Broadband observations of the X-ray burster 4U 1705-44 with BeppoSAX

4U 1705-44 is one of the most-studied type I X-ray burster and Atoll sources. This source represents a perfect candidate to test different models proposed to self-consistently track the physical changes occurring between different spectral states because it shows clear spectral state transitions. The broadband coverage, the sensitivity and energy resolution of the BeppoSAX satellite offers the opportunity to disentangle the components that form the total X-ray spectrum and to study their changes according to the spectral state. Using two BeppoSAX observations carried out in August and October 2000, respectively, for a total effective exposure time of about 100 ks, we study the spectral evolution of the source from a soft to hard state. Energy spectra are selected according to the source position in the color-color diagram (CCD) Results. We succeeded in modeling the spectra of the source using a physical self-consistent scenario for both the island and banana branches (the double Comptonization scenario). The components observed are the soft Comptonization and hard Comptonization, the blackbody, and a reflection component with a broad iron line. When the source moves from the banana state to the island state, the parameters of the two Comptonization components change significantly and the blackbody component becomes too weak to be detected. We interpret the soft Comptonization component as emission from the hot plasma surrounding the neutron star, hard Comptonization as emission from the disk region, and the blackbody component as emission from the inner accretion disk. The broad feature in the iron line region is compatible with reflection from the inner accretion disk.Comment: 8 pages, 10 figures, accepted for publication by A&

A relativistic iron emission line from the neutron star low-mass X-ray binary GX 3+1

We present the results of a spectroscopic study of the Fe K{\alpha} emission of the persistent neutron-star atoll low-mass X-ray binary and type I X-ray burster GX 3+1 with the EPIC-PN on board XMM-Newton. The source shows a flux modulation over several years and we observed it during its fainter phase, which corresponds to an X-ray luminosity of Lx~10^37 ergs/s. When fitted with a two-component model, the X-ray spectrum shows broad residuals at \sim6-7 keV that can be ascribed to an iron K{\alpha} fluorescence line. In addition, lower energy features are observed at \sim3.3 keV, \sim3.9 keV and might originate from Ar XVIII and Ca XIX. The broad iron line feature is well fitted with a relativistically smeared profile. This result is robust against possible systematics caused by instrumental pile-up effects. Assuming that the line is produced by reflection from the inner accretion disk, we infer an inner disk radius of \sim25 Rg and a disk inclination of 35{\deg} < i < 44{\deg}.Comment: 4 pages, 3 figures Accepted for publication in Astronomy and Astrophysic

Testing reflection features in 4U 1705-44 with XMM-Newton, BeppoSAX and RXTE in the hard and soft state

We use data of the bright atoll source 4U 1705-44 taken with XMM-Newton, BeppoSAX and RXTE both in the hard and in the soft state to perform a self-consistent study of the reflection component in this source. Although the data from these X-ray observatories are not simultaneous, the spectral decomposition is shown to be consistent among the different observations, when the source flux is similar. We therefore select observations performed at similar flux levels in the hard and soft state in order to study the spectral shape in these two states in a broad band (0.1-200 keV) energy range, with good energy resolution, and using self-consistent reflection models. These reflection models provide a good fit for the X-ray spectrum both in the hard and in the soft state in the whole spectral range. We discuss the differences in the main spectral parameters we find in the hard and the soft state, respectively, providing evidence that the inner radius of the optically thick disk slightly recedes in the hard state.Comment: Accepted for publication in A&A, 20 pages, 12 figure

Long Term Study of the Double Pulsar J0737-3039 with XMM-Newton: pulsar timing

The relativistic double neutron star binary PSR J0737-3039 shows clear evidence of orbital phase-dependent wind-companion interaction, both in radio and X-rays. In this paper we present the results of timing analysis of PSR J0737-3039 performed during 2006 and 2011 XMM-Newton Large Programs that collected ~20,000 X-ray counts from the system. We detected pulsations from PSR J0737-3039A (PSR A) through the most accurate timing measurement obtained by XMM-Newton so far, the spin period error being of 2x10^-13 s. PSR A's pulse profile in X-rays is very stable despite significant relativistic spin precession that occurred within the time span of observations. This yields a constraint on the misalignment between the spin axis and the orbital momentum axis Delta_A ~6.6^{+1.3}_{-5.4} deg, consistent with estimates based on radio data. We confirmed pulsed emission from PSR J0737-3039B (PSR B) in X-rays even after its disappearance in radio. The unusual phenomenology of PSR B's X-ray emission includes orbital pulsed flux and profile variations as well as a loss of pulsar phase coherence on time scales of years. We hypothesize that this is due to the interaction of PSR A's wind with PSR B's magnetosphere and orbital-dependent penetration of the wind plasma onto PSR B closed field lines. Finally, the analysis of the full XMM-Newton dataset provided evidences of orbital flux variability (~7%) for the first time, involving a bow-shock scenario between PSR A's wind and PSR B's magnetosphere.Comment: Comments: 16 Pages, 6 Figures. Accepted for publication in Astrophysical Journal (Draft Version

