Search

The ePix detector family provides multiple variants of hybrid pixel detectors to support a wide range of applications at free electron laser facilities. We present the results of a systematic study of the influence of radiation induced damage on the performance and lifetime of an ePix100a detector module using a direct attenuated beam of the EuXFEL at 9 keV photon energy and an average power of 10 $\mu$W. An area of 20 x 20 pixels was irradiated with an average photon flux of approx. 7 x $10^{9}$ photons/s to a dose of approximately 760$\pm$65 kGy at the location of the Si/SiO$_2$ interfaces in the sensor. A dose dependent increase in both offset and noise of the ePix100a detector have been observed originating from an increase of the sensor leakage current. Moreover, we observed an effect directly after irradiation resulting in the saturation of individual pixels by their dark current. Changes in gain are evaluated one and half hours post irradiation and suggest damage to occur also on the ASIC level. Based on the obtained results, thresholds for beam parameters are deduced and the detector lifetime is estimated with respect to the requirements to the data quality in order to satisfy the scientific standards defined by the experiments. We conclude the detector can withstand a beam with an energy up to 1 $\mu$J at a photon energy of 9 keV impacting on an area of 1 mm$^2$. The detector can be used without significant degradation of its performance for several years if the incident photon beam intensities do not exceed the detector's dynamic range by at least three orders of magnitude. Our results provide valuable input for the operation of the ePix100a detector at FEL facilities and for the design of future detector technology., Comment: 25 pages, 15 figures, accepted for publication by the Journal of Instrumentation

The European X-ray Free Electron Laser (EuXFEL) is a research facility providing spatially coherent X-ray flashes in the energy range from 0.25keV to 25keV of unprecedented brilliance and with unique time structure: X-ray pulses with a 4.5 MHz repetition rate arranged in trains with 2700 pulses every 100 ms. The facility operates three photon beamlines called SASE 1, SASE 2 and SASE 3. Each of the beamlines is hosting two scientific experiments. The SASE 1 beamline started its user operation in September 2017, followed by successful first lasing at the SASE 2 beamline in May 2018. Early user experiments are planned to start in 2019 at this beamline, while early user experiments for the SASE 3 beamline are scheduled for the end of 2018. The quality of the experimental data will gain substantial benefits from an accurate characterization and calibration of the X-ray detectors. Supplementing high repetition rate detectors at MHz speeds, slower detectors such as the ePix100a and the FastCCD will be operated at the train repetition rate of 10 Hz. These 2D silicon pixelized detectors use fast parallel column-wise readout implemented as a CCD or as a hybrid pixel detector. In the following, characterization and analysis approaches for the FastCCD and the ePix100a detectors are discussed and the performance of the detectors is evaluated using appropriate state-of-the-art analysis techniques., Comment: 9 pages to be published in the proceedings of the 20th International Workshop on Radiation Imaging Detectors in Journal of Instrumentation

The European X-ray Free Electron Laser (XFEL.EU) will provide unprecedented peak brilliance and ultra-short and spatially coherent X-ray pulses in an energy range of 0.25 to 25 keV . The pulse timing structure is unique with a burst of 2700 pulses of 100 fs length at a temporal distance of 220 ns followed by a 99.4 ms gap. To make optimal use of this timing structure and energy range a great variety of detectors are being developed for use at XFEL.EU, including 2D X-ray imaging cameras that are able to detect images at a rate of 4.5 MHz, provide dynamic ranges up to 10$^5$ photons per pulse per pixel under different operating conditions and covering a large range of angular resolution. In order to characterize, commission and calibrate this variety of detectors and for testing of detector prototypes the XFEL.EU detector group is building up an X-ray test laboratory that allows testing of detectors with X-ray photons under conditions that are as similar to the future beam line conditions at the XFEL.EU as is possible with laboratory sources. A total of four test environments provide the infrastructure for detector tests and calibration: two portable setups that utilize low power X-ray sources and radioactive isotopes, a test environment where a commercial high power X-ray generator is in use, and a pulsed X-ray/electron source which will provide pulses as short as 25 ns in XFEL.EU burst mode combined with target anodes of different materials. The status of the test environments, three of which are already in use while one is in commissioning phase, will be presented as well as first results from performance tests and characterization of the sources., Comment: 8 pages, 7 figures including corrected captions with included copyright information also to be published in an erratum

