Papers by Andreas Heusler
Physical Review C, 2014
In the doubly magic nucleus 208 Pb, many states contain fractions of particle-hole configurations... more In the doubly magic nucleus 208 Pb, many states contain fractions of particle-hole configurations whose strength can be determined from experiment. However, some configurations are not excited in a directly detectable way. Their strengths can be determined by observing an ensemble of states which consists entirely of an equivalent set of configurations with a given spin and parity among which only one or two configurations are not detected. Examples for spins 2 −-5 − are evaluated. Excitation energies of states in 208 Pb are determined with a precision down to 100 eV by experiments on the 208 Pb(p,p) and 207 Pb(d,p) reactions with the Q3D magnetic spectrograph (Maier-Leibnitz-Laboratorium, Garching, Germany). Six doublets with distances between the states of less than 2 keV are resolved. 72 negative parity states below E x = 6.1 MeV are identified. They correspond to 70 states predicted by the schematic shell model without residual interaction below E x = 6361 keV. The 1 − and 3 − yrast states appear in addition. Six new spins are assigned to negative parity states, three new spins to positive parity states, and two spins suggested by the Nuclear Data Sheets are verified. The state at E x = 4953 keV is identified as the 3 + member of the configuration g 9/2 i 13/2. Among about hundred states, the configuration mixing for unnatural parity is shown to be less than for natural parity.
Physical Review C, 2017
We study the spectral fluctuations of the 208 Pb nucleus using the complete experimental spectrum... more We study the spectral fluctuations of the 208 Pb nucleus using the complete experimental spectrum of 151 states up to excitation energies of 6.20 MeV recently identified at the Maier-Leibnitz-Laboratorium at Garching, Germany. For natural parity states the results are very close to the predictions of Random Matrix Theory (RMT) for the nearest-neighbor spacing distribution. A quantitative estimate of the agreement is given by the Brody parameter ω, which takes the value ω = 0 for regular systems and ω ≃ 1 for chaotic systems. We obtain ω = 0.85 ± 0.02 which is, to our knowledge, the closest value to chaos ever observed in experimental bound states of nuclei. By contrast, the results for unnatural parity states are far from RMT behavior. We interpret these results as a consequence of the strength of the residual interaction in 208 Pb, which, according to experimental data, is much stronger for natural than for unnatural parity states. In addition our results show that chaotic and non-chaotic nuclear states coexist in the same energy region of the spectrum.
Journal of Physics: Conference Series, 2015
Using the Q3D magnetic spectrograph of the Maier-Leibnitz-Labaratorium at Garching (Germany), exp... more Using the Q3D magnetic spectrograph of the Maier-Leibnitz-Labaratorium at Garching (Germany), experiments on the 208 Pb(d, d ′), the 207 Pb(d, p), the resonant and the non-resonant 208 Pb(p, p ′) reactions were performed. The 208 Pb(p, p ′) reaction was investigated near all seven known isobaric analog resonances in 209 Bi. The excitation energies of about 300 states in 208 Pb at Ex < 8 MeV were determined with an accuracy of about 100 eV. The mean distance between states in 208 Pb is about 10 keV up to the neutron threshold (S(n) = 7368 keV). The number of 1 − states observed by particle spectroscopy up to the proton threshold (S(p) = 8.00 MeV) is close to the number predicted by the schematic shell model without residual interaction. The structure of several 1 − states is deduced from particle spectroscopy. Four out of the seven lowest 1 − states contain about 80% strength of a single configuration.
EPJ Web of Conferences, 2014
Using the Q3D magnetic spectrograph at the Maier-Leibnitz-Laboratorium, Garching, experiments wit... more Using the Q3D magnetic spectrograph at the Maier-Leibnitz-Laboratorium, Garching, experiments with the 208 Pb(p, p ′) reaction via isobaric analog resonances and using the 207 Pb(d, p) reaction have been performed with a HWHM of 1.5 keV on the low energy side. All 70 particle-hole states with negative parity predicted by the schematic shell model without residual interaction below E x = 6.3 MeV are identified. Except for the states with spins 1 − and 2 − , more than 80% of the strength in each state can be described by at most four configurations; for spins 0 − , 4 − , 6 − , 7 − , and 8 − two configurations or even one configuration describe more than 95% of the strength. Natural parity configurations are more strongly mixed than unnatural parity configurations.