The reflection component in NS LMXBs

Thanks to the good spectral resolution and large effective area of the EPIC/PN instrument on board of XMM-Newton, we have at hand a large number of observations of accreting low-mass X-ray binaries, that allow for the fist time a comprehensive view on the characteristics of the reflection component at different accretion regimes and to probe the effects of a magnetosphere on its formation. We focus here on a comparative analysis of the reflection component from a series of spectroscopic studies on selected sources: 4U 1705-44, observed both in the soft and hard state, the pulsating ms pulsars SAX J1808.4-3658 and IGR J17511-3057, and the intermittent pulsar HETE J1900-2455. Although the sources can present very similar accretion rates and continuum shapes, the reflection parameters do not generally result the same, moreover the effect of a magnetosphere on the formation of the reflection component appears elusive. \ua9 2014 Owned by the authors

Imaging of SNR IC443 and W44 with the Sardinia Radio Telescope at 1.5 GHz and 7 GHz

Observations of supernova remnants (SNRs) are a powerful tool for investigating the later stages of stellar evolution, the properties of the ambient interstellar medium, and the physics of particle acceleration and shocks. For a fraction of SNRs, multi-wavelength coverage from radio to ultra high-energies has been provided, constraining their contributions to the production of Galactic cosmic rays. Although radio emission is the most common identifier of SNRs and a prime probe for refining models, high-resolution images at frequencies above 5 GHz are surprisingly lacking, even for bright and well-known SNRs such as IC443 and W44. In the frameworks of the Astronomical Validation and Early Science Program with the 64-m single-dish Sardinia Radio Telescope, we provided, for the first time, single-dish deep imaging at 7 GHz of the IC443 and W44 complexes coupled with spatially-resolved spectra in the 1.5-7 GHz frequency range. Our images were obtained through on-the-fly mapping techniques, providing antenna beam oversampling and resulting in accurate continuum flux density measurements. The integrated flux densities associated with IC443 are S_1.5GHz = 134 +/- 4 Jy and S_7GHz = 67 +/- 3 Jy. For W44, we measured total flux densities of S_1.5GHz = 214 +/- 6 Jy and S_7GHz = 94 +/- 4 Jy. Spectral index maps provide evidence of a wide physical parameter scatter among different SNR regions: a flat spectrum is observed from the brightest SNR regions at the shock, while steeper spectral indices (up to 0.7) are observed in fainter cooling regions, disentangling in this way different populations and spectra of radio/gamma-ray-emitting electrons in these SNRs.Comment: 13 pages, 9 figures, accepted for publication to MNRAS on 18 May 201

Sardinia Radio Telescope wide-band spectral-polarimetric observations of the galaxy cluster 3C 129

We present new observations of the galaxy cluster 3C 129 obtained with the Sardinia Radio Telescope in the frequency range 6000-7200 MHz, with the aim to image the large-angular-scale emission at high-frequency of the radio sources located in this cluster of galaxies. The data were acquired using the recently-commissioned ROACH2-based backend to produce full-Stokes image cubes of an area of 1 deg x 1 deg centered on the radio source 3C 129. We modeled and deconvolved the telescope beam pattern from the data. We also measured the instrumental polarization beam patterns to correct the polarization images for off-axis instrumental polarization. Total intensity images at an angular resolution of 2.9 arcmin were obtained for the tailed radio galaxy 3C 129 and for 13 more sources in the field, including 3C 129.1 at the galaxy cluster center. These data were used, in combination with literature data at lower frequencies, to derive the variation of the synchrotron spectrum of 3C 129 along the tail of the radio source. If the magnetic field is at the equipartition value, we showed that the lifetimes of radiating electrons result in a radiative age for 3C 129 of t_syn = 267 +/- 26 Myrs. Assuming a linear projected length of 488 kpc for the tail, we deduced that 3C 129 is moving supersonically with a Mach number of M=v_gal/c_s=1.47. Linearly polarized emission was clearly detected for both 3C 129 and 3C 129.1. The linear polarization measured for 3C 129 reaches levels as high as 70% in the faintest region of the source where the magnetic field is aligned with the direction of the tail.Comment: 19 pages, 17 figures, accepted for publication in MNRA

Methods for detection and analysis of weak radio sources with single-dish radio telescopes

The detection of mJy/sub-mJy point sources is a significant challenge for single-dish radio telescopes. Detection or upper limits on the faint afterglow from GRBs or other sources at cosmological distances are important means of constraining the source modeling. Using the Sardinia Radio Telescope (SRT), we compare the sensitivity and robustness of three methods applied to the detection of faint radio sources from raster maps around a known source position: the smart quick-look method, the source extraction method (typical of high-energy astronomy), and the fit with a 2-D Gaussian. We developed a Python code specific for the analysis of point-like radio sources applied to the SRT C-band (6.9 GHz) observations of both undetected sources (GRB afterglows of 181201A and 190114C) and the detected Galactic X-ray binary GRS 1915+105. Our comparative analysis of the different detection methods made extensive use of simulations as a useful complement to actual radio observations. The best method for the SRT data analysis is the fit with a 2-D Gaussian, as it pushes down the sensitivity limits of single-dish observations -- with respect to more traditional techniques -- to ~ 1.8 mJy, improving by ~ 40 % compared with the initial value. This analysis shows that -- especially for faint sources -- good maps of the scanned region pre- or post-outburst are essential.Comment: 22 pages, 11 figures, 3 tables, pre-print of an article published in Experimental Astronomy; v2: updated abstract and reference