The European X-ray Free Electron Laser is a high-intensity X-ray light source currently being constructed in the area of Hamburg, that will provide spatially coherent X-rays in the energy range between $0.25\,\mathrm{keV}$ and $25\,\mathrm{keV}$. The machine will deliver $10\,\mathrm{trains/s}$, consisting of up to $2700\,\mathrm{pulses}$, with a $4.5\,\mathrm{MHz}$ repetition rate. The LPD, DSSC and AGIPD detectors are being developed to provide high dynamic-range Mpixel imaging capabilities at the mentioned repetition rates. A consequence of these detector characteristics is that they generate raw data volumes of up to $15\,\mathrm{Gbyte/s}$. In addition the detector's on-sensor memory-cell and multi-/non-linear gain architectures pose unique challenges in data correction and calibration, requiring online access to operating conditions and control settings. We present how these challenges are addressed within XFEL's control and analysis framework Karabo, which integrates access to hardware conditions, acquisition settings (also using macros) and distributed computing. Implementation of control and calibration software is mainly in Python, using self-optimizing (py) CUDA code, numpy and iPython parallels to achieve near-real time performance for calibration application., Comment: Proceeding ICALEPS 2015

The CERN Axion Solar Telescope (CAST) searches for $a\to\gamma$ conversion in the 9 T magnetic field of a refurbished LHC test magnet that can be directed toward the Sun. Two parallel magnet bores can be filled with helium of adjustable pressure to match the X-ray refractive mass $m_\gamma$ to the axion search mass $m_a$. After the vacuum phase (2003--2004), which is optimal for $m_a\lesssim0.02$ eV, we used $^4$He in 2005--2007 to cover the mass range of 0.02--0.39 eV and $^3$He in 2009--2011 to scan from 0.39--1.17 eV. After improving the detectors and shielding, we returned to $^4$He in 2012 to investigate a narrow $m_a$ range around 0.2 eV ("candidate setting" of our earlier search) and 0.39--0.42 eV, the upper axion mass range reachable with $^4$He, to "cross the axion line" for the KSVZ model. We have improved the limit on the axion-photon coupling to $g_{a\gamma}< 1.47\times10^{-10} {\rm GeV}^{-1}$ (95% C.L.), depending on the pressure settings. Since 2013, we have returned to vacuum and aim for a significant increase in sensitivity., Comment: CAST Collaboration 6 pages 3 figures

The CERN Axion Solar Telescope (CAST) has finished its search for solar axions with 3^He buffer gas, covering the search range 0.64 eV < m_a <1.17 eV. This closes the gap to the cosmological hot dark matter limit and actually overlaps with it. From the absence of excess X-rays when the magnet was pointing to the Sun we set a typical upper limit on the axion-photon coupling of g_ag < 3.3 x 10^{-10} GeV^{-1} at 95% CL, with the exact value depending on the pressure setting. Future direct solar axion searches will focus on increasing the sensitivity to smaller values of g_a, for example by the currently discussed next generation helioscope IAXO., Comment: 5 pages, 2 figures. Last version uploaded

In non-hadronic axion models, which have a tree-level axion-electron interaction, the Sun produces a strong axion flux by bremsstrahlung, Compton scattering, and axio-recombination, the "BCA processes." Based on a new calculation of this flux, including for the first time axio-recombination, we derive limits on the axion-electron Yukawa coupling g_ae and axion-photon interaction strength g_ag using the CAST phase-I data (vacuum phase). For m_a < 10 meV/c2 we find g_ag x g_ae< 8.1 x 10^-23 GeV^-1 at 95% CL. We stress that a next-generation axion helioscope such as the proposed IAXO could push this sensitivity into a range beyond stellar energy-loss limits and test the hypothesis that white-dwarf cooling is dominated by axion emission.

