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Miller J.J.,West Virginia University | McLaughlin M.A.,West Virginia University | Rea N.,Institute Of Ciencies Of Lespai Ieec Csic Campus Uab | Lazaridis K.,Max Planck Institute for Radio Astronomy | And 4 more authors.
Astrophysical Journal | Year: 2013

We present the results of simultaneous radio and X-ray observations of PSR J1819-1458. Our 94 ks XMM-Newton observation of the high magnetic field (∼5 × 1013 G) pulsar reveals a blackbody spectrum (kT ∼ 130 eV) with a broad absorption feature, possibly composed of two lines at ∼1.0 and ∼1.3 keV. We performed a correlation analysis of the X-ray photons with radio pulses detected in 16.2 hr of simultaneous observations at 1-2 GHz with the Green Bank, Effelsberg, and Parkes telescopes, respectively. Both the detected X-ray photons and radio pulses appear to be randomly distributed in time. We find tentative evidence for a correlation between the detected radio pulses and X-ray photons on timescales of less than 10 pulsar spin periods, with the probability of this occurring by chance being 0.46%. This suggests that the physical process producing the radio pulses may also heat the polar-cap. © 2013. The American Astronomical Society. All rights reserved. Source

Papitto A.,Institute Of Ciencies Of Lespai Ieec Csic Campus Uab | D'Ai A.,University of Palermo | Di Salvo T.,University of Palermo | Egron E.,University of Cagliari | And 5 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

We present a study of the accretion flow to the intermittent accreting millisecond pulsar, HETE J1900.1-2455, based on observations made simultaneously by XMM-Newton and RXTE. The 0.33-50 keV energy spectrum is described by the sum of a hard Comptonized component produced in an optically thin τ (1 corona, a soft thermal kTin (0.2 keV component interpreted as accretion disc emission, and of disc reflection of the hard component. Two emission features are detected at energies of 0.98(1) and 6.58(7) keV, respectively. The latter is identified as Kα transition of Fe XXIII-XXV. A simultaneous detection in the European Photon Imaging Camera (EPIC-pn), EPIC-MOS2 and Reflection Grating Spectrometer (RGS) spectra favours an astrophysical origin also for the latter, which has an energy compatible with Fe-Lα and helium-like Ne-Kα transitions. The broadness of the two features, σ/E ( 0.1, suggests a common origin, resulting from reflection in an accretion disc with inclination of (30-3 +4) , and extending down to Rin = 25-11 +16 gravitational radii from the compact object. However, the strength of the feature at lower energy measured by EPIC-pn cannot be entirely reconciled with the amplitude of the Fe-Kα line, hampering the possibility of describing it in terms of a broadband reflection model, and preventing a firm identification. Pulsations at the known 377.3 Hz spin frequency could not be detected with an upper limit of 0.4 per cent at 3σ confidence level on the pulsed fractional amplitude. We interpret the value of the inner disc radius estimated from spectral modelling and the lack of significant detection of coherent X-ray pulsations as an indication of a disc accretion flow truncated by some mechanism connected to the overall evolution of the accretion disc, rather than by the neutron star magnetic field. This is compatible with the extremely close similarity of spectral and temporal properties of this source with respect to other, non-pulsing atoll sources in the hard state. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Source

Caliandro G.A.,Institute Of Ciencies Of Lespai Ieec Csic Campus Uab | Torres D.F.,Institute Of Ciencies Of Lespai Ieec Csic Campus Uab | Torres D.F.,Catalan Institution for Research and Advanced Studies | Rea N.,Institute Of Ciencies Of Lespai Ieec Csic Campus Uab
Monthly Notices of the Royal Astronomical Society | Year: 2012

The detection of pulsations from an X-ray binary is an unambiguous signature of the presence of a neutron star in the system. When the pulsations are missed in the radio band, their detection at other wavelengths, such as X-ray or gamma-rays, requires orbital demodulation, since the length of the observations is often comparable to, or longer than, the system orbital period. A detailed knowledge of the orbital parameters of binary systems plays a crucial role in the detection of the spin period of pulsars since any uncertainty in their determination translates into a loss in the coherence of a signal during the demodulation process. In this paper, we present an analytical study aimed at unveiling how the uncertainties in the orbital parameters might impact on periodicity searches. We find a correlation between the power of the signal in the demodulated arrival time series and the uncertainty in each of the orbital parameters. This correlation is also a function of the pulsar frequency. We test our analytical results with numerical simulations, finding good agreement between them. Finally, we apply our study to the cases of LS 5039 and LS I +61 303 and consider the current level of uncertainties in the orbital parameters of these systems and their impact on a possible detection of a hosted pulsar. We also discuss the possible appearance of a sideband ambiguity in real data. The latter can occur when, due to the use of uncertain orbital parameters, the power of a putative pulsar is distributed in frequencies lying near the pulsar period. Even if the appearance of a sideband is already a signature of a pulsar component, it may introduce an ambiguity in the determination of its period. We present here a method to solve the sideband issue. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS. Source

Martin J.,Institute Of Ciencies Of Lespai Ieec Csic Campus Uab | Torres D.F.,Institute Of Ciencies Of Lespai Ieec Csic Campus Uab | Torres D.F.,Catalan Institution for Research and Advanced Studies | Rea N.,Institute Of Ciencies Of Lespai Ieec Csic Campus Uab
Monthly Notices of the Royal Astronomical Society | Year: 2012

In this work, we present a leptonic, time-dependent model of pulsar wind nebulae (PWNe). The model seeks a solution for the lepton distribution function considering the full time-energy-dependent diffusion-loss equation. The time-dependent lepton population is balanced by injection, energy losses and escape. We include synchrotron, inverse-Compton (IC; with the cosmic-microwave background as well as with IR/optical photon fields), self-synchrotron Compton, and bremsstrahlung processes, all devoid of any radiative approximations. With this model in place we focus on the Crab nebula as an example and present its time-dependent evolution. Afterwards, we analyse the impact of different approximations made at the level of the diffusion-loss equation, as can be found in the literature. Whereas previous models ignored the escape term, e.g. with the diffusion-loss equation becoming advective, others approximated the losses as catastrophic, so that the equation has only time derivatives. Additional approximations are also described and computed. We study what the impact of these approaches is on the determination of the PWN evolution. In particular, we find the time-dependent deviation of the multi-wavelength spectrum and the best-fitting parameters obtained with the complete and the approximate models. © 2012 The Authors Monthly Notices of the Royal Astronomical Society. © 2012 RAS. Source

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