Aperiodic variability of low-mass X-ray binaries at very low frequencies

We have obtained discrete Fourier power spectra of a sample of persistent low-mass neutron-star X-ray binaries using long-term light curves from the All Sky Monitor on board the Rossi X-ray Timing Explorer. Our aim is to investigate their aperiodic variability at frequencies in the range 1 x 10^{-7}-5 x 10^{-6} Hz and compare their properties with those of the black-hole source Cyg X-1. We find that the classification scheme that divides LMXBs into Z and atoll sources blurs at very low frequencies. Based on the long-term (~ years) pattern of variability and the results of power-law fits (P ~ v^{-a}) to the 1 x 10^{-7}-5 x 10^{-6} Hz power density spectra, low-mass neutron-star binaries fall into three categories. Type I includes all Z sources, except Cyg X-2, and the atoll sources GX9+1 and GX13+1. They show relatively flat power spectra (a < 0.9) and low variability (rms < 20%). Type II systems comprise 4U 1636-53, 4U 1735-44 and GX3+1. They are more variable (20% < rms < 30%) and display steeper power spectra (0.9 < a < 1.2) than Type I sources. Type III systems are the most variable (rms > 30%) and exhibit the steepest power spectra (a > 1.2). The sources 4U 1705-44, GX354-0 and 4U 1820-30 belong to this group. GX9+9 and Cyg X-2 appear as intermediate systems in between Type I and II and Type II and III sources, respectively. We speculate that the differences in these systems may be caused by the presence of different types of mass-donor companions. Other factors, like the size of the accretion disc and/or the presence of weak magnetic fields, are also expected to affect their low-frequency X-ray aperiodic varibility.Comment: 9 pages, 6 figures. To be published in A&

Broad-band X-ray spectra of anomalous X-ray pulsars and soft γ\gamma-ray repeaters: pulsars in a weak-accretion regime ?

We present the results from the analysis of the broad-band X-ray spectra of 5 Anomalous X-ray Pulsars (AXPs) and Soft $\gamma$-ray Repeaters (SGRs). We fit their Suzaku and INTEGRAL spectra with models appropriate for the X-ray emission from the accretion flow onto a pulsar. We find that their X-ray spectra can be well described with this model. In particular we find that: (a) the radius of the accretion column is $\sim150-350$ m resulting in a transverse optical depth of $\sim 1$; (b) the vertical Thompson optical depth is $\approx 50-400$, and (c) their luminosity translates in accretion rates $\approx10^{15}\rm{g\, s^{-1}}$. These results are in good agreement with the predictions from the fall-back disk model, providing further support in the interpretation of AXPs and SGRs as accreting pulsars.Comment: Accepted for publication in MNRAS, 10 pages, 2 figure

Is the Galactic submillimeter dust emissivity underestimated?

We present detailed modeling of the spectral energy distribution (SED) of the spiral galaxies NGC 891, NGC 4013, and NGC 5907 in the far-infrared and submm wavelengths. The model takes into account the emission produced by the diffuse dust and the star forming HII complexes. The dust mass is constrained by radiative transfer simulations in the optical (Xilouris et al. 1999). We find that the submm emission predicted by our model cannot account for the observed fluxes. Two scenarios may account for the "missing" submm flux. In the first scenario (Popescu et al. 2000), additional dust (to that derived from the optical, and associated with young stars) is embedded in the galaxy in the form of a thin disk and gives rise to additional submm emission. The other scenario investigates whether the average submm emissivity of the dust grains is higher than the values widely used in Galactic environments. In this case, the dust mass is equal to that derived from the optical observations, and the submm emissivity is treated as a free parameter calculated by fitting our model to the observed SED. We find the submm emissivity value to be ~3 times that often used for our Galaxy. While both scenarios reproduce the observed 850 micron surface brightness, the extra embedded dust model is not supported by the near infrared observations. We, thus, find that the enhanced dust submm emissivity scenario is the most plausible. [abridged]Comment: 12 pages, 10 figures, accepted for publication in Astronomy and Astrophysic

Modelling the dust content of spiral galaxies: More dust mass vs. enhanced dust grain emissivity

We present detailed modelling of the spectral energy distribution (SED) of the spiral galaxies NGC 891, NGC 4013, and NGC 5907 in the far-infrared (FIR) and sub-millimeter (submm) wavelengths. The model takes into account the emission of the diffuse dust component, which is heated by the UV and optical radiation fields produced by the stars, as well as the emission produced locally in star forming HII complexes. The radiative transfer simulations of Xilouris et al. (1999) in the optical bands are used to constrain the stellar and dust geometrical parameters, as well as the total amount of dust. We find that the submm emission predicted by our model can not account for the observed fluxes at these wavelengths. We examine two cases, one having more dust embedded in a second thin disk and another allowing for an enhanced submillimeter emissivity of the dust grains. We argue that both cases can equally well reproduce the observed SED. The case of having more dust embedded in a second disk though, is not supported by the near-infrared observations and thus more realistic distributions of the dust (i.e., in spiral arms and clumps) have to be examined in order to better fit the surface brightness of each galaxy.Comment: To appear in the proceedings of: "The Spectral Energy Distribution of Gas-Rich Galaxies: Confronting Models with Data", Heidelberg, 4-8 Oct. 2004, eds. C.C. Popescu and R.J. Tuffs, AIP Conf. Ser., in pres

Radiation Transfer Models in Galaxies

The dust in galaxies makes radiation transport calculations in them absolutely necessary. It is not only common practice in Astrophysics, but also wisdom, to try to make as simple models as possible to simulate physical systems. For spiral galaxies, however, this turned out to be catastrophic. For years, the major question of the opacity of spiral galaxies kept the community divided, because the models were too simple. A spiral galaxy appears, to first order, to have exponential distributions of stars and dust, which cannot be approximated with uniform distributions. We will review the radiative transfer methods used in galaxies and we will comment on their pluses and minuses. We will also present some of the main results of the application of one of the methods to the observations.Comment: To appear in the proceedings of: "The Spectral Energy Distribution of Gas-Rich Galaxies: Confronting Models with Data", Heidelberg, 4-8 Oct. 2004, eds. C.C. Popescu and R.J. Tuffs, AIP Conf. Ser., in pres

A jet model for black-hole X-ray sources

A jet model for Galactic black-hole X-ray binaries will be presented that appears to explain several observational characteristics. In particular, it explains the energy spectrum from radio to hard X-rays, the time-lags as a function of Fourier frequency, the increase of the variability amplitude (QPO and high frequency) with increasing photon energy, and the narrowing of the autocorrelation function with increasing photon energy. On the other hand, there are additional observational constraints that no model has tried to explain yet. It is important that we all try to address these constraints if we are to make any progress in understanding black-hole X-ray source