15136851
Broad-band X-ray measurements of the black hole candidate XTE J1908+094
XTE J1908+094 is an X-ray transient that went into outburst in February 2002. After two months it reached a 2- 250 keV peak fl ux of 1t o 2 10 8 erg cm 2 s 1 . Circumstantial evidence points to an accreting galactic black hole as the origin of the X-radiation: pulsations nor thermonuclear flashes were detected that would identify a neutron star and the spectrum was unusually hard for a neutron star at the outburst onset. We report on BeppoSAX and RXTE All Sky Monitor observations of the broad-band spectrum of XTE J1908+094. The spectrum is consistent with a model consisting of a Comptonization component by a40 keV plasma, a multicolor accretion disk blackbody component with a temperature just below 1 keV and a broad emission line at about 6 keV. The spectrum is heavily absorbed by cold interstellar matter with an equivalent hydrogen column density of 2:5 10 22 cm 2 , which makes it dicult to study the black body component in detail. The black body component exhibits strong evolution about 6 weeks into the outburst. Two weeks later this is followed by a swift decay of the power law component. The broadness of the 6 keV feature may be due to relativistic broadening or Compton scattering of a narrow Fe-K line.
14970990
The X-ray emission from Nova V382 Velorum – I. The hard component observed with BeppoSAX
We present BeppoSAX observations of Nova Velorum 1999 (V382 Vel), carried out in a broad X-ray band covering 0.1–300 keV only 15 d after the discovery and again after 6 months. The nova was detected at day 15 with the BeppoSAX instruments which measured a flux Fx≃1.8×10−11 erg cm−2 s−1 in the 0.1–10 keV range and a 2σ upper limit Fx<6.7×10−12 erg cm−2 s−1 in the 15–60 keV range. We attribute the emission to shocked nebular ejecta at a plasma temperature kT≃6 keV. At six months no bright component emerged in the 15–60 keV range, but a bright central supersoft X-ray source appeared. The hot nebular component previously detected had cooled to a plasma temperature kT<1 keV. There was strong intrinsic absorption of the ejecta in the first observation and not in the second, because the column density of neutral hydrogen decreased from N(H)≃1.7×1023 to N(H)≃1021 cm−2 (close to the interstellar value). The unabsorbed X-ray flux also decreased from Fx=4.3×10−11 to Fx≃10−12 erg cm−2 s−1.
18267251
A study of the dipping low mass X-ray binary X 1624-490 from the broadband BeppoSAX observation
We present results of a study of the luminous dipping low mass X-ray binary X 1624-490 made using BeppoSAX .A n interval of deep and rapidly varying dipping was included in the observation. The radial intensity profile of the source ob- tained using the MECS instruments revealed excesses in inten- sity above the instrument point spread function below 5k eV demonstrating the presence of a dust-scattering halo. From mod- elling of the radial profile in several energy bands, halo intensity fractions rising to 30% in the lowest band 2.5-3.5 keV were obtained. From these data, the optical depth to dust scatter- ing at 1 keV was found to be 2:4 0:4. The non-dip spectrum of X 1624-490 in the energy band 1-100 keV is shown to be well-described by the emission model consisting of point-like blackbody radiation assumed to be from the neutron star plus extended Comptonized emission from an ADC. The blackbody temperature was 1:31 0:07 keV and the Comptonized emis- sion had photon power law index 2:0 +0:5 0:7 and cut-off energy 12 keV. The spectra of several dip levels were shown to con- tain an unabsorbed component below 5 keV. Good fits to the dip spectra were obtained by allowing the Comptonized emission to be progressively covered by an extended absorber while the blackbody was rapidly absorbed and a constant halo component accounted for dust scattering into the line-of-sight. It is shown that the unabsorbed component consists of the uncovered part of the Comptonized emission plus a halo contribution which in deepest dipping dominates the spectrum below 4.5 keV. From the dip ingress time, we have derived a diameter of the extended Comptonized emission region of 5:3 0:8 10 10 cm, consis- tent with a hot, X-ray emitting corona extending to 50% of the accretion disk radius. The source luminosity for a distance of 15 kpc is 7:3 10 37 erg s 1 , an appreciable fraction of the Ed- dington limit making X 1624-490 the most luminous dipping LMXB. The half-height of the blackbody emitting region on the neutron star of 6:8 1:8 km agrees with the half-height of the radiatively supported inner accretion disk of 6:3 2:9 km, which together with similar agreement recently obtained for 13 other LMXB strongly supports the identification of the neutron star as the origin of the blackbody emission in LMXB. Finally,
6043968
The broad band spectral properties of galactic X-ray binary pulsars
Abstract BeppoSAX observed several galactic binary X-ray pulsars during the Science Verification Phase and in the first year of the regular program. The complex emission spectra of these sources are an ideal target for the BeppoSAX instrumentation, that can measure the emission spectra in an unprecedented broad energy band. Using this capability of BeppoSAX a detailed observational work can be done on the galactic X-ray pulsars. In particular the 0.1–200 keV energy band allows the shape of the continuum emission to be tightly constrained. A better determination of the underlying continuum allows an easier detection of features superimposed onto it, both at low energy (Fe K and L, Ne lines) and at high energies (cyclotron features). We report on the spectral properties of a sample of X-ray pulsars observed with BeppoSAX comparing the obtained results. Some ideas of common properties are also discussed and compared with our present understanding of the emission mechanisms and processes.
