Alexander Stefanescu

Optical polarization of the Crab pulsar: precision measurements and comparison to the radio emission

The linear polarization of the Crab pulsar and its close environment was derived from observations with the high-speed photopolarimeter Optical Pulsar TIMing Analyser at the 2.56-m Nordic Optical Telescope in the optical spectral range (400-750 nm). Time resolution as short as 11 μs, which corresponds to a phase interval of 1/3000 of the pulsar rotation, and high statistics allow the derivation of polarization details never achieved before. The degree of optical polarization and the position angle correlate in surprising details with the light curves at optical wavelengths and at radio frequencies of 610 and 1400 MHz. Our observations show that there exists a subtle connection between presumed non-coherent (optical) and coherent (radio) emissions. This finding supports previously detected correlations between the optical intensity of the Crab and the occurrence of giant radio pulses. Interpretation of our observations requires more elaborate theoretical models than those currently available in the literature.

Fast infrared variability from a relativistic jet in GX 339−4

We present the discovery of fast infrared/X-ray correlated variability in the black hole transient GX 339-4. The source was observed with subsecond time resolution simultaneously with Very Large Telescope/Infrared Spectrometer And Array Camera and Rossi X-ray Timing Explorer/Proportional Counter Array in 2008 August, during its persistent low-flux highly variable hard state. The data show a strong correlated variability, with the infrared emission lagging the X-ray emission by 100 ms. The short time delay and the nearly symmetric cross-correlation function, together with the measured brightness temperature of similar to 2.5 x 10(6) K, indicate that the bright and highly variable infrared emission most likely comes from a jet near the black hole. Under standard assumptions about jet physics, the measured time delay can provide us a lower limit of Gamma > 2 for the Lorentz factor of the jet. This suggests that jets from stellar-mass black holes are at least mildly relativistic near their launching region. We discuss implications for future applications of this technique.

DEPFET Macropixel Detectors for MIXS: Integration and Qualification of the Flight Detectors

The Mercury imaging X-ray spectrometer (MIXS) on board of ESAs fifth cornerstone mission BepiColombo will be the first space instrument using DEpleted P-channel FET (DEPFET) based detectors. The MIXS spectrometer comprises two channels with identical focal plane detectors and is dedicated to energy resolved imaging of X-ray fluorescence from the mercurial surface. We report on the characterization, integration, and spectroscopic qualification of MIXS flight detectors. Detector chips were precharacterized at die level in order to select the best dies for integration and to do homogeneity and yield studies. Then, the detector chips were integrated to MIXS Detector Plane Arrays (DPAs), a complicated process due to the sophisticated mechanical structure, which allows the thermal decoupling of the detector from its readout and control chips. After integration, spectroscopic qualification measurements were done in order to analyze the detector performance and to prove the excellent spectroscopic performance of the DEPFET Macropixel detectors over a wide temperature range. The integration and spectroscopic qualification of all flight grade modules is now successfully completed.

Spectral performance of DEPFET and gateable DEPFET macropixel devices

Future x-ray observatories, such as the proposed ATHENA+ mission, will investigate bright and rapidly evolving radiation sources. To reach the scientific goals, high speed, spatial resolving sensors with excellent spectroscopic performance are mandatory. Well suited for this task are matrices of Depleted P-channel Field Effect Transistors (DEPFETs). DEPFETs provide intrinsic signal amplification, 100 percent fill factor, charge storage capability and a low read noise. Previous studies of DEPFET matrices of 256 × 256 pixels demonstrated an excellent energy resolution of 126 eV FWHM at 5.9 keV (compared to the theoretical Fano limit 120 eV). Usually these matrices are read out on demand, using e.g. the ASTEROID ASIC. Because the DEPFET is always sensitive, charge collected during the readout, causes so called misfits, which increase the background. For low frame rates this can be neglected. However, for fast timings, as suggested for ATHENA+, this effect reduces the spectral performance. We will present measurements on DEPFET macropixel structures, read out using a semi-Gaussian shaper, which demonstrate the excellent spectroscopic performance of these devices. Furthermore we will investigate the effect of misfits on the spectral background of DEPFET devices read out on demand. These measurements show the necessity to suppress misfits when the devices are operated for fast timing modes. As will be shown this can be done using so called gateable DEPFETs. The general advantage of gateable DEPFETs at fast timings, in terms of peak-to-background ratio will be demonstrated.

New simulation and measurement results on gateable DEPFET devices

To improve the signal to noise level, devices for optical and x-ray astronomy use techniques to suppress background events. Well known examples are e.g. shutters or frame-store Charge Coupled Devices (CCDs). Based on the DEpleted P-channel Field Effect Transistor (DEPFET) principle a so-called Gatebale DEPFET detector can be built. Those devices combine the DEPFET principle with a fast built-in electronic shutter usable for optical and x-ray applications. The DEPFET itself is the basic cell of an active pixel sensor build on a fully depleted bulk. It combines internal amplification, readout on demand, analog storage of the signal charge and a low readout noise with full sensitivity over the whole bulk thickness. A Gatebale DEPFET has all these benefits and obviates the need for an external shutter. Two concepts of Gatebale DEPFET layouts providing a built-in shutter will be introduced. Furthermore proof of principle measurements for both concepts are presented. Using recently produced prototypes a shielding of the collection anode up to 1 • 10−4 was achieved. Predicted by simulations, an optimized geometry should result in values of 1 • 10−5 and better. With the switching electronic currently in use a timing evaluation of the shutter opening and closing resulted in rise and fall times of 100ns.

Results of a pnCCD Detector System for High-Speed Optical Imaging

We present the design and optical imaging performance of a pnCCD detector system for highest frame rates and excellent sensitivity over a wide wavelength range from the UV to near IR region. To achieve frame rates higher than one thousand frames per second with an exceptionally low noise level, the devices are based on proven technology with column parallel readout and operated in a split-frame transfer mode. The CCDs are back illuminated and coated with an Anti-Reflective- Coating. The sensitivity over their full thickness of 450 &mgr;m allows for a quantum efficiency near 100% over a wide spectral range. At an optical test bench we determined the photon transfer curve, quantum efficiency and point-spread function within a wavelength region between 300 nm to 1100 nm for various detector parameter. To demonstrate the ability of a pnCCD to perform high-speed optical differential photometry, the crab nebula with the crab pulsar as central object were observed at the 1.3m SKINAKAS telescope on crete. For these observations the pnCCD was operated at a speed of 2000 frames per second. The high speed, low noise and high quantum efficiency makes this detector an ideal instrument to be used as a wavefront sensor in adaptive optics systems.

Development and characterization of new 256 × 256 pixel DEPFET detectors for X-ray astronomy

DEPFET detectors are silicon active pixel sensors for X-ray imaging spectroscopy. They will be used for the MIXS instrument of BepiColombo planetary mission and they are foreseen for the Wide Field Imager of the International X-ray Observatory currently in study with ESA, NASA and JAXA. New DEPFET matrixes with 256 × 256 pixels of 75 μm pitch have been produced, mounted on ceramic boards with dedicated front-end electronics and integrated in a new set-up able to acquire large-format images and spectra. Excellent homogeneity has been observed on two samples. Energy resolution as low as 129 eV FWHM at 5.9 keV has been obtained including all single events of the matrix back illuminated at −40°C and read out at a 300 frames per second rate. Experimental methods and results are reported.

OPTIMA: A High Time Resolution Optical Photo-Polarimeter

A high-speed photo-polarimeter, “OPTIMA” short for Optical Pulsar Timing Analyzer, has been designed and developed in the group for gamma-ray astronomy of the Max-Planck-Institut fur Extraterrestrische Physik. This sensitive, portable detector is used to observe optical emissions of sources that radiate mainly at X- and gamma-ray energies, like pulsars and other highly variable compact sources. The single photon counting instrument is based on fiber fed avalanche photodiodes (APDs), a GPS timing receiver,a CCD camera for target acquisition and a stand-alone data acquisition and control system. Several configurations are available: for photometry a hexagonal bundle with seven channels and one fiber offset for sky background monitoring; for polarimetry a rotating polarization filter in front of the photometer or a newly developed 4-channel double Wollaston system; and for coarse spectroscopy a 4-colour prism spectrograph.

Electrical characterization of different DEPFET designs on die level

For the future X-ray astronomy project Advanced Telescope for High ENergy Astrophysics plus (ATHENA+) wafer-scale DEpleted P-channel Field Effect Transistor (DEPFET) detectors are proposed as Focal Plane Array (FPA) for the Wide Field Imager (WFI). Prototype structures with different pixel layouts, each consisting of 64 x 64 pixels, were fabricated to study four different DEPFET designs. We report on the results of the electrical characterization of the different DEPFET designs. The transistor properties of the DEPFET structures are investigated in order to determine whether the design intentions are reflected in the transistor characteristics. In addition yield and homogeneity of the prototypes can be studied on die, wafer and batch level for further improvement of the production technology with regard to wafer-scale devices. These electrical characterization measurements prove to be a reliable tool to pre-select the best detector dies for further integration into full detector systems.

64-channel ASTEROID ASIC: Experimental performance and measurements with Macro Pixel arrays for X-ray astronomy

The 64 channel ASTEROID ASIC has been designed to readout DEPFET pixel matrices for astronomy. In particular ASTEROID will be the readout ASIC of the DEPFET based Macro Pixel X-ray detector for the MIXS instrument on the European Mercury exploration mission BepiColombo. For this mission a detector system with high speed readout, high energy resolution and radiation hardness properties is required. Each of the 64 analog channels implements a trapezoidal weighting function. This is the optimum time limited filter for the white series noise, which is dominant at the foreseen readout speed. The ASIC is implemented in the 0.35 μm 3.3 V AMS CMOS technology. The spectroscopic performance of ASTEROID coupled with a 64×64 Macro Pixel array prototype with pixel area of 500×500 μm2 has been tested. An energy resolution of 128.5eV FWHM with the reconstruction of all split events has been measured on the Mn-ka of a 55Fe source operating ASTEROID with a total readout time of 4.8μs at -80ºC. Furthermore radiation hardness measurement on the digital section of the ASIC, both with a 10 MeV proton source and with a 5.5 MeV a particles source, proved the SEU immunity of the DICE based SRAM cells.