Gamma-ray bursts from magnetized collisionally-heated jets

Jets producing gamma-ray bursts (GRBs) are likely to carry a neutron component that drifts with respect to the proton component. The neutron-proton collisions strongly heat the jet and generate electron-positron pairs. We investigate radiation produced by this heating using a new numerical code. Our results confirm the recent claim that collisional heating generates the observed Band-type spectrum of GRBs. We extend the model to study the effects of magnetic fields on the emitted spectrum. We find that the spectrum peak remains near 1 MeV for the entire range of the magnetization parameter $0<\epsilon_B<2$ that is explored in our simulations. The low-energy part of the spectrum softens with increasing $\epsilon_B$, and a visible soft excess appears in the keV band. The high-energy part of the spectrum extends well above the GeV range and can contribute to the prompt emission observed by Fermi/LAT. Overall, the radiation spectrum created by the collisional mechanism appears to agree with observations, with no fine-tuning of parameters.Comment: 13 pages, 6 figures, accepted to Ap

Spectral analysis of a large sample of BeppoSAX Seyfert spectra with Comptonization models: Preliminary results

We present preliminary results of the spectral analysis of a large sample of Seyfert galaxies observed by BeppoSAX. The only selection criterium was a sufficiently large S/N ratio (>10) in the PDS band (12-200 keV) to allow good detection up to the highest energy. The resulting sample is composed of 28 objects (17 Seyfert 1, 11 Seyfert 2) and 50 observations. Our main effort here is to adopt Comptonization models to fit the different spectra on a truly broad band basis (0.1-200 keV). We use two distinct disc-corona configurations, an anisotropic slab and an isotropic spherical one. We discuss the distributions of the physical parameters, like temperature and optical depth of the corona and the reflection component, among this sample. We also discussed the existence (or inexistence) of correlations between these parameters.Comment: 4 pages, 2 figures. Proc. of the meeting: "The Restless High-Energy Universe" (Amsterdam, The Netherlands), E.P.J. van den Heuvel, J.J.M. in 't Zand, and R.A.M.J. Wijers Ed

Model atmospheres of X-ray bursting neutron stars

We present an extended set of model atmospheres and emergent spectra of X-ray bursting neutron stars in low mass X-ray binaries. Compton scattering is taken into account. The models were computed in LTE approximation for six different chemical compositions: pure hydrogen and pure helium atmospheres, and atmospheres with a solar mix of hydrogen and helium and various heavy elements abundances: Z = 1, 0.3, 0.1, and 0.01 Z_sun, for three values of gravity, log g =14.0, 14.3, and 14.6 and for 20 values of relative luminosity l = L/L_Edd in the range 0.001 - 0.98. The emergent spectra of all models are fitted by diluted blackbody spectra in the observed RXTE/PCA band 3 - 20 keV and the corresponding values of color correction factors f_c are presented. We also show how to use these dependencies to estimate the neutron star's basic parameters.Comment: 2 pages, 1 figure, conference "Astrophysics of Neutron Stars - 2010" in honor of M. Ali Alpar, Izmir, Turke

Photon breeding mechanism in relativistic jets: astrophysical implications

Photon breeding in relativistic jets involves multiplication of high-energy photons propagating from the jet to the external environment and back with the conversion into electron-positron pairs. The exponential growth of the energy density of these photons is a super-critical process powered by the bulk energy of the jet. The efficient deceleration of the jet outer layers creates a structured jet morphology with the fast spine and slow sheath. In initially fast and high-power jets even the spine can be decelerated efficiently leading to very high radiative efficiencies of conversion of the jet bulk energy into radiation. The decelerating, structured jets have angular distribution of radiation significantly broader than that predicted by a simple blob model with a constant Lorentz factor. This reconciles the discrepancy between the high Doppler factors determined by the fits to the spectra of TeV blazars and the low apparent velocities observed at VLBI scales as well as the low jet Lorentz factors required by the observed statistics and luminosity ratio of Fanaroff-Riley I radio galaxies and BL Lac objects. Photon breeding produces a population of high-energy leptons in agreement with the constraints on the electron injection function required by spectral fits of the TeV blazars. Relativistic pairs created outside the jet and emitting gamma-rays by inverse Compton process might explain the relatively high level of the TeV emission from the misaligned jet in the radio galaxies. The mechanism reproduces basic spectral features observed in blazars including the blazar sequence (shift of the spectral peaks towards lower energies with increasing luminosity). The mechanism is very robust and can operate in various environments characterised by the high photon density.Comment: 6 pages, 3 figures, to appear in the proceedings of the HEPRO conference, September 24-28, 2007, Dublin, Irelan

