arXiv subjects

M. Pohl

Publications and source records attributed to M. Pohl.

At least 127 records · Page 7Linked to original sources

arXiv2014

The Large Observatory For x-ray Timing

The Large Observatory For x-ray Timing (LOFT) was studied within ESA M3 Cosmic Vision framework and participated in the final down-selection for a launch slot in 2022-2024. Thanks to the unprecedented combination of effective area and spectral resolution of its main instrument, LOFT will study the behaviour of matter under extreme conditions, such as the strong gravitational field in the innermost regions of accretion flows close to black holes and neutron stars, and the supra-nuclear densities in the interior of neutron stars. The science payload is based on a Large Area Detector (LAD, 10 m 2 effective area, 2-30 keV, 240 eV spectral resolution, 1 deg collimated field of view) and a WideField Monitor (WFM, 2-50 keV, 4 steradian field of view, 1 arcmin source location accuracy, 300 eV spectral resolution). The WFM is equipped with an on-board system for bright events (e.g. GRB) localization. The trigger time and position of these events are broadcast to the ground within 30 s from discovery. In this paper we present the status of the mission at the end of its Phase A study.

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arXiv2014

The Large Area Detector of LOFT: the Large Observatory for X-ray Timing

LOFT (Large Observatory for X-ray Timing) is one of the five candidates that were considered by ESA as an M3 mission (with launch in 2022-2024) and has been studied during an extensive assessment phase. It is specifically designed to perform fast X-ray timing and probe the status of the matter near black holes and neutron stars. Its pointed instrument is the Large Area Detector (LAD), a 10 m 2 -class instrument operating in the 2-30keV range, which holds the capability to revolutionise studies of variability from X-ray sources on the millisecond time scales. The LAD instrument has now completed the assessment phase but was not down-selected for launch. However, during the assessment, most of the trade-offs have been closed leading to a robust and well documented design that will be re- proposed in future ESA calls. In this talk, we will summarize the characteristics of the LAD design and give an overview of the expectations for the instrument capabilities.

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arXiv2014

Radiation tests of the Silicon Drift Detectors for LOFT

During the three years long assessment phase of the LOFT mission, candidate to the M3 launch opportunity of the ESA Cosmic Vision programme, we estimated and measured the radiation damage of the silicon drift detectors (SDDs) of the satellite instrumentation. In particular, we irradiated the detectors with protons (of 0.8 and 11 MeV energy) to study the increment of leakage current and the variation of the charge collection efficiency produced by the displacement damage, and we "bombarded" the detectors with hypervelocity dust grains to measure the effect of the debris impacts. In this paper we describe the measurements and discuss the results in the context of the LOFT mission.

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arXiv2014

The design of the wide field monitor for LOFT

LOFT (Large Observatory For x-ray Timing) is one of the ESA M3 missions selected within the Cosmic Vision program in 2011 to carry out an assessment phase study and compete for a launch opportunity in 2022-2024. The phase-A studies of all M3 missions were completed at the end of 2013. LOFT is designed to carry on-board two instruments with sensitivity in the 2-50 keV range: a 10 m 2 class Large Area Detector (LAD) with a <1° collimated FoV and a wide field monitor (WFM) making use of coded masks and providing an instantaneous coverage of more than 1/3 of the sky. The prime goal of the WFM will be to detect transient sources to be observed by the LAD. However, thanks to its unique combination of a wide field of view (FoV) and energy resolution (better than 500 eV), the WFM will be also an excellent monitoring instrument to study the long term variability of many classes of X-ray sources. The WFM consists of 10 independent and identical coded mask cameras arranged in 5 pairs to provide the desired sky coverage. We provide here an overview of the instrument design, configuration, and capabilities of the LOFT WFM. The compact and modular design of the WFM could easily make the instrument concept adaptable for other missions.

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arXiv2014

Optimisation of the design for the LOFT Large Area Detector Module

LOFT (Large Observatory for X-ray Timing) is an X-ray timing observatory that, with four other candidates, was considered by ESA as an M3 mission (with launch in 2022-2024) and has been studied during an extensive assessment phase. Its pointed instrument is the Large Area Detector (LAD), a 10 m 2 -class instrument operating in the 2-30 keV range, which is designed to perform X-ray timing of compact objects with unprecedented resolution down to millisecond time scales. Although LOFT was not downselected for launch, during the assessment most of the trade-offs have been closed, leading to a robust and well documented design that will be reproposed in future ESA calls. The building block of the LAD instrument is the Module, and in this paper we summarize the rationale for the module concept, the characteristics of the module and the trade-offs/optimisations which have led to the current design.