118471764
Variations in the dip properties of the low-mass X-ray binary XB 1254-690 observed with XMM-Newton and INTEGRAL
We have analysed data from five XMM-Newton observations of XB 1254-690, one of them simultaneous with INTEGRAL, to investigate the mechanism responsible for the highly variable dip durations and depths seen from this low-mass X-ray binary. Deep dips were present during two observations, shallow dips during one and no dips were detected during the remaining two observations. At high (1–4 s) time resolution “shallow dips” are seen to include a few very rapid, deep dips whilst the “deep” dips consist of many similar very rapid, deep fluctuations. The folded V -band Optical Monitor light curves obtained when the source was undergoing deep, shallow and no detectable dipping exhibit sinusoid-like variations with different amplitudes and phases. We fit EPIC spectra obtained from “persistent” or dip-free intervals with a model consisting of disc-blackbody and thermal Comptonisation components together with Gaussian emission features at 1 and 6.6 keV modified by absorption due to cold and photo-ionised material. None of the spectral parameters appears to be strongly correlated with the dip depth except for the temperature of the disc blackbody which is coolest ( kT  ~ 1.8 keV) when deep dips are present and warmest ( kT ~ 2.1 keV) when no dips are detectable. We propose that the changes in both disc temperature and optical modulation could be explained by the presence of a tilted accretion disc in the system. We provide a revised estimate of the orbital period of 0.16388875 ± 0.00000017 day.
6052935
BeppoSAX observation of the eclipsing dipping X-ray binary X 1658-298
Results of a 2000 August 12{13 BeppoSAX observation of the 7.1 hr eclipsing, dipping, bursting, transient, low-mass X-ray binary (LMXRB) X 1658-298 are presented. The spectrum outside of eclipses, dips and bursts can be modeled by the combination of a soft disk-blackbody and a harder Comptonized component with a small amount (1:3 10 21 atom cm 2 ) of low-energy absorption. In contrast, an RXTE observation 18 months earlier during the same outburst, measured an absorption of 5:0 10 22 atom cm 2 . Such a change is consistent with a thinning of the accretion disk as the outburst progresses. Structured residuals from the best-t spectral model are present which are tentatively identied with Ne-K/Fe-L and Fe-K shell emission. The spectral changes during dips are complex and may be modeled by a strong (3 10 23 atom cm 2 ) increase in absorption of the Comptonized component only, together with reductions in normalizations of both spectral components. This behavior is in contrast to the \complex continuum" model for X-ray dip sources, where the softer blackbody component rapidly suers strong absorption. It is however, similar to that found during recent XMM-Newton observations of the eclipsing, dipping, LMXRB EXO 0748-676. An updated orbital ephemeris is provided which does not require a quadratic term, in contrast to that of Wachter et al. (2000).
18828707
Production of Large Transverse Momentum W Bosons at the Tevatron
We discuss the production of W bosons at large transverse momentum in p pbar collisions at the Tevatron Run I and II. W boson hadroproduction is a process of relevance in testing the Standard Model and in estimates of backgrounds to new physics. The next-to-leading order cross section in the region of large transverse momentum is dominated by threshold soft-gluon corrections which arise from incomplete cancellations near partonic threshold between graphs with real emission and virtual graphs due to the limited phase space available for real gluon emission. In this contribution it is shown how, by including next-to-next-to-leading-order (NNLO) soft-gluon corrections, the transverse momentum distribution of the W at the Tevatron is modestly enhanced and the dependence on the factorization and renormalization scales is significantly reduced.
