Teresa Mineo

XIPE: the X-ray imaging polarimetry explorer

Abstract X-ray polarimetry, sometimes alone, and sometimes coupled to spectral and temporal variability measurements and to imaging, allows a wealth of physical phenomena in astrophysics to be studied. X-ray polarimetry investigates the acceleration process, for example, including those typical of magnetic reconnection in solar flares, but also emission in the strong magnetic fields of neutron stars and white dwarfs. It detects scattering in asymmetric structures such as accretion disks and columns, and in the so-called molecular torus and ionization cones. In addition, it allows fundamental physics in regimes of gravity and of magnetic field intensity not accessible to experiments on the Earth to be probed. Finally, models that describe fundamental interactions (e.g. quantum gravity and the extension of the Standard Model) can be tested. We describe in this paper the X-ray Imaging Polarimetry Explorer (XIPE), proposed in June 2012 to the first ESA call for a small mission with a launch in 2017. The proposal was, unfortunately, not selected. To be compliant with this schedule, we designed the payload mostly with existing items. The XIPE proposal takes advantage of the completed phase A of POLARIX for an ASI small mission program that was cancelled, but is different in many aspects: the detectors, the presence of a solar flare polarimeter and photometer and the use of a light platform derived by a mass production for a cluster of satellites. XIPE is composed of two out of the three existing JET-X telescopes with two Gas Pixel Detectors (GPD) filled with a He-DME mixture at their focus. Two additional GPDs filled with a 3-bar Ar-DME mixture always face the Sun to detect polarization from solar flares. The Minimum Detectable Polarization of a 1 mCrab source reaches 14 % in the 2–10 keV band in 105 s for pointed observations, and 0.6 % for an X10 class solar flare in the 15–35 keV energy band. The imaging capability is 24 arcsec Half Energy Width (HEW) in a Field of View of 14.7 arcmin × 14.7 arcmin. The spectral resolution is 20 % at 6 keV and the time resolution is 8 μs. The imaging capabilities of the JET-X optics and of the GPD have been demonstrated by a recent calibration campaign at PANTER X-ray test facility of the Max-Planck-Institut für extraterrestrische Physik (MPE, Germany). XIPE takes advantage of a low-earth equatorial orbit with Malindi as down-link station and of a Mission Operation Center (MOC) at INPE (Brazil). The data policy is organized with a Core Program that comprises three months of Science Verification Phase and 25 % of net observing time in the following 2 years. A competitive Guest Observer program covers the remaining 75 % of the net observing time.

Study of the accreting pulsar 4U 0115+63 using a bulk and thermal Comptonization model

Context. Highly magnetized pulsars accreting matter in a binary system are bright sources in the X-ray band (0.1–100 keV). Despite the early comprehension of the basic emission mechanism, their spectral energy distribution is generally described by phenomenological or simplified models. Aims. We propose a study of the spectral emission from the high mass X-ray binary pulsar 4U 0115+63 by means of thermal and bulk Comptonization models based on the physical properties of such objects. Methods. For this purpose, we analyze the BeppoSAX data in the energy range 0.7–100 keV of the 1999 giant outburst, 12 days after the maximum. We focus first on the phase averaged emission, and then on the phase resolved spectra, by modeling the system using a two-component continuum. Results. At higher energy, above ∼7 keV, the emission is due to thermal and bulk Comptonization of the seed photons produced by cyclotron cooling of the accretion column, and at lower energy, the emission is due to thermal Comptonization of a blackbody source in a diffuse halo close to the stellar surface. Phase resolved analysis establishes that most of the emission in the main peak comes from the column, while the low energy component gives a nearly constant contribution throughout the phase. Conclusions. From the best fit parameters, we argue that the cyclotron emission is produced ∼1.7 km above the stellar surface, and escapes from the column near its base, where the absorption features are generated by the interaction with the magnetic field in the halo. We find that in 4U 0115+63, the observed spectrum is dominated by reprocessed cyclotron radiation, whereas in other bright sources with stronger magnetic fields such as Her X-1, the spectrum is dominated by reprocessed bremsstrahlung.

