arXiv subjects

P. Friedrich

Publications and source records attributed to P. Friedrich.

24 records · Page 2Linked to original sources

arXiv2009

Probing eV-scale axions with CAST

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 $m_γ$, thereby achieving the maximum possible conversion rate for those axion masses \ma that match $m_γ$. 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.

hep-ex↗

arXiv2007

The X-ray Telescope of CAST

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.

physics.ins-det↗

arXiv2005

The X-ray Telescope of the CAST Experiment

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.

astro-ph↗

arXiv2005

XMM-Newton observations of bright ROSAT selected active galactic nuclei with low intrinsic absorption

We present a sample of twenty-one ROSAT bright active galactic nuclei (AGN), representing a range of spectral classes, and selected for follow-up snap-shot observations with XMM-Newton. The typical exposure was between 5-10 ks. The objects were primarily selected on the bases of X-ray brightness and not on hardness ratio; thus the sample cannot be strictly defined as a `soft' sample. One of the main outcomes from the XMM-Newton observations was that all of the AGN, including eleven type 1.8-2 objects, required low levels of intrinsic absorption (nh < 10^21 cm^-2). The low absorption in type 2 systems is a challenge to account for in the standard orientation-based unification model, and we discuss possible physical and geometrical models which could elucidate the problem. Moreover, there does not appear to be any relation between the strength and shape of the soft excess, and the spectral classification of the AGN in this sample. We further identify a number of AGN which deserve deeper observations or further analysis: for example, the LINERs NGC 5005 and NGC 7331, where optically thin thermal and extended emission is detected, and the narrow-line Seyfert 1 II Zw 177, which shows a broad emission feature at 5.8keV.

astro-ph↗

arXiv2005

Health and cleanliness of the XMM-Newton science payload since launch

On December 10th 2004 the XMM-Newton observatory celebrated its 5th year in orbit. Since the beginning of the mission steady health and contamination monitoring has been performed by the XMM-Newton SOC and the instrument teams. Main targets of the monitoring, using scientific data for all instruments on board, are the behaviour of the Charge Transfer Efficiency, the gain, the effective area and the bad, hot and noisy pixels. The monitoring is performed by combination of calibration observations using internal radioactive calibration sources with observations of astronomical targets. In addition a set of housekeeping parameters is continuously monitored reflecting the health situation of the instruments from an engineering point of view. We show trend behaviour over the 5 years especially in combination with events like solar flares and other events affecting the performance of the instruments.

astro-ph↗

arXiv1997

UV to X-ray Spectra of Radio-Quiet Quasars. Comparison With Accretion Disk Models

We present UV to soft X-ray spectra of 31 radio-quiet quasars, comprising data from the IUE ULDA database and the ROSAT pointed observation phase. 90 % of the sample members show a soft X-ray excess above an underlying hard X-ray power law spectrum. Particularly for the steep X-ray spectrum (alpha_energy > 1.5), low redshift subsample (17 objects) the X-ray spectral power law index is strongly correlated with the optical to X-ray broad-band spectral index alpha_ox, indicating that the main contribution to the soft X-ray and optical emission is due to the same emission component. We model the UV/soft X-ray spectra in terms of thermal emission from a geometrically thin alpha-accretion disk. The structure and radiation field of the disk is calculated self-consistently and Compton scattering is treated by the Kompaneets equation. All relativistic effects on the disk structure and the emergent disk spectrum are included. Satisfactory spectral fits of the UV and soft X-ray continuum spectra are achieved when additional non-thermal hard X-ray and IR power law emission components are taken into account. The UV and soft X-ray spectra are well described by emission resulting from accretion rates in the range ~0.1 to ~0.3 times the Eddington accretion rate. Low mass/low redshift objects are found to accret at < 0.15 Mdot_Eddington. Correlations of the accretion disk parameters with alpha_ox are discussed.

astro-ph↗