The status of the solar axion search with the CERN Axion Solar Telescope (CAST) will be presented. Recent results obtained by the use of $^3$He as a buffer gas has allowed us to extend our sensitivity to higher axion masses than our previous measurements with $^4$He. With about 1 h of data taking at each of 252 different pressure settings we have scanned the axion mass range 0.39 eV$ \le m_{a} \le $ 0.64 eV. From the absence of an excess of x rays when the magnet was pointing to the Sun we set a typical upper limit on the axion-photon coupling of g$_{a\gamma} \le 2.3\times 10^{-10}$ GeV$^{-1}$ at 95% C.L., the exact value depending on the pressure setting. CAST published results represent the best experimental limit on the photon couplings to axions and other similar exotic particles dubbed WISPs (Weakly Interacting Slim Particles) in the considered mass range and for the first time the limit enters the region favored by QCD axion models. Preliminary sensitivities for axion masses up to 1.16 eV will also be shown reaching mean upper limits on the axion-photon coupling of g$_{a\gamma} \le 3.5\times 10^{-10}$ GeV$^{-1}$ at 95% C.L. Expected sensibilities for the extension of the CAST program up to 2014 will be presented. Moreover long term options for a new helioscope experiment will be evoked., Comment: 4 pages, 2 pages, to appear in the proceedings of the 24th Rencontres de Blois V2 A few affiliations were not corrected in previous version V3 Author added

Ongoing investigations for the improvement of Geant4 accuracy and computational performance resulting by refactoring and reengineering parts of the code are discussed. Issues in refactoring that are specific to the domain of physics simulation are identified and their impact is elucidated. Preliminary quantitative results are reported., Comment: To be published in the Proc. CHEP (Computing in High Energy Physics) 2012

Recent efforts for the improvement of the accuracy of physics data libraries used in particle transport are summarized. Results are reported about a large scale validation analysis of atomic parameters used by major Monte Carlo systems (Geant4, EGS, MCNP, Penelope etc.); their contribution to the accuracy of simulation observables is documented. The results of this study motivated the development of a new atomic data management software package, which optimizes the provision of state-of-the-art atomic parameters to physics models. The effect of atomic parameters on the simulation of radioactive decay is illustrated. Ideas and methods to deal with physics models applicable to different energy ranges in the production of data libraries, rather than at runtime, are discussed., Comment: To be published in the Proc. of CHEP (Computing in High Energy Physics) 2012

The assessment of the reliability of Monte Carlo simulations is discussed, with emphasis on uncertainty quantification and the related impact on experimental results. Methods and techniques to account for epistemic uncertainties, i.e. for intrinsic knowledge gaps in physics modeling, are discussed with the support of applications to concrete experimental scenarios. Ongoing projects regarding the investigation of epistemic uncertainties in the Geant4 simulation toolkit are reported., Comment: To be published in the Proceedings of the 13th ICATPP Conference on Astroparticle, Particle, Space Physics and Detectors for Physics Applications, Villa Olmo, Como, 3-7 October 2011

The CERN Axion Solar Telescope (CAST) has extended its search for solar axions by using 3He as a buffer gas. At T=1.8 K this allows for larger pressure settings and hence sensitivity to higher axion masses than our previous measurements with 4He. With about 1 h of data taking at each of 252 different pressure settings we have scanned the axion mass range 0.39 eV < m_a < 0.64 eV. From the absence of excess X-rays when the magnet was pointing to the Sun we set a typical upper limit on the axion-photon coupling of g_ag < 2.3 x 10^{-10} GeV^{-1} at 95% CL, the exact value depending on the pressure setting. KSVZ axions are excluded at the upper end of our mass range, the first time ever for any solar axion search. In future we will extend our search to m_a < 1.15 eV, comfortably overlapping with cosmological hot dark matter bounds., Comment: 5 pages, 2 figures, updated author list, revised arguments, results unchanged

We present {\gamma} spectroscopy validation measurements for the Geant4 radioactive decay simulation for a selected range of isotopes using a simple experimental setup. Using these results we point out problems in the decay simulation and where they may originate from., Comment: 4 pages, to appear in proceedings of the Joint International Conference on Supercomputing in Nuclear Applications and Monte Carlo 2010

The role of data libraries as a collaborative tool across Monte Carlo codes is discussed. Some new contributions in this domain are presented; they concern a data library of proton and alpha ionization cross sections, the development in progress of a data library of electron ionization cross sections and proposed improvements to the EADL (Evaluated Atomic Data Library), the latter resulting from an extensive data validation process., Comment: 4 pages, to appear in proceedings of the Nuclear Science Symposium and Medical Imaging Conference 2010, Knoxville

Ongoing investigations to introduce software techniques suitable to support new experimental requirements for multi-scale simulation are discussed., Comment: 2 pages, to appear in proceedings of the Nuclear Science Symposium and Medical Imaging Conference 2010, Knoxville

