ray electron energy density is 3.6 x10^[-12] ergs cm-3 and the average magnetic field is 0.034 muGauss in the largest radio emitting region. The ratio of cosmic-ray electron energy density to magnetic field energy density, ~2.5x10^4, is significantly out of equipartition and is therefore atypical of radio lobes. In addition, the groups small size and low non-thermal energy density strongly contradicts the size-energy relationship found for radio lobes. Thus, it is unlikely to the related to the active galaxy and is most likely a relic of the merger. The energy in cosmic-rays and magnetic field is consistent with simulations of cosmic-ray acceleration by merger shocks.
After the positive detection by BeppoSAX of hard X-ray radiation up to ~80 keV in the Coma cluster spectrum, we present evidence for nonthermal emission from A2256 in excess of thermal emission at a 4.6sigma confidence level. In addition to this power law component, a second nonthermal component already detected by ASCA could be present in the X-ray spectrum of the cluster, not surprisingly given the complex radio morphology of the cluster central region. The spectral index of the hard tail detected by the PDS onboard BeppoSAX is marginally consistent with that expected by the inverse Compton model. A value of ~0.05 microG is derived for the intracluster magnetic field of the extended radio emission in the northern regions of the cluster, while a higher value of ~0.5 microG could be present in the central radio halo, likely related to the hard tail detected by ASCA.
We report the results of a long BeppoSAX observation of Abell 3667, one of the most spectacular galaxy cluster in the southern sky. A clear detection of hard X-ray radiation up to ~ 35 keV is reported, while a hard excess above the thermal gas emission is present at a marginal level that should be considered as an upper limit to the presence of nonthermal radiation. The strong hard excesses reported by BeppoSAX in Coma and A2256 and the only marginal detection of nonthermal emission in A3667 can be explained in the framework of the inverse Compton model. We argue that the nonthermal X-ray detections in the PDS energy range are related to the radio index structure of halos and relics present in the observed clusters of galaxie.
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