On the Origin of Polarization near the Lyman Edge in Quasars

Optical/UV radiation from accretion disks in quasars is likely to be partly scattered by a hot plasma enveloping the disk. We investigate whether the scattering may produce the steep rises in polarization observed blueward of the Lyman limit in some quasars. We suggest and assess two models. In the first model, primary disk radiation with a Lyman edge in absorption passes through a static ionized "skin" covering the disk, which has a temperature about 3 keV and a Thomson optical depth about unity. Electron scattering in the skin smears out the edge and produces a steep rise in polarization at lambda < 912 A. In the second model, the scattering occurs in a hot coronal plasma outflowing from the disk with a mildly relativistic velocity. We find that the second model better explains the data. The ability of the models to fit the observed rises in polarization is illustrated with the quasar PG 1630+377.Comment: submitted to ApJ Letter

Spectral and timing properties of the accreting X-ray millisecond pulsar IGR J17498-2921

We analyze the spectral and timing properties of IGR J17498-2921 and the characteristics of X-ray bursts to constrain the physical processes responsible for the X-ray production in this class of sources. The broad-band average spectrum is well-described by thermal Comptonization with an electron temperature of kT_e ~ 50 keV, soft seed photons of kT_bb ~ 1 keV, and Thomson optical depth \taut ~ 1 in a slab geometry. The slab area corresponds to a black body radius of R_bb ~9 km. During the outburst, the spectrum stays remarkably stable with plasma and soft seed photon temperatures and scattering optical depth that are constant within the errors. This behavior has been interpreted as indicating that the X-ray emission originates above the neutron star (NS) surface in a hot slab (either the heated NS surface or the accretion shock). The INTEGRAL, RXTE, and Swift data reveal the X-ray pulsation at a period of 2.5 milliseconds up to ~65 keV. The pulsed fraction is consistent with being constant, i.e. energy independent and has a typical value of 6-7%. The nearly sinusoidal pulses show soft lags that seem to saturate near 10 keV at a rather small value of ~ -60\mu s with those observed in other accreting pulsars. The short burst profiles indicate that there is a hydrogen-poor material at ignition, which suggests either that the accreted material is hydrogen-deficient, or that the CNO metallicity is up to a factor of about two times solar. However, the variation in the burst recurrence time as a function of \dot{m} (inferred from the X-ray flux) is much smaller than predicted by helium-ignition models.Comment: 9 pages, 8 figures, accepted for publication in A&A. arXiv admin note: text overlap with arXiv:1012.022

The transitional millisecond pulsar IGR J18245-2452 during its 2013 outburst at X-rays and soft gamma-rays

IGR~J18245--2452/PSR J1824--2452I is one of the rare transitional accreting millisecond X-ray pulsars, showing direct evidence of switches between states of rotation powered radio pulsations and accretion powered X-ray pulsations, dubbed transitional pulsars. IGR~J18245--2452 is the only transitional pulsar so far to have shown a full accretion episode, reaching an X-ray luminosity of $\sim10^{37}$~erg~s$^{-1}$ permitting its discovery with INTEGRAL in 2013. In this paper, we report on a detailed analysis of the data collected with the IBIS/ISGRI and the two JEM-X monitors on-board INTEGRAL at the time of the 2013 outburst. We make use of some complementary data obtained with the instruments on-board XMM-Newton and Swift in order to perform the averaged broad-band spectral analysis of the source in the energy range 0.4 -- 250~keV. We have found that this spectrum is the hardest among the accreting millisecond X-ray pulsars. We improved the ephemeris, now valid across its full outburst, and report the detection of pulsed emission up to $\sim60$ keV in both the ISGRI ($10.9 \sigma$) and Fermi/GBM ($5.9 \sigma$) bandpass. The alignment of the ISGRI and Fermi GBM 20 -- 60 keV pulse profiles are consistent at a $\sim25\ \mu$s level. We compared the pulse profiles obtained at soft X-rays with \xmm\ with the soft \gr-ray ones, and derived the pulsed fractions of the fundamental and first harmonic, as well as the time lag of the fundamental harmonic, up to $150\ \mu$s, as a function of energy. We report on a thermonuclear X-ray burst detected with \Integ, and using the properties of the previously type-I X-ray burst, we show that all these events are powered primarily by helium ignited at a depth of $y_{\rm ign} \approx 2.7\times10^8$ g cm${}^{-2}$. For such a helium burst the estimated recurrence time of $\Delta t_{\rm rec}\approx5.6$ d is in agreement with the observations.Comment: 10 pages, 6 Figures, 3 Tables Astronomy and Astrophysics Journal, accepted for publication on the 13th of April 201

Reflection and noise in the low spectral state of GX339-4

We analyze RXTE/PCA observations of GX339-4 in the low spectral state from 1996--1997 and show that the pattern of its spectral and temporal variability is nearly identical to that of Cyg X-1. In particular, a tight correlation exists between the QPO centroid frequency and the spectral parameters. An increase of the QPO centroid frequency is accompanied with an increase of the amplitude of the reflected component and a steepening the slope of the underlying power law. Fourier frequency resolved spectral analysis showed, that the variability of the reflected component at frequencies higher than ~1-10 Hz is suppressed in comparison with that of the primary emission.Comment: 4 pages, 4 figures. Accepted in Astronomy and Astrophysics Main Journa