The Geant4 toolkit offers a rich variety of electromagnetic physics models; so far the evaluation of this Geant4 domain has been mostly focused on its physics functionality, while the features of its design and their impact on simulation accuracy, computational performance and facilities for verification and validation have not been the object of comparable attention yet, despite the critical role they play in many experimental applications. A new project is in progress to study the application of new design concepts and software techniques in Geant4 electromagnetic physics, and to evaluate how they can improve on the current simulation capabilities. The application of a policy-based class design is investigated as a means to achieve the objective of granular decomposition of processes; this design technique offers various advantages in terms of flexibility of configuration and computational performance. The current Geant4 physics models have been re-implemented according to the new design as a pilot project. The main features of the new design and first results of performance improvement and testing simplification are presented; they are relevant to many Geant4 applications, where computational speed and the containment of resources invested in simulation production and quality assurance play a critical role., Comment: 4 pages, 4 figures and images, to appear in proceedings of the Nuclear Science Symposium and Medical Imaging Conference 2009, Orlando

A R&D project has been launched in 2009 to address fundamental methods in radiation transport simulation and revisit Geant4 kernel design to cope with new experimental requirements. The project focuses on simulation at different scales in the same experimental environment: this set of problems requires new methods across the current boundaries of condensed-random-walk and discrete transport schemes. An exploration is also foreseen about exploiting and extending already existing Geant4 features to apply Monte Carlo and deterministic transport methods in the same simulation environment. An overview of this new R&D associated with Geant4 is presented, together with the first developments in progress., Comment: 4 pages, to appear in proceedings of the Nuclear Science Symposium and Medical Imaging Conference 2009, Orlando

The anticipated high sensitivity and the science goals of the next generation X-ray space missions, like the International X-ray Observatory or Simbol-X, rely on a low instrumental background, which in turn requires optimized shielding concepts. We present Geant4 based simulation results on the IXO Wide Field Imager cosmic ray proton induced background in comparison with previous results obtained for the Simbol-X LED and HED focal plane detectors. Our results show that an improvement in mean differential background flux compared to actually operating X-ray observatories may be feasible with detectors based on DEPFET technology. In addition we present preliminary results concerning the validation of Geant4 based radioactive decay simulation in space applications as a part of the Nano5 project., Comment: 6 pages, 7 figures and images, to appear in proceedings of the Nuclear Science Symposium and Medical Imaging Conference 2009, Orlando

We have searched for 14.4 keV solar axions or more general axion-like particles (ALPs), that may be emitted in the M1 nuclear transition of 57Fe, by using the axion-to-photon conversion in the CERN Axion Solar Telescope (CAST) with evacuated magnet bores (Phase I). From the absence of excess of the monoenergetic X-rays when the magnet was pointing to the Sun, we set model-independent constraints on the coupling constants of pseudoscalar particles that couple to two photons and to a nucleon g_{a\gamma} |-1.19 g_{aN}^{0}+g_{aN}^{3}|<1.36\times 10^{-16} GeV^{-1} for m_{a}<0.03 eV at the 95% confidence level., Comment: 23 pages, 5 figures. Revised version of the paper after referee`s comments. Main changes in the Measurement and data analysis section

We present the results of a search for a high-energy axion emission signal from 7Li (0.478 MeV) and D(p,gamma)3He (5.5 MeV) nuclear transitions using a low-background gamma-ray calorimeter during Phase I of the CAST experiment. These so-called "hadronic axions" could provide a solution to the long-standing strong-CP problem and can be emitted from the solar core from nuclear M1 transitions. This is the first such search for high-energy pseudoscalar bosons with couplings to nucleons conducted using a helioscope approach. No excess signal above background was found., Comment: 20 pages, 8 figures, final version to be published in JCAP