A detection of TeV γ-rays from Mkn 501 is reported, based on observations made between March and ... more A detection of TeV γ-rays from Mkn 501 is reported, based on observations made between March and August 1996 with the first HEGRA Cherenkov telescope (CT1). From the image analysis, 351 excess candidate γray events are obtained from the 147 h dataset. The statistical significance of the excess is 5.2 σ. The average excess rate is 2.4 ± 0.5 h −1 above the ≈ 1.5 TeV threshold of CT1. Under the assumption that the spectrum of Mkn 501 follows a power law we find a differential spectral index of 2.6±0.5 and obtain a time-averaged integral flux above 1.5 TeV of 2.3(±0.4) Stat (+1.5−0.6) Syst ×10 −12 cm −2 s −1. Comparison with our near contemporary observations of the Crab Nebula, used as a standard candle to test CT1 after upgrading to a 127 pixel camera, indicates that Mkn 501 has a spectrum similar to that of the Crab Nebula above 1.5 TeV. The integral flux above 1.5 TeV from Mkn 501 is found to have been between 2.2 and 3.6 times smaller than that from the Crab Nebula. HEGRA is the second experiment to have detected Mkn 501 in the TeV range.
Astrophysical Journal, 2000
We present exactly simultaneous X-ray and TeV monitoring with RXT E and HEGRA of the TeV blazar M... more We present exactly simultaneous X-ray and TeV monitoring with RXT E and HEGRA of the TeV blazar Mrk 501 during 15 days in 1998 June. After an initial period of very low Ñux at both wavelengths, the source underwent a remarkable Ñare in the TeV and X-ray energy bands, lasting for about 6 days and with a larger amplitude at TeV energies than in the X-ray band. At the peak of the TeV Ñare, rapid TeV Ñux variability on subhour timescales is found. Large spectral variations are observed at X-rays, with the 3È20 keV photon index of a pure power-law continuum Ñattening from ! \ 2.3 to ! \ 1.8 on a timescale of 2È3 days. This implies that during the maximum of the TeV activity the synchrotron peak shifted to energies keV, a behavior similar to that observed during the longer lasting, Z50 more intense Ñare in 1997 April. The TeV spectrum during the Ñare is described by a power law with photon index ! \ 1.9 and an exponential cuto † at D4 TeV ; an indication for spectral softening during the Ñare decay is observed in the TeV hardness ratios. Our results generally support a scenario in which the TeV photons are emitted via inverse Compton scattering of ambient seed photons by the same electron population responsible for the synchrotron X-rays. The simultaneous spectral energy distributions can be Ðt with a one-zone synchrotron self-Compton model assuming a substantial increase of the magnetic Ðeld and the electron energy by factors of 3 and 10, respectively.
The Astrophysical Journal, 2001
In this paper we present the results of simultaneous observations of the TeV blazar Markarian 421... more In this paper we present the results of simultaneous observations of the TeV blazar Markarian 421 (Mrk 421) at X-ray and TeV gamma-ray energies with the Rossi X-Ray Timing Explorer (RXTE) and the stereoscopic Cherenkov Telescope system of the HEGRA (High Energy Gamma Ray Astronomy) experiment, respectively. The source was monitored from February 2nd to February 16th and from May 3rd to May 8th, 2000.
Physical Review D, 1999
The HEGRA system of 4 Imaging Atmospheric Cherenkov Telescopes (IACTs) has been used to determine... more The HEGRA system of 4 Imaging Atmospheric Cherenkov Telescopes (IACTs) has been used to determine the flux and the spectrum of cosmic ray protons over a limited energy range around 1.5 TeV. Although the IACT system is designed for the detection of γ-rays with energies above 500 GeV, it has also a large detection area of 10 6 m 2 • 3 msr for primary protons of energies above 1 TeV and the capability to reconstruct the primary proton energy with a reasonable accuracy ∆E/E of 50% near this threshold. Furthermore, the principle of stereoscopic detection of air showers permits the effective suppression of air showers induced by heavier primaries already on the trigger level, and in addition on the software level by analysis of the stereoscopic images. The combination of both capabilities permits a determination of the proton spectrum almost independently of the cosmic ray chemical composition. The accuracy of our estimate of the spectral index at 1.5 TeV is limited by systematic uncertainties and is comparable to the accuracy achieved with recent balloon and space borne experiments. In this paper we describe in detail the analysis tools, namely the detailed Monte Carlo simulation, the analysis procedure and the results. We determine the local (i.e. in the range of 1.5 to 3 TeV) differential spectral index to be γ p = 2.72±0.02 stat. ±0.15 syst. and obtain an integral flux above 1.5 TeV of F (> 1.5 TeV) = 3.1±0.6 stat. ±1.2 syst. •10 −2 /s sr m 2 .
