The X-ray emission of magnetic cataclysmic variables in the XMM-Newton era

We review the X-ray spectral properties of magnetic cataclysmic binaries derived from observations obtained during the last decade with the large X-ray observatories XMM-Newton, Chandra and Suzaku. We focus on the signatures of the different accretion modes which are predicted according to the values of the main physical parameters (magnetic field, local accretion rate and white dwarf mass). The observed large diversity of spectral behaviors indicates a wide range of parameter values in both intermediate polars and polars, in line with a possible evolutionary link between both classes.Comment: To appear in the Proceedings of "The Golden Age of Cataclysmic Variables (Palermo 2011)", in Mem. Soc. Astron. It. (7 pages, 3 figures

The eclipsing bursting X-ray binary EXO 0748-676 revisited by XMM-Newton

The bright eclipsing and bursting low-mass X-ray binary EXO 0748-676 has been observed at several occasions by XMM-Newton during the initial calibration and performance verification (CAL/PV) phase. We present here the results obtained from observations with the EPIC cameras. Apart from several type-I X-ray bursts, the source shows a high degree of variability with the presence of soft flares. The wide energy coverage and high sensitivity of XMM-Newton allows for the first time a detailed description of the spectral variability. The source is found to be the superposition of a central (~2 10^8 cm) Comptonized emission, most probably a corona surrounding the inner edge of an accretion disk, associated with a more extended (~3 10^10 cm) thermal halo at a typical temperature of ~0.6 keV with an indication of non-solar abundances. Most of the variations of the source can be accounted for by a variable absorption affecting only the central comptonized component and reaching up to NH ~1.3 10^23 cm^{-2}. The characteristics of the surrounding halo are found compatible with an irradiated atmosphere of an accretion disc which intercepts the central emission due to the system high inclination.Comment: 6 pages, 4 figures, accepted for publication in A&A Letters, XMM special issu

QPTAS and Subexponential Algorithm for Maximum Clique on Disk Graphs

A (unit) disk graph is the intersection graph of closed (unit) disks in the plane. Almost three decades ago, an elegant polynomial-time algorithm was found for Maximum Clique on unit disk graphs [Clark, Colbourn, Johnson; Discrete Mathematics '90]. Since then, it has been an intriguing open question whether or not tractability can be extended to general disk graphs. We show the rather surprising structural result that a disjoint union of cycles is the complement of a disk graph if and only if at most one of those cycles is of odd length. From that, we derive the first QPTAS and subexponential algorithm running in time 2^{O~(n^{2/3})} for Maximum Clique on disk graphs. In stark contrast, Maximum Clique on intersection graphs of filled ellipses or filled triangles is unlikely to have such algorithms, even when the ellipses are close to unit disks. Indeed, we show that there is a constant ratio of approximation which cannot be attained even in time 2^{n^{1-epsilon}}, unless the Exponential Time Hypothesis fails

Reducing the Number of Sputum Samples Examined and Thresholds for Positivity: An Opportunity to Optimise Smear Microscopy.

SETTING: Urban health clinic, Nairobi. OBJECTIVE: To evaluate the impact on tuberculosis (TB) case detection and laboratory workload of reducing the number of sputum smears examined and thresholds for diagnosing positive smears and positive cases. DESIGN: In this prospective study, three Ziehl-Neelsen stained sputum smears from consecutive pulmonary TB suspects were examined blind. The standard approach (A), > or = 2 positive smears out of 3, using a cut-off of 10 acid-fast bacilli (AFB)/100 high-power fields (HPF), was compared with approaches B, > or = 2 positive smears (> or = 4 AFB/100 HPF) out of 3, one of which is > or = 10 AFB/100 HPF; C, > or = 2 positive smears (> or = 4 AFB/100 HPF) out of 3; D, > or = 1 positive smear (> or = 10 AFB/100 HPF) out of 2; and E, > or = 1 positive smear (> or = 4 AFB/100 HPF) out of 2. The microscopy gold standard was detection of at least one positive smear (> or = 4 AFB/100 HPF) out of 3. RESULTS: Among 644 TB suspects, the alternative approaches detected from 114 (17.7%) (approach B) to 123 cases (19.1%) (approach E) compared to 105 cases (16.3%) for approach A (P < 0.005). Sensitivity ranged between 82.0% (105/128) for A and 96.1% (123/128) for E. The single positive smear approaches reduced the number of smears by 36% compared to approach A. CONCLUSION: Reducing the number of specimens and the positivity threshold to define a positive case increased the sensitivity of microscopy and reduced laboratory workload

The ephemeris, orbital decay, and masses of 10 eclipsing HMXBs

We take advantage of more than 10 years of monitoring of the eclipsing HMXB systems LMC X-4, Cen X-3, 4U 1700-377, 4U 1538-522, SMC X-1, IGR J18027-2016, Vela X-1, IGR J17252-3616, XTE J1855-026, and OAO 1657-415 with the ASM on-board RXTE and ISGRI on-board INTEGRAL to update their ephemeris. These results are used to refine previous measurements of the orbital period decay of all sources (where available) and provide the first accurate values of the apsidal advance in Vela X-1 and 4U 1538-522. Updated values for the masses of the neutron stars hosted in the ten HMXBs are also provided, as well as the long-term lightcurves folded on the sources best determined orbital parameters. These lightcurves reveal complex eclipse ingresses and egresses, that are understood mostly as being due to the presence of accretion wakes. The results reported in this paper constitute a database to be used for population and evolutionary studies of HMXBs, as well as theoretical modelling of long-term accretion in wind-fed X-ray binaries.Comment: Accepted for publication on A&

Investigation of collective radial expansion and stopping in heavy ion collisions at Fermi energies

We present an analysis of multifragmentation events observed in central Xe+Sn reactions at Fermi energies. Performing a comparison between the predictions of the Stochastic Mean Field (SMF) transport model and experimental data, we investigate the impact of the compression-expansion dynamics on the properties of the final reaction products. We show that the amount of radial collective expansion, which characterizes the dynamical stage of the reaction, influences directly the onset of multifragmentation and the kinematic properties of multifragmentation events. For the same set of events we also undertake a shape analysis in momentum space, looking at the degree of stopping reached in the collision, as proposed in recent experimental studies. We show that full stopping is achieved for the most central collisions at Fermi energies. However, considering the same central event selection as in the experimental data, we observe a similar behavior of the stopping power with the beam energy, which can be associated with a change of the fragmentation mechanism, from statistical to prompt fragment emission.Comment: 15 page

Memory Effects in the Standard Model for Glasses

The standard model of glasses is an ensemble of two-level systems interacting with a thermal bath. The general origin of memory effects in this model is a quasi-stationary but non-equilibrium state of a single two-level system, which is realized due to a finite-rate cooling and very slow thermally activated relaxation. We show that single particle memory effects, such as negativity of the specific heat under reheating, vanish for a sufficiently disordered ensemble. In contrast, a disordered ensemble displays a collective memory effect [similar to that described by Kovacs for glassy polymers], where non-equilibrium features of the ensemble are monitored via a macroscopic observable. An experimental realization of the effect can be used to further assess the consistency of the model.Comment: 4 pages, 6 figure