We have searched for solar axions or other pseudoscalar particles that couple to two photons by using the CERN Axion Solar Telescope (CAST) setup. Whereas we previously have reported results from CAST with evacuated magnet bores (Phase I), setting limits on lower mass axions, here we report results from CAST where the magnet bores were filled with \hefour gas (Phase II) of variable pressure. The introduction of gas generated a refractive photon mass $m_\gamma$, thereby achieving the maximum possible conversion rate for those axion masses \ma that match $m_\gamma$. With 160 different pressure settings we have scanned \ma up to about 0.4 eV, taking approximately 2 h of data for each setting. From the absence of excess X-rays when the magnet was pointing to the Sun, we set a typical upper limit on the axion-photon coupling of $\gag\lesssim 2.17\times 10^{-10} {\rm GeV}^{-1}$ at 95% CL for $\ma \lesssim 0.4$ eV, the exact result depending on the pressure setting. The excluded parameter range covers realistic axion models with a Peccei-Quinn scale in the neighborhood of $f_{\rm a}\sim10^{7}$ GeV. Currently in the second part of CAST Phase II, we are searching for axions with masses up to about 1.2 eV using \hethree as a buffer gas., Comment: 18 pages, 7 figures. Revised version of the paper after referee's comments. Main changes on the gas section

The CAST (CERN Axion Solar Telescope) experiment is searching for solar axions by their conversion into photons inside the magnet pipe of an LHC dipole. The analysis of the data recorded during the first phase of the experiment with vacuum in the magnet pipes has resulted in the most restrictive experimental limit on the coupling constant of axions to photons. In the second phase, CAST is operating with a buffer gas inside the magnet pipes in order to extent the sensitivity of the experiment to higher axion masses. We will present the first results on the $^{4}{\rm He}$ data taking as well as the system upgrades that have been operated in the last year in order to adapt the experiment for the $^{3}{\rm He}$ data taking. Expected sensitivities on the coupling constant of axions to photons will be given for the recent $^{3}{\rm He}$ run just started in March 2008., Comment: Proceedings of the ICHEP 2008 conference

We have started the development of a detector system, sensitive to single photons in the eV energy range, to be suitably coupled to one of the CAST magnet ports. This system should open to CAST a window on possible detection of low energy Axion Like Particles emitted by the sun. Preliminary tests have involved a cooled photomultiplier tube coupled to the CAST magnet via a Galileian telescope and a switched 40 m long optical fiber. This system has reached the limit background level of the detector alone in ideal conditions, and two solar tracking runs have been performed with it at CAST. Such a measurement has never been done before with an axion helioscope. We will present results from these runs and briefly discuss future detector developments., Comment: Paper submitted to the proceedings of the "4th Patras Workshop on Axions, WIMPs and WISPs", DESY, Hamburg Site - Germany, 18-21 June 2008. Author affiliations are reported on the title page of the paper. In version 2: 1 affiliation change, 3 references added

Using the Geant4 toolkit, a Monte-Carlo code to simulate the detector background of the Simbol-X focal plane instrument has been developed with the aim to optimize the design of the instrument. Structural design models of the mirror and detector satellites have been built and used as baseline for our simulations, to evaluate the different background contributions that must be taken into account to determine the sensitivity of the Simbol-X detectors. We work towards a simulation based background and mass model which can be used before and during the mission. For different material compositions, material thicknesses, locations etc. the response of the instrument to the diffuse cosmic hard X-ray background and to the cosmic proton induced background have been calculated. As a result we present estimates of the background count rate expected in the low and high energy detector, and anti-coincidence rates. The effect of induced radioactivity in the detector and shielding materials and soft proton scattering in the mirror shells are also under study., Comment: 5 pages, 5 figures and images, to appear in proceedings of "Simbol-X: the hard X-ray Universe in focus", held in Bologna (Italy), May 14-16, 2007

Theoretical axion models state that axions are very weakly interacting particles. In order to experimentally detect them, the use of colorful and inspired techniques becomes mandatory. There is a wide variety of experimental approaches that were developed during the last 30 years, most of them make use of the Primakoff effect, by which axions convert into photons in the presence of an electromagnetic field. We review the experimental techniques used to search for axions and will give an outlook on experiments planned for the near future., Comment: 38 pages, 26 figures and images, to appear in the Lecture Notes in Physics volume on Axions (Springer Verlag)

The CERN Axion Solar Telescope (CAST) experiment searches for axions from the Sun converted into photons with energies up to around 10 keV via the inverse Primakoff effect in the high magnetic field of a superconducting Large Hadron Collider (LHC) prototype magnet. A backside illuminated pn-CCD detector in conjunction with an X-ray mirror optics is one of the three detectors used in CAST to register the expected photon signal. Since this signal is very rare and different background components (environmental gamma radiation, cosmic rays, intrinsic radioactive impurities in the set-up, ...) entangle it, a detailed study of the detector background has been undertaken with the aim to understand and further reduce the background level of the detector. The analysis is based on measured data taken during the Phase I of CAST and on Monte Carlo simulations of different background components. This study will show that the observed background level (at a rate of (8.00+-0.07)10^-5 counts/cm^2/s/keV between 1 and 7 keV) seems to be dominated by the external gamma background due to usual activities at the experimental site, while radioactive impurities in the detector itself and cosmic neutrons could make just smaller contribution., Comment: Comments: 10 pages, 9 figures and images, submitted to Astroparticle Physics

