Joao Braga

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.

GRB 010921: Localization and Observations by the High Energy Transient Explorer Satellite

On 2001 September 21 at 05:15:50.56 UT, the French Gamma Telescope (FREGATE) on the High Energy Transient Explorer (HETE) detected a bright gamma-ray burst (GRB). The burst was also seen by the X-detector on the Wide-field X-ray Monitor (WXM) instrument and was therefore well localized in the X-direction; however, the burst was outside the fully coded field of view of the WXM Y-detector, and therefore information on the Y-direction of the burst was limited. Cross-correlation of the HETE and Ulysses time histories yielded an Interplanetary Network (IPN) annulus that crosses the HETE error strip at an ~45° angle. The intersection of the HETE error strip and the IPN annulus produces a diamond-shaped error region for the location of the burst having an area of 310 arcmin2. Based on the FREGATE and WXM light curves, the duration of the burst is characterized by t90 = 34.2 s in the WXM 4-25 keV energy range, and 23.8 and 21.8 s in the FREGATE 6-40 and 32-400 keV energy ranges, respectively. The fluence of the burst in these same energy ranges is 4.8 × 10-6, 5.5 × 10-6, and 11.4 × 10-6 ergs cm-2, respectively. Subsequent optical and radio observations by ground-based observers have identified the afterglow of GRB 010921 and determined an apparent redshift of z = 0.450.

Physical implementation of an antimask in URA based coded mask systems

Abstract X- and gamma-ray astronomy experiments which employ rectangular URA coded masks alone show artifacts in the images reconstructed due to nonuniform background levels in the detector plane. The employment of a separate antimask in addition to the mask in observations is useful to eliminate this problem. We propose here a method to implement the antimask with the same mask, utilizing the antisymmetric properties in the mask pattern, thereby avoiding the need for a separate antimask in an experiment. Simulations performed with this mask-antimask system are presented to show its advantages.

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.

Development of an attitude control system for a balloon-borne gamma ray telescope

Abstract We describe the attitude control system employed by the MASCO balloon-borne gamma ray telescope and present the first laboratory tests of this system. The MASCO experiment is a low energy gamma ray imaging telescope that employs a MURA coded mask. Its angular resolution is 14′ over a 13° field of view, which requires a pointing accuracy of some arcminutes in order to allow the accomplishment of the scientific goals. The attitude control system was designed to provide pointing and stabilization of a 1400 kg, 7 m-high, 2 m-wide, 2 m-deep gondola that encompasses the telescope detector system and its associated electronics. The sensors used are an electronic magnetic compass, an encoder, a two-axis solar sensor, a sun-tracker, two accelerometers, a gyroscope and three CCD stellar sensors. A GPS system is also employed. The actuators are a reaction wheel for azimuth control, an azimuth motor located in the balloon-gondola decoupling mechanism for desaturation of the reaction wheel, and a telescope elevation motor with a harmonic drive reduction. The software architecture and the operation modes are also presented.

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.

Confirming the thermal Comptonization model for black hole X-ray emission in the low-hard state

Hard X-ray spectra of black hole binaries in the low/hard state are well modeled by thermal Comptonization of soft seed photons by a corona-type region with

THE CCD STELLAR SENSOR OF THE MASCO TELESCOPE POINTING SYSTEM

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INTEGRAL observations of Scorpius X-1: evidence for Comptonization up to 200 keV

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