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arXiv2014

Magnetic Field Generation in Core-Sheath Jets via the Kinetic Kelvin-Helmholtz Instability

We have investigated magnetic field generation in velocity shears via the kinetic Kelvin-Helmholtz instability (kKHI) using a relativistic plasma jet core and stationary plasma sheath. Our three-dimensional particle-in-cell simulations consider plasma jet cores with Lorentz factors of 1.5, 5, and 15 for both electron-proton and electron-positron plasmas. For electron-proton plasmas we find generation of strong large-scale DC currents and magnetic fields which extend over the entire shear-surface and reach thicknesses of a few tens of electron skin depths. For electron-positron plasmas we find generation of alternating currents and magnetic fields. Jet and sheath plasmas are accelerated across the shear surface in the strong magnetic fields generated by the kKHI. The mixing of jet and sheath plasmas generates transverse structure similar to that produced by the Weibel instability.

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arXiv2014

Measurement of the effect of Non Ionising Energy Losses on the leakage current of Silicon Drift Detector prototypes for the LOFT satellite

The silicon drift detectors are at the basis of the instrumentation aboard the Large Observatory For x-ray Timing (LOFT) satellite mission, which underwent a three year assessment phase within the "Cosmic Vision 2015 - 2025" long-term science plan of the European Space Agency. Silicon detectors are especially sensitive to the displacement damage, produced by the non ionising energy losses of charged and neutral particles, leading to an increase of the device leakage current and thus worsening the spectral resolution. During the LOFT assessment phase, we irradiated two silicon drift detectors with a proton beam at the Proton Irradiation Facility in the accelerator of the Paul Scherrer Institute and we measured the increase in leakage current. In this paper we report the results of the irradiation and we discuss the impact of the radiation damage on the LOFT scientific performance.

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arXiv2014

Investigating the TeV Morphology of MGRO J1908+06 with VERITAS

We report on deep observations of the extended TeV gamma-ray source MGRO J1908+06 made with the VERITAS very high energy (VHE) gamma-ray observatory. Previously, the TeV emission has been attributed to the pulsar wind nebula (PWN) of the Fermi-LAT pulsar PSR J1907+0602. We detect MGRO J1908+06 at a significance level of 14 standard deviations (14 sigma) and measure a photon index of 2.20 +/- 0.10_stat +/- 0.20_sys. The TeV emission is extended, covering the region near PSR J1907+0602 and also extending towards SNR G40.5--0.5. When fitted with a 2-dimensional Gaussian, the intrinsic extension has a standard deviation of sigma_src = 0.44 +/- 0.02 degrees. In contrast to other TeV PWNe of similar age in which the TeV spectrum softens with distance from the pulsar, the TeV spectrum measured near the pulsar location is consistent with that measured at a position near the rim of G40.5--0.5, 0.33 degrees away.

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arXiv2014

Deep Broadband Observations of the Distant Gamma-ray Blazar PKS 1424+240

We present deep VERITAS observations of the blazar PKS 1424+240, along with contemporaneous Fermi Large Area Telescope, Swift X-ray Telescope and Swift UV Optical Telescope data between 2009 February 19 and 2013 June 8. This blazar resides at a redshift of $z\ge0.6035$, displaying a significantly attenuated gamma-ray flux above 100 GeV due to photon absorption via pair-production with the extragalactic background light. We present more than 100 hours of VERITAS observations from three years, a multiwavelength light curve and the contemporaneous spectral energy distributions. The source shows a higher flux of (2.1$\pm0.3$)$\times10^{-7}$ ph m$^{-2}$s$^{-1}$ above 120 GeV in 2009 and 2011 as compared to the flux measured in 2013, corresponding to (1.02$\pm0.08$)$\times10^{-7}$ ph m$^{-2}$s$^{-1}$ above 120 GeV. The measured differential very high energy (VHE; $E\ge100$ GeV) spectral indices are $Γ=$3.8$\pm$0.3, 4.3$\pm$0.6 and 4.5$\pm$0.2 in 2009, 2011 and 2013, respectively. No significant spectral change across the observation epochs is detected. We find no evidence for variability at gamma-ray opacities of greater than $τ=2$, where it is postulated that any variability would be small and occur on longer than year timescales if hadronic cosmic-ray interactions with extragalactic photon fields provide a secondary VHE photon flux. The data cannot rule out such variability due to low statistics.