Physical Review C, 2006
Inelastic proton scattering via isobaric analog resonances allows to derive rather complete infor... more Inelastic proton scattering via isobaric analog resonances allows to derive rather complete information about neutron particle-hole states. We applied this method to the doubly-magic nucleus 208 Pb by measuring angular distributions of 208 Pb(p, p') on top of the isobaric analog resonances in 209 Bi with the Q3D magnetic spectrograph at München. We identify the six states of the i 11/2 f 5/2 multiplet and the four states of the i 11/2 p 3/2 multiplet in the energy range 4.6 M eV < Ex < 5.3 MeV. Firm spin assignments for the ten states are given, some of them new. Additional measurements of the reaction 207 Pb(d, p) confirm the fragmented i 11/2 p 1/2 multiplet.
Arxiv preprint astro-ph/9903386, 1999
During the several months of the outburst of Mkn 501 in 1997 the source has been monitored in TeV... more During the several months of the outburst of Mkn 501 in 1997 the source has been monitored in TeV γ-rays with the HEGRA stereoscopic system of imaging atmospheric Cherenkov telescopes. Quite remarkably it turned out that the shapes of the daily γ-ray energy spectra remained essentially stable throughout the entire state of high activity despite dramatic flux variations during this period. The derivation of a long term time-averaged energy spectrum, based on more than 38,000 detected TeV photons, is therefore physically meaningful. The unprecedented γ-ray statistics combined with the 20% energy resolution of the instrument resulted in the first detection of γ-rays from an extragalactic source well beyond 10 TeV, and the first high accuracy measurement of an exponential cutoff in the energy region above 5 TeV deeply into the exponential regime. From 500 GeV to 24 TeV the differential pho
The European Physical Journal A, 2010
From 208 Pb(p, p) via isobaric analog resonances in 209 Bi, at Ex = 6420 keV in the doubly magic ... more From 208 Pb(p, p) via isobaric analog resonances in 209 Bi, at Ex = 6420 keV in the doubly magic nucleus 208 Pb, a new state with spin 2 − with half the s 1/2 p 3/2 strength and major g 7/2 f 5/2 , d 3/2 f 5/2 fractions is identified. Ten more 2 − states up to Ex = 7.0 MeV are identified and their structure discussed, among them six states with new spin assignments. The known 6314 1 − state is shown to contain almost the entire s 1/2 p 3/2 strength.
The European Physical Journal A, 2011
Arxiv preprint astro-ph/9905032, 1999
Since its commissioning in fall 1996, the stereoscopic system of Imaging Atmospheric Cherenkov Te... more Since its commissioning in fall 1996, the stereoscopic system of Imaging Atmospheric Cherenkov Telescope (IACTs) of HEGRA with an energy threshold of 500 GeV, an angular resolution of 0.1 degree and an energy resolution of 20% per individual photon, and an energy flux sensitivity nu F_nu at 1 TeV of 1E-11erg/cm**2 s (S/N = 5 sigma) for one hour of
Astronomy & Astrophysics, 2005
We present results of a search for TeV γ-ray emission from the microquasar SS-433 and the surroun... more We present results of a search for TeV γ-ray emission from the microquasar SS-433 and the surrounding region covering a ∼8 • × 8 • field of view. Analysis of data taken with the HEGRA stereoscopic system of imaging atmosphericČerenkov imaging telescopes reveals no evidence of steady or variable emission from any position. Observation times of over 100 h have been achieved in central parts of the field of view. We set 99% confidence level upper limits to a number of a-priorichosen objects of interest, including SS-443 and its interaction regions, 32 pulsars, 3 supernova remnants and the GeV source GeV J1907+0537. Our upper limit of 3.2% Crab flux (for energies E > 0.8 TeV) for the eastern-lobe region e3 of SS-433 permits, after comparison with X-ray fluxes, a lower limit of B ≥ 19 µG on the post-shocked magnetic field in this region. An ensemble upper limit at 0.3% Crab flux (E > 0.7 TeV) from a subset (11) of the 32 pulsars implies a maximum of 4.5% of the spin-down pulsar power is available for TeV γ-ray production. For one of the SNR in our FoV, 3C 396, recent Chandra observations suggest that a central pulsar-driven wind nebula may be the source of X-ray emission. Our upper limit implies that a maximum of 0.1% of the spin-down power from the central source of 3C 396 would be available for TeV γ-rays.