The Cern Axion Solar Telescope (CAST) is in operation and taking data since 2003. The main objective of the CAST experiment is to search for a hypothetical pseudoscalar boson, the axion, which might be produced in the core of the sun. The basic physics process CAST is based on is the time inverted Primakoff effect, by which an axion can be converted into a detectable photon in an external electromagnetic field. The resulting X-ray photons are expected to be thermally distributed between 1 and 7 keV. The most sensitive detector system of CAST is a pn-CCD detector combined with a Wolter I type X-ray mirror system. With the X-ray telescope of CAST a background reduction of more than 2 orders off magnitude is achieved, such that for the first time the axion photon coupling constant g_agg can be probed beyond the best astrophysical constraints g_agg < 1 x 10^-10 GeV^-1., Comment: 19 pages, 25 figures and images, replaced by the revised version accepted for publication in New Journal of Physics

The CERN Axion Solar Telescope (CAST) searches for solar axions employing a 9 Tesla superconducting dipole magnet equipped with 3 independent detection systems for X-rays from axion-photon conversions inside the 10 m long magnetic field. Results of the first 6 months of data taking in 2003 imply a 95 % CL upper limit on the axion-photon coupling constant of 1.16x10(-10) GeV(-1) for axion masses < 0.02 eV. The most sensitive detector of CAST is a X-ray telescope consisting of a Wolter I type mirror system and a fully depleted pn-CCD as focal plane detector. Exploiting the full potential of background suppression by focussing X-rays emerging from the magnet bore, the axion sensitivity obtained with telescope data taken in 2004, for the first time in a controlled laboratory experiment, will supersede axion constraints derived from stellar energy loss arguments., Comment: 5 pages, 9 figures

The CAST experiment at CERN (European Organization of Nuclear Research) searches for axions from the sun. The axion is a pseudoscalar particle that was motivated by theory thirty years ago, with the intention to solve the strong CP problem. Together with the neutralino, the axion is one of the most promising dark matter candidates. The CAST experiment has been taking data during the last two years, setting an upper limit on the coupling of axions to photons more restrictive than from any other solar axion search in the mass range below 0.1 eV. In 2005 CAST will enter a new experimental phase extending the sensitivity of the experiment to higher axion masses. The CAST experiment strongly profits from technology developed for high energy physics and for X-ray astronomy: A superconducting prototype LHC magnet is used to convert potential axions to detectable X-rays in the 1-10 keV range via the inverse Primakoff effect. The most sensitive detector system of CAST is a spin-off from space technology, a Wolter I type X-ray optics in combination with a prototype pn-CCD developed for ESA's XMM-Newton mission. As in other rare event searches, background suppression and a thorough shielding concept is essential to improve the sensitivity of the experiment to the best possible. In this context CAST offers the opportunity to study the background of pn-CCDs and its long term behavior in a terrestrial environment with possible implications for future space applications. We will present a systematic study of the detector background of the pn-CCD of CAST based on the data acquired since 2002 including preliminary results of our background simulations., Comment: 11 pages, 8 figures, to appear in Proc. SPIE 5898, UV, X-Ray, and Gamma-Ray Space Instrumentation for Astronomy XIV

We present the analysis of Rossi X-ray Timing Explorer (RXTE) observations of the turn-on phase of a 35 day cycle of the X-ray binary Her X-1. During the early phases of the turn-on, the energy spectrum is composed of X-rays scattered into the line of sight plus heavily absorbed X-rays. The energy spectra in the 3-17 keV range can be described by a partial covering model, where one of the components is influenced by photoelectric absorption and Thomson scattering in cold material plus an iron emission line at 6.5 keV. In this paper we show the evolution of spectral parameters as well as the evolution of the pulse profile during the turn-on. We describe this evolution using Monte Carlo simulations which self-consistently describe the evolution of the X-ray pulse profile and of the energy spectrum., Comment: 16 pages, 13 figures, accepted for publication in A&A