2038251
Time-dependent Ionization in Radiatively Cooling Gas
We present new computations of the equilibrium and nonequilibrium cooling efficiencies and ionization states for low-density radiatively cooling gas containing the elements H, He, C, N, O, Ne, Mg, Si, S, and Fe. We present results for gas temperatures between 10^4 and 10^8 K, assuming dust-free and optically thin conditions, and no external radiation. For nonequilibrium cooling we solve the coupled time-dependent ionization and energy loss equations for a radiating gas cooling from an initially hot, ≳5 × 10^6 K, equilibrium state, down to 10^4 K. We present results for heavy element compositions ranging from 10^(-3) to 2 times the elemental abundances in the Sun. We consider gas cooling at constant density (isochoric) and at constant pressure (isobaric). We calculate the critical column densities and temperatures at which radiatively cooling clouds make the dynamical transition from isobaric to isochoric evolution. We construct ion ratio diagnostics for the temperature and metallicity in radiatively cooling gas. We provide numerical estimates for the maximal cloud column densities for which the gas remains optically thin to the cooling radiation. We present our computational results in convenient online figures and tables.
17397524
Reionization of Hydrogen and Helium by Early Stars and Quasars
We compute the reionization histories of hydrogen and helium caused by the ionizing radiation fields produced by stars and quasars. For the quasars we use a model based on halo-merger rates that reproduces all known properties of the quasar luminosity function at high redshifts. The less constrained properties of the ionizing radiation produced by stars are modeled with two free parameters: (i) a transition redshift, ztran, above which the stellar population is dominated by massive, zero-metallicity stars and below which it is dominated by a Scalo mass function; and (ii) the product of the escape fraction of stellar ionizing photons from their host galaxies and the star formation efficiency, fescf*. We constrain the allowed range of these free parameters at high redshifts on the basis of the lack of the H I Gunn-Peterson trough at z 6 and the upper limit on the total intergalactic optical depth for electron scattering, τes < 0.18, from recent cosmic microwave background (CMB) experiments. We find that quasars ionize helium by a redshift z ~ 4, but cannot reionize hydrogen by themselves before z ~ 6. A major fraction of the allowed combinations of fescf* and ztran leads to an early peak in the ionized fraction because of the presence of metal-free stars at high redshifts. This sometimes results in two reionization epochs, namely, an early H II or He III overlap phase followed by recombination and a second overlap phase. Even if early overlap is not achieved, the peak in the visibility function for scattering of the CMB often coincides with the early ionization phase rather than with the actual reionization epoch. Consequently, τes does not correspond directly to the reionization redshift. We generically find values of τes 7%, which should be detectable by the MAP satellite.
31215380
X-ray orbital modulations in intermediate polars
We present an analysis of 30 archival ASCA and RXTE X-ray observations of 16 intermediate polars to investigate the nature of their orbital modulation. We show that X-ray orbital modulation is widespread amongst these systems, but not ubiquitous as indicated by previous studies that included fewer objects. Only seven of the sixteen systems show a clearly statistically significant modulation depth whose amplitude decreases with increasing X-ray energy. Interpreting this as due to photoelectric absorption in material at the edge of an accretion disc would imply that such modulations are visible for all system inclination angles in excess of 60 degrees. However, it is also apparent that the presence of an X-ray orbital modulation can appear and disappear on a timescale of ∼years or months in an individual system. This may be evidence for the presence of a precessing, tilted accretion disc, as inferred in some low mass X-ray binaries.
118707424
A new rare type of supernovae: hypervelocity stellar collisions at galactic centers
When a binary star system is tidally disrupted by a supermassive black hole at a galactic nucleus, one star is ejected at a high speed while the other remains in a tightly bound orbit around the black hole. The cluster of tightly bound stars builds over time, eventually creating a steady state in which the rate of collisions between these stars is similar to the rate of capturing new stars. A large fraction of the collisions occur near the periapsis of the orbits around the black hole, where the kinetic energies are sufficient to generate an explosive disruption of the two stars involved. The typical flare brightens for several days, with a peak luminosity that is comparable to the lower-luminosity end of known supernovae. The explosion lightcurve is followed by a longer flare due to accretion of ejected matter onto the black hole. Dedicated searches in the near universe could observe several such "collisional-supernovae" per year.