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arXiv2014

Observations of the unidentified gamma-ray source TeV J2032+4130 by VERITAS

TeV J2032+4130 was the first unidentified source discovered at very high energies (VHE; E $>$ 100 GeV), with no obvious counterpart in any other wavelength. It is also the first extended source to be observed in VHE gamma rays. Following its discovery, intensive observational campaigns have been carried out in all wavelengths in order to understand the nature of the object, which have met with limited success. We report here on a deep observation of TeV J2032+4130, based on 48.2 hours of data taken from 2009 to 2012 by the VERITAS (Very Energetic Radiation Imaging Telescope Array System) experiment. The source is detected at 8.7 standard deviations ($σ$) and is found to be extended and asymmetric with a width of 9.5$^{\prime}$$\pm$1.2$^{\prime}$ along the major axis and 4.0$^{\prime}$$\pm$0.5$^{\prime}$ along the minor axis. The spectrum is well described by a differential power law with an index of 2.10 $\pm$ 0.14$_{stat}$ $\pm$ 0.21$_{sys}$ and a normalization of (9.5 $\pm$ 1.6$_{stat}$ $\pm$ 2.2$_{sys}$) $\times$ 10$^{-13}$TeV$^{-1}$ cm$^{-2}$ s$^{-1}$ at 1 TeV. We interpret these results in the context of multiwavelength scenarios which particularly favor the pulsar wind nebula (PWN) interpretation.

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arXiv2014

A search for enhanced very high energy gamma-ray emission from the March 2013 Crab Nebula flare

In March 2013, a flaring episode from the Crab Nebula lasting ~2 weeks was detected by the Fermi-LAT (Large Area Telescope on board the Fermi Gamma-ray Space Telescope). VERITAS provides simultaneous observations throughout this period. During the flare, the Fermi-LAT detected a 20-fold increase in flux above the average synchrotron flux >100 MeV seen from the Crab Nebula. Simultaneous measurements with VERITAS are consistent with the non-variable long-term average Crab Nebula flux at TeV energies. Assuming a linear correlation between the very-high-energy flux change >1 TeV and the flux change seen in the Fermi-LAT band >100 MeV during the period of simultaneous observations, the linear correlation factor can be constrained to be at most 8.6 * 10^-3 with 95% confidence.

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arXiv2013

A Three-Year Multi-Wavelength Study of the Very High Energy Gamma-ray Blazar 1ES 0229+200

The high-frequency-peaked BL Lacertae object 1ES 0229+200 is a relatively distant (z = 0.1396), hard-spectrum (Gamma ~ 2.5), very-high-energy-emitting (E > 100 GeV) gamma-ray blazar. Very-high-energy measurements of this active galactic nucleus have been used to place constraints on the intensity of the extragalactic background light and the intergalactic magnetic field. A multi-wavelength study of this object centered around very-high-energy observations by VERITAS is presented. This study obtained, over a period of three years, an 11.7 standard deviation detection and an average integral flux F(E>300 GeV) = (23.3 +- 2.8_stat +- 5.8_sys) x 10^-9 photons m^-2 s^-1, or 1.7% of the Crab Nebula's flux (assuming the Crab Nebula spectrum measured by H.E.S.S). Supporting observations from Swift and RXTE are analyzed. The Swift observations are combined with previously published Fermi observations and the very-high-energy measurements to produce an overall spectral energy distribution which is then modeled assuming one-zone synchrotron-self-Compton emission. The chi^2 probability of the TeV flux being constant is 1.6%. This, when considered in combination with measured variability in the X-ray band, and the demonstrated variability of many TeV blazars, suggests that the use of blazars such as 1ES 0229+200 for intergalactic magnetic field studies may not be straightforward and challenges models that attribute hard TeV spectra to secondary gamma-ray production along the line of sight.

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arXiv2013

Long-term TeV and X-ray Observations of the Gamma-ray Binary HESS J0632+057

HESS J0632+057 is the only gamma-ray binary known so far whose position in the sky allows observations with ground-based observatories both in the northern and southern hemispheres. Here we report on long-term observations of HESS J0632+057 conducted with the VERITAS and H.E.S.S. Cherenkov Telescopes and the X-ray Satellite Swift, spanning a time range from 2004 to 2012 and covering most of the system's orbit. The VHE emission is found to be variable, and is correlated with that at X-ray energies. An orbital period of $315 ^{+6}_{-4}$ days is derived from the X-ray data set, which is compatible with previous results, $P = (321 \pm 5$) days. The VHE light curve shows a distinct maximum at orbital phases close to 0.3, or about 100 days after periastron passage, which coincides with the periodic enhancement of the X-ray emission. Furthermore, the analysis of the TeV data shows for the first time a statistically significant ($> 6.5 σ$) detection at orbital phases 0.6--0.9. The obtained gamma-ray and X-ray light curves and the correlation of the source emission at these two energy bands are discussed in the context of the recent ephemeris obtained for the system. Our results are compared to those reported for other gamma-ray binaries.

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arXiv2013

VERITAS Observations of the Microquasar Cygnus X-3

We report results from TeV gamma-ray observations of the microquasar Cygnus X-3. The observations were made with the Very Energetic Radiation Imaging Telescope Array System (VERITAS) over a time period from 2007 June 11 to 2011 November 28. VERITAS is most sensitive to gamma rays at energies between 85 GeV to 30 TeV. The effective exposure time amounts to a total of about 44 hours, with the observations covering six distinct radio/X-ray states of the object. No significant TeV gamma-ray emission was detected in any of the states, nor with all observations combined. The lack of a positive signal, especially in the states where GeV gamma rays were detected, places constraints on TeV gamma-ray production in Cygnus X-3. We discuss the implications of the results.