Astronomy & Astrophysics, 2001
Using the HEGRA system of imaging atmospheric Cherenkov telescopes, a region of the Galactic plan... more Using the HEGRA system of imaging atmospheric Cherenkov telescopes, a region of the Galactic plane (−10 • < b < 5 • , 38 • < l < 43 •) was surveyed for TeV gamma-ray emission, both from point sources and of diffuse nature. The region covered includes 15 known pulsars, 6 known supernova remnants (SNR) and one unidentified EGRET source. No evidence for emission from point sources was detected; upper limits are typically below 0.1 Crab units for the flux above 1 TeV. For the diffuse gamma-ray flux from the Galactic plane, an upper limit of 6.1 × 10 −15 ph cm −2 s −1 sr −1 MeV −1 was derived under the assumption that the spatial distribution measured by the EGRET instrument extends to the TeV regime. This upper flux limit is a factor of about 1.5 larger than the flux expected from the ensemble of gamma-ray unresolved Galactic cosmic ray sources.
Astronomy & Astrophysics, 2003
TeV γ-rays from the BL Lac object 1ES 1959+650 have been measured during the years 2000 and 2001 ... more TeV γ-rays from the BL Lac object 1ES 1959+650 have been measured during the years 2000 and 2001 with a significance of 5.2 σ at a value of 5.3% of the Crab flux and in May 2002 during strong outbursts with >23 σ at a flux level of up to 2.2 Crab, making 1ES 1959+650 the TeV Blazar with the third best event statistics. The deep observation of 197.4 h has been performed with the HEGRA stereoscopic system of 5 imaging atmospheric Cherenkov telescopes (IACT system). 1ES 1959+650 is located at a redshift of z = 0.047, providing an intermediate distance between the nearby Blazars Mkn 421 and Mkn 501, and the much more distant object H1426+428. This makes 1ES 1959+650 an important member of the class of TeV Blazars in view of the absorption of TeV photons by the diffuse extragalactic background radiation (DEBRA). The differential energy spectrum of 1ES 1959+650 during the flares can be fitted by a power law with a spectral index of 2.83 ± 0.14 stat ± 0.08 sys or by a power law with an exponential cutoff at (4.2 +0.8 −0.6 stat ± 0.9 sys) TeV and a spectral index of 1.83 ± 0.15 stat ± 0.08 sys. The low state differential energy spectrum obtained with lower statistics can be described by a pure power law with a spectral index of 3.18 ± 0.17 stat ± 0.08 sys .
Astronomy & Astrophysics, 2005
The unidentified TeV source in Cygnus is now confirmed by follow-up observations from 2002 with t... more The unidentified TeV source in Cygnus is now confirmed by follow-up observations from 2002 with the HEGRA stereoscopic system of Cherenkov Telescopes. Using all data (1999 to 2002) we confirm this new source as steady in flux over the four years of data taking, extended with radius 6.2 (±1.2 stat ± 0.9 sys) and exhibiting a hard spectrum with photon index −1.9. It is located in the direction of the dense OB stellar association, Cygnus OB2. Its integral flux above energies E > 1 TeV amounts to ∼5% of the Crab assuming a Gaussian profile for the intrinsic source morphology. There is no obvious counterpart at radio, optical nor X-ray energies, leaving TeV J2032+4130 presently unidentified. Observational parameters of this source are updated here and some astrophysical discussion is provided. Also included are upper limits for a number of other interesting sources in the FoV, including the famous microquasar Cygnus X-3.
Astronomy & Astrophysics, 2002
A search for potential point sources of very high energy gamma rays has been carried out on the d... more A search for potential point sources of very high energy gamma rays has been carried out on the data taken simultaneously by the HEGRA AIROBICC and Scintillator arