Flavio DAmico

Optical polarimetry and infrared photometry of two AM Her binaries: 1RXS J161008.0+035222 and 1RXS J231603.9−052713

We present the first optical circular and linear polarizatio n measurements of two polar candidates from ROSAT: 1RXS J161008.0+035222 and 1RXS J231603.9 052713. We also present H band near-infrared photometry of the last object. The presence of strong circular polarization confirms them as AM Her systems. 1RXS J231603.9 052713 was observed in two different brightness states. The orbital phase dependence of the flux and polarization of 1RXS J161008.0+035222 is reasonably fitted with a simple mod el in which the binary is observed at a small inclination and the magnetic field axis is al most parallel to the white-dwarf rotation axis resulting in the accretion column axis being s een from top during the whole orbital revolution. An alternative geometry with intermediate inclination can fit the observed flux and circular polarization. However, in this case, the mo del produces a linear polarization peak which is not corroborated by the data. The estimated magnetic field is in the 10 to 20 MG range. The circular polarization of 1RXS J231603.9 052713 is complex and highly variable. The light-curves of that object have been fitted using a model which includes the white-dwarf, a heated secondary and a point-like accretion region. The secondary emission contributes significantly even in optical wavelengths. This model also repr oduces the main features of the optical polarization of 1RXS J231603.9 052713. We estimate the main parameters of the binary, of the accretion region and the distance to the syste m. An improved description of this system should include an extended and inhomogeneous accreting region as well as non-radial accretion.

A new mask-antimask coded-aperture telescope for hard X-ray astronomy

A new imaging balloon-borne telescope for hard X-rays in the energy range from 30 to 100 keV is described. The imaging capability is provided by the use of an extended URA-based coded-mask. With only one motor and suitable stop pins, we can rotate a carbon-fiber wheel with most of the mask elements attached to it by 180°, and a bar, which is also part of the mask pattern and is allowed to rotate freely over the wheel, by 90°; this combined rotation creates an antimask of the original mask, except for the central element. This is a novel and elegant manner of providing an antimask without additional weight and complex mechanical manipulations. We show that the use of antimasks is a very effective method of eliminating systematic variations in the background map over the position-sensitive detector area. The expected sensitivity of the instrument for the 30–100 keV range is of the order of 7 × 10-5 photons cm-2 s-1 keV-1, for an integration time of 104 seconds at a residual atmosphere of 3.5 g cm-2. This telescope will provide imaging observations of bright galactic hard X-ray sources with an angular resolution of ∼2° in a 10° by 10° FOV, which is defined by a collimator placed in front of the detector system. We are particularly interested in the galactic center region, where recent imaging results in X-rays have shown the presence of an interesting source field. Results of computer simulations of the imaging system are reported.

MIRAX: a Brazilian X-ray astronomy satellite mission

We describe the “Monitor e Imageador de Raios-X” (MIRAX), an X-ray astronomy satellite mission proposed by the high-energy astrophysics group at the National Institute for Space Research (INPE) in Brazil to the Brazilian Space Agency. MIRAX is an international collaboration that includes, besides INPE, the University of California San Diego, the University of Tubingen in Germany, the Massachusetts Institute of Technology and the Space Research Organization Netherlands. The payload of MIRAX will consist of two identical hard X-ray cameras (10–200 keV) and one soft X-ray camera (2–28 keV), both with angular resolution of ∼5–7′. The basic objective of MIRAX is to carry out continuous broadband imaging spectroscopy observations of a large source sample (∼9 months/yr) in the central Galactic plane region. This will allow the detection, localization, possible identification, and spectral/temporal study of the entire history of transient phenomena to be carried out in one single mission. MIRAX will have sensitivities of ∼ 5 mCrab/day in the 2–10 keV band (∼2 times better than the All Sky Monitor on Rossi X-ray Timing Explorer) and 2.6 mCrab/day in the 10–100 keV band (∼40 times better than the Earth Occultation technique of the Burst and Transient Source Experiment on the Compton Gamma-Ray Observatory). The MIRAX spacecraft will weigh about 200 kg and is expected to be launched in a low-altitude (∼600 km) circular equatorial orbit around 2007/2008.

A mura-based coded mask telescope

Abstract A high-energy telescope that employs a Modified Uniformly Redundant Array (MURA) coded mask is described. The imaging device is a 19×19 element square MURA-based extended mask mounted in a single mask-antimask configuration. This is the first experiment to use such a mask pattern and configuration for astrophysical purposes. The detector system consists of a 41 cm diameter, 5 cm thick NaI(Tl) crystal coupled to 19 photomultipliers. The anticoincidence system is composed of plastic scintillators on the sides and a NaI(Tl) crystal at the bottom. The angular resolution is approximately 14′ over a 13° field of view. The expected 3 σ sensitivity for an on-axis source observed for 104 s at a residual atmosphere of 3.5 g.cm−2 is 1.4 × 10−5 photons cm−2 s−1 keV−1 at 100 keV and 1.0 × 10−6 photons cm−2 s−1 keV−1 at 1 MeV.

Telescope performance and image simulations of the balloon-borne coded-mask protoMIRAX experiment

Abstract In this work we present the results of imaging simulations performed with the help of the GEANT4 package for the protoMIRAX hard X-ray balloon experiment. The instrumental background was simulated taking into account the various radiation components and their angular dependence, as well as a detailed mass model of the experiment. We modeled the meridian transits of the Crab Nebula and the Galactic Centre region during balloon flights in Brazil ( ∼ − 23 ° of latitude and an altitude of ∼ 40 km ) and introduced the correspondent spectra as inputs to the imaging simulations. We present images of the Crab and of three sources in the Galactic Centre region: 1E 1740.7–2942, GRS 1758–258 and GX 1+4. The results show that the protoMIRAX experiment is capable of making spectral and timing observations of bright hard X-ray sources as well as important imaging demonstrations that will contribute to the design of the MIRAX satellite mission.

An overview of the MASCO balloon-borne gamma-ray experiment

We present a detailed description of the technical characteristics of the MASCO balloon-borne gammaray experiment, which is an instrument designed to obtain images of cosmic sources with a 14-arcminute resolution over an ~ 14-degree circular field of view. It operates between 50 keV and 1.8 MeV. The MASCO experiment employs a Modified Uniformly Redundant Array coded mask and a NaI(T1) position sensitive detector. The attitude control system was designed to provide a few arcminutes pointing and stabilization for a - 2,000 kg, 7-m high, 2-m wide, 2-m deep gondola that encompasses the telescope and its associated electronics. We describe the imaging system, the attitude control system, and the gondola where the telescope is mounted on. We also show some results of new tests performed with the instrument: the 1-a detector spatial resolution in the X-axis is 1.04 cm and 1.14 cm in the Y-axis for an Am 241 source; for a Na 22 source, the spatial resolution in the X-axis is 1.67 cm and 1.39 cm in the Y-axis; the attitude control system tests showed that the instrument is able to maintain a target locked with at least 5-arcminute accuracy.

PREFLIGHT TESTS OF THE MASCO TELESCOPE

We present some of the preflight tests carried out with the MASCO balloon-borne imaging gamma ray experiment in order to test the detector system and associated electronics employed by the telescope. The detector system is composed by a 41 cm-diameter, 5 cm-thick NaI(Tl) crystal surrounded by plastic scintillators on the top and on the sides, for shielding. A similar NaI(Tl) is used at the bottom with the same purpose. The imaging device capability is provided by an extended 19×19 MURA-based coded mask placed 305 cm away from the detector plane which is mounted in such a way that it becomes an antimask with a 90 degree rotation. The tests determined the position sensitive detector diameter to be approximately 24 cm and showed that it is possible to do imaging in a fully coded circular (14.2 degree-diameter) field of view with a positioning power of 4.5 arcminutes for a 5 σ source. The measured energy resolution was about 10% at 662 keV and the spatial resolution was approximately 10 mm at 100 keV. We have made a series of laboratory images using a 320 mCi 137Cs radioactive source to test the effectiveness of the mask-antimask subtraction technique and obtained a 60% improvement in signal-to-noise ratio of the images. Temperature tests of the on board electronics were carried out and the results of the peak detector circuits tests are presented.

Development of the hard X-ray imaging experiment timax: Laboratory images and first balloon flight

We present calibration results and laboratory images produced by the balloon-borne hard X-ray imaging telescope TIMAX. The images were produced with an241Am radioactive source placed 45 m away from the detector plane, in the center of the field of view. It is shown that the mask 3-antimask imaging reconstruction process, when combined with flat-fielding techniques, is very effective at recovering signal-to-noise ratio lost due to systematic non-uniformity in the background measured by the 35 detectors. The experiment was launched in June 8th, 1993 from Birigüi, SP, Brazil, onboard a 186,000 m3 stratospheric balloon, and remained at an atmospheric depth of ∼2 g cm−2su for ∼8 hours. Even though no scientific data were gathered in this first flight, we obtained valuable engineering data and could also calculate the sensitivity of the experiment based on the instrumental background spectrum at balloon altitudes. In the 60–70 keV energy band, the experiment can detect 3σ sources at a level of ∼1.2 x 10−4 photons cm−2 s−1 keV−1 for an integration of 6 hours at 2.1 g cm−2.

The high resolution X-ray imaging detector planes for the MIRAX mission

The MIRAX X-ray observatory, the first Brazilian-led astrophysics space mission, is designed to perform an unprecedented wide-field, wide-band hard X-ray (5?200 keV) survey of Galactic X-ray transient sources. In the current configuration, MIRAX will carry a set of four coded-masks telescopes with high spatial resolution Cadmium Zinc Telluride (CZT) detector planes, each one consisting of an array of 64 closely tiled CZT pixelated detectors. Taken together, the four telescopes will have a total detection area of 959 cm2, a large field of view (60? ? 60? FWHM), high angular resolution for this energy range (6 arcmin) and very good spectral resolution ( ~ 2 keV @ 60 keV). A stratospheric balloon-borne prototype of one of the MIRAX telescopes has been developed, tested and flown by the Harvard-Smithsonian Center for Astrophysics (CfA) as part of the ProtoEXIST program. In this paper we show results of validation and calibration tests with individual CZT detectors of the ProtoEXIST second generation experiment (P2). Each one of 64 detector units of the P2 detector plane consists of an ASIC, developed by Caltech for the NuSTAR telescope, hybridized to a CZT crystal with 0.6 mm pixel size. The performance of each detector was evaluated using radioactive sources in the laboratory. The calibration results show that the P2 detectors have average energy resolution of ~ 2.1 keV @ 60 keV and 2.3 @ 122 keV. P2 was also successfully tested on near-space environment on a balloon flight, demonstrating the detector unit readiness for integration on a space mission telescope, as well as satisfying all MIRAX mission requirements.

Background and imaging simulations for the hard X-ray camera of the MIRAX mission

We report the results of detailed Monte Carlo simulations of the performance expected both at balloon altitudes and at the probable satellite orbit of a hard X-ray coded-aperture camera being developed for the MIRAX mission. Based on a thorough mass model of the instrument and detailed specifications of the spectra and angular dependence of the various relevant radiation fields at both the stratospheric and orbital environments, we have used the well-known package GEANT4 to simulate the instrumental background of the camera. We also show simulated images of source fields to be observed and calculated the detailed sensitivity of the instrument in both situations. The results reported here are especially important to researchers in this field considering that we provide important information, not easily found in the literature, on how to prepare input files and calculate crucial instrumental parameters to perform GEANT4 simulations for high-energy astrophysics space experiments.