Swift XRT Observations of the Afterglow of GRB 050319

Swift discovered the high-redshift GRB 050319 with the Burst Alert Telescope (BAT) and began observing with its narrow-field instruments only 225 s after the burst onset. The afterglow X-ray emission was monitored by the XRT up to 28 days after the burst. The light curve shows a decay with three different phases, each characterized by a distinct slope: an initial steep decay with a power-law index of ~5.5, a second phase characterized by a flat decay slope of ~0.54, and a third phase with a decay slope of ~1.14. During the first phase the spectral energy distribution is softer than in the following two phases, and the photon index is consistent with the GRB prompt spectrum. The extrapolation of the BAT light curve to the XRT band suggests that the initial fast-decaying phase of the XRT afterglow might be the low-energy tail of the prompt emission. The second break in the afterglow light curve occurs about 27,000 s after the burst. The spectral energy distribution before and after the second break does not change, and it can be tentatively interpreted as a jet break or the end of a delayed or continuous energy injection phase.

The phase of the radio and X-ray pulses of PSR B1937+21

We present timing and spectral results of PSR B1937+21, the fastest known millisecond pulsar (P � 1.56 ms), observed with RXTE. The pulse profile, detected up to ∼20 keV, shows a double peak with the main component much stronger than the other. The peak phase separation is 0.526 ± 0.002 and the pulsed spectrum over the energy range 2-25 keV is well described by a power law with a photon index equal to 1.14 ± 0.07. We find that the X-ray pulses are closely aligned in phase with the giant pulses observed in the radio band. This results suggest that giant radio pulses and X-ray pulses originate in the same region of the magnetosphere due to a high and fluctuating electron density that occasionally emits coherently in the radio band. The X-ray events, however, do not show any clustering in time indicating that no X-ray flares are produced.

SWIFT XRT OBSERVATIONS OF THE AFTERGLOW OF XRF 050416A

Swift discovered XRF 050416Awith the Burst Alert Telescope and began observing it with its narrow-field instrumentsonly64.5saftertheburst onset.Itsverysoftspectrumclassifiesthisevent asanX-rayflash.TheafterglowX-ray emissionwasmonitoredupto74daysaftertheburst.TheX-raylightcurveinitiallydecaysveryfast(decayslope � � 2:4), subsequently flattens (� � 0:44), and eventually steepens again (� � 0:88), similar to many X-ray afterglows. The first and second phases end � 172 and � 1450 s after the burst onset, respectively. We find evidence of spectral evolution from a softer emission with photon index � � 3:0 during the initial steep decay, to a harder emission with � � 2:0 during the following evolutionary phases. The spectra show intrinsic absorption in the host galaxy with column density of � 6:8 ; 10 21 cm � 2 . The consistency of the initial photon index with the high-energy BAT photon index suggests that the initial fast decaying phase of the X-ray light curve may be the low-energy tail of the prompt emission. The lack of jet break signatures in the X-ray afterglow light curve is not consistent with empirical relations between the source rest-frame peak energy and the collimation-corrected energy of the burst. The standard uniform jet model can give apossible descriptionof the XRF 050416A X-ray afterglow for anopeningangle largerthan a few tens of degrees, although numerical simulations show that the late-time decay is slightly flatter than expected from on-axis viewing of a uniform jet. A structured Gaussian-type jet model with uniform Lorentz factor distribution and viewing angle outside the Gaussian core is another possibility, although a full agreement with data is not achieved with the numerical models explored. Subject headingg gamma rays: bursts — X-rays: individual (XRF 050416A)

X-ray observations of the Large Magellanic Cloud pulsar PSR B0540-69 and its pulsar wind nebula

PSR B0540-69 is a young pulsar in the Large Magellanic Cloud that has similar properties with respect to the Crab Pulsar, and is embedded in a pulsar wind nebula (PWN). We have analysed the complete archival Rossi X-Ray Timing Explorer (RXTE) data set of observations of this source, together with new Swift/XRT (X-Ray Telescope) and International Gamma-Ray Astrophysics Laboratory (INTEGRAL)/IBIS (Imager on Board INTEGRAL Satellite) data. Accurate light curves are produced in various energy bands between 2 and 60 keV, showing no significant energy variations of the pulse shape. The spectral analysis shows that the pulsed spectrum is curved, and is best fitted up to 100 keV by a log-parabolic model: this strengthens the similarities with the Crab Pulsar, and is discussed in the light of a phenomenologic multicomponent model. The total emission from this source is studied, the relative contributions of the pulsar and the PWN emission are derived and discussed in the context of other INTEGRAL-detected pulsar/PWN systems.

INTEGRAL observation of the accreting pulsar 1E1145.1-6141

Aims. We analyze 1050 ks of INTEGRAL data of the high mass X-ray binary pulsar IE 1145.1-6141 to study its properties over a long time baseline, from June 2003 to June 2004, with wide spectral coverage. Methods. We study three high luminosity episodes, two of them at the system apoastron, three brightening with lower intensity, two at the periastron, and one extended period of intermediate luminosity spanning one orbital cycle. We perform a timing analysis to determine the pulse period and pulse profiles at different energy ranges. We also analyze the broad band phase averaged spectrum of different luminosity states and perform phase resolved spectroscopy for the first flare. Results. From the timing analysis, we find a pulse period of ∼297 s around MJD 53 000 with a significant scatter around the mean value. From the spectral analysis we find that the source emission can be described by an absorbed bremsstrahlung model in which the electron temperature varies between ∼25 and ∼37 keV, without any correlation with luminosity, and the intrinsic absorbing column is constantly of the order of 10 23 cm -2 . Phase resolved spectral analysis reveals a different temperature of the plasma in the ascending and descending edges of the pulse during the first flare. This justifies the pulse maximum shift by ∼0.4 phase units between 20 and 100 keV observed in the pulse profiles. Conclusions. The comparison with the previous period measurements reveals that the source is currently spinning down, in contrast to the long term secular trend observed so far, indicating that at least a temporary accretion disk is formed. The study of the spectral property variations with respect to time and spin phase suggests the presence of two emitting components at different temperatures whose relative intensity varies with time.

Background simulations for the Large Area Detector onboard LOFT

The Large Observatory For X-ray Timing (LOFT), currently in an assessment phase in the framework the ESA M3 Cosmic Vision programme, is an innovative medium-class mission specifically designed to answer fundamental questions about the behaviour of matter, in the very strong gravitational and magnetic fields around compact objects and in supranuclear density conditions. Having an effective area of ∼10 m2 at 8 keV, LOFT will be able to measure with high sensitivity very fast variability in the X-ray fluxes and spectra. A good knowledge of the in-orbit background environment is essential to assess the scientific performance of the mission and optimize the design of its main instrument, the Large Area Detector (LAD). In this paper the results of an extensive Geant-4 simulation of the instrumentwillbe discussed, showing the main contributions to the background and the design solutions for its reduction and control. Our results show that the current LOFT/LAD design is expected to meet its scientific requirement of a background rate equivalent to 10 mCrab in 2‒30 keV, achieving about 5 mCrab in the most important 2–10 keV energy band. Moreover, simulations show an anticipated modulation of the background rate as small as 10 % over the orbital timescale. The intrinsic photonic origin of the largest background component also allows for an efficient modelling, supported by an in-flight active monitoring, allowing to predict systematic residuals significantly better than the requirement of 1 %, and actually meeting the 0.25 % science goal.

The particle background of the X-IFU instrument

In this paper we are going to review the latest estimates for the particle background expected on the X-IFU instrument onboard of the ATHENA mission. The particle background is induced by two different particle populations: the so called “soft protons” and the Cosmic rays. The first component is composed of low energy particles (< 100s keV) that get funnelled by the mirrors towards the focal plane, losing part of their energy inside the filters and inducing background counts inside the instrument sensitivity band. The latter component is induced by high energy particles (> 100 MeV) that possess enough energy to cross the spacecraft and reach the detector from any direction, depositing a small fraction of their energy inside the instrument. Both these components are estimated using Monte Carlo simulations and the latest results are presented here.

Broad band spectral properties of Seyfert 1 galaxies observed with BeppoSAX

Abstract We will present some results on the broad-band observations of BeppoSAX of the bright Seyfert galaxies NGC 4151 and NGC 5548.