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arXiv2013

Observation of Markarian 421 in TeV gamma rays over a 14-year time span

The variability of the blazar Markarian 421 in TeV gamma rays over a 14-year time period has been explored with the Whipple 10 m telescope. It is shown that the dynamic range of its flux variations is large and similar to that in X-rays. A correlation between the X-ray and TeV energy bands is observed during some bright flares and when the complete data sets are binned on long timescales. The main database consists of 878.4 hours of observation with the Whipple telescope, spread over 783 nights. The peak energy response of the telescope was 400 GeV with 20% uncertainty. This is the largest database of any TeV-emitting active galactic nucleus (AGN) and hence was used to explore the variability profile of Markarian 421. The time-averaged flux from Markarian 421 over this period was 0.446$\pm$0.008 Crab flux units. The flux exceeded 10 Crab flux units on three separate occasions. For the 2000-2001 season the average flux reached 1.86 Crab units, while in the 1996-1997 season the average flux was only 0.23 Crab units.

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arXiv2013

Multiwavelength Observations of The TeV Binary LS I +61 303 with VERITAS, Fermi-LAT and Swift-XRT During a TeV Outburst

We present the results of a multiwavelength observational campaign on the TeV binary system LS I +61 303 with the VERITAS telescope array (>200 GeV), Fermi-LAT (0.3-300 GeV), and Swift-XRT (2-10 keV). The data were taken from December 2011 through January 2012 and show a strong detection in all three wavebands. During this period VERITAS obtained 24.9 hours of quality selected livetime data in which LS I +61 303 was detected at a statistical sig- nificance of 11.9 sigma. These TeV observations show evidence for nightly variability in the TeV regime at a post-trial significance of 3.6 sigma. The combination of the simultaneously obtained TeV and X-ray fluxes do not demonstrate any evidence for a correlation between emission in the two bands. For the first time since the launch of the Fermi satellite in 2008, this TeV detection allows the construction of a detailed MeV-TeV spectral energy distribution from LS I +61 303. This spectrum shows a distinct cutoff in emission near 4 GeV, with emission seen by the VERITAS observations following a simple power-law above 200 GeV. This feature in the spectrum of LS I +61 303, obtained from overlapping observations with Fermi-LAT and VERITAS, may indicate that there are two distinct populations of accelerated particles producing the GeV and TeV emission.

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arXiv2013

Long term observations of B2 1215+30 with VERITAS

We report on VERITAS observations of the BL Lac object B2 1215+30 between 2008 and 2012. During this period, the source was detected at very high energies (VHE; E > 100 GeV) by VERITAS with a significance of $8.9σ$ and showed clear variability on time scales larger than months. In 2011, the source was found to be in a relatively bright state and a power-law fit to the differential photon spectrum yields a spectral index of $3.6 \pm 0.4_{\mathrm{stat}} \pm 0.3_{\mathrm{syst}}$ with an integral flux above 200 GeV of $(8.0 \pm 0.9_{\mathrm{stat}} \pm 3.2_{\mathrm{syst}}) \times 10^{-12}\, \mathrm{cm}^{-2} \mathrm{s}^{-1}$. No short term variability could be detected during the bright state in 2011. Multi-wavelength data were obtained contemporaneous with the VERITAS observations in 2011 and cover optical (Super-LOTIS, MDM, Swift-UVOT), X-ray (Swift-XRT), and gamma-ray (Fermi-LAT) frequencies. These were used to construct the spectral energy distribution (SED) of B2 1215+30. A one-zone leptonic model is used to model the blazar emission and the results are compared to those of MAGIC from early 2011 and other VERITAS-detected blazars. The SED can be well reproduced with model parameters typical for VHE-detected BL Lacs.

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arXiv2013

Nuclear astrophysics with radioactive ions at FAIR

The nucleosynthesis of elements beyond iron is dominated by neutron captures in the s and r processes. However, 32 stable, proton-rich isotopes cannot be formed during those processes, because they are shielded from the s-process flow and r-process beta-decay chains. These nuclei are attributed to the p and rp process. For all those processes, current research in nuclear astrophysics addresses the need for more precise reaction data involving radioactive isotopes. Depending on the particular reaction, direct or inverse kinematics, forward or time-reversed direction are investigated to determine or at least to constrain the desired reaction cross sections. The Facility for Antiproton and Ion Research (FAIR) will offer unique, unprecedented opportunities to investigate many of the important reactions. The high yield of radioactive isotopes, even far away from the valley of stability, allows the investigation of isotopes involved in processes as exotic as the r or rp processes.

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