The X-ray binary pulsar Her X-1 shows a wide variety of long and short term variabilities in the X-ray light curve. The 35 d variability of the source is interpreted as the influence of a warped, inclined, and twisted accretion disk periodically covering the line of sight to the central neutron star. In 1997 September we observed the ``turn-on'' of a 35 d cycle with the Rossi X-ray Timing Explorer (RXTE). Spectral analysis reveals that during early phases of the turn-on the overall spectrum is composed of X-rays scattered into the line of sight plus heavily absorbed X-rays. This interpretation is consistent with the variation of the pulse profile observed at the same time. The overall shape of the pulse profile is not changing, but towards earlier phases of the turn-on the pulse signature is steadily ``washed out''. This behavior can be understood as an influence of scattering and absorption due to the presence of the accretion disk rim. Using a Monte Carlo code we simulate the influence of both processes on a time variable, beamed emission characteristic, similar to the pulse profile of Her X-1. By comparing the results of the simulation with the observed profiles we determine the amount of scattered radiation, absorbed radiation, and the size of the scattering region., Comment: 3 pages, 4 Figures, To appear in the Proceedings of the Symposium on `New Visions of the X-ray Universe in the XMM-Newton and Chandra Era', 26-30 November 2001, ESTEC, The Netherlands

We present the XMM-Newton first light image, taken in January 2000 with the EPIC pn camera during the instrument's commissioning phase, when XMM-Newton was pointing towards the Large Magellanic Cloud (LMC). The field is rich in different kinds of X-ray sources: point sources, supernova remnants (SNRs) and diffuse X-ray emission from LMC interstellar gas. The observations are of unprecedented sensitivity, reaching a few 10^32 erg/s for point sources in the LMC. We describe how these data sets were analysed and discuss some of the spectroscopic results. For the SNR N157B the power law spectrum is clearly steeper than previously determined from ROSAT and ASCA data. The existence of a significant thermal component is evident and suggests that N157B is not a Crab-like but a composite SNR. Most puzzling is the spectrum of the LMC hot interstellar medium, which indicates a significant overabundance of Ne and Mg of a few times solar., Comment: to be published in A&A 365

We present XMM-Newton observations of the high redshift z=3.104, radio-loud quasar PKS 0537-286. The EPIC CCD cameras provide the highest signal-to-noise spectrum of a high-z quasar to date. The EPIC observations show that PKS 0537-286 is extremely X-ray luminous (Lx=2x10^47 erg/s), with an unusually hard X-ray spectrum (Gamma=1.27+/-0.02). The flat power-law emission extends over the whole observed energy range; there is no evidence of intrinsic absorption, which has been claimed in PKS 0537-286 and other high z quasars. However, there is evidence for weak Compton reflection. A redshifted iron K line, observed at 1.5 keV - corresponding to 6.15 keV in the quasar rest frame - is detected at 95% confidence. If confirmed, this is the most distant iron K line known. The line equivalent width is small (33eV), consistent with the `X-ray Baldwin effect' observed in other luminous quasars. The reflected continuum is also weak (R=0.25). We find the overall spectral energy distribution of PKS 0537-286 is dominated by the X-ray emission, which, together with the flat power-law and weak reflection features, suggests that the X-radiation from PKS 0537-286 is dominated by inverse Compton emission associated with a face-on relativistic jet., Comment: 6 pages, accepted for publication in Astronomy and Astrophysics Letters

A careful analysis of X-ray light curves and pulse profiles of Her X-1 obtained over more than 20 years strongly evidences for free precession of a magnetized neutron star with rotational axis inclined to the orbital plane as a central clock underlying the observed 35-day period. Strong asymmetric X-ray illumination of the optical star atmosphere leads to the formation of gaseous streams coming out of the orbital plane and forming a tilted accretion disk around the neutron star. Such a disk precesses due to tidal forces and dynamical action of gaseous streams from the secondary companion. The locking of these torques with neutron star precession makes the net disk precession period to be very close to that of the neutron star free precession., Comment: 14 pages, 7 Postscript figures, uses epsf.sty, rotate.sty. Invited talk presented at workshop of Bogolyubov Theoretical Physics Laboratory of JINR (Dubna) "Hot topics in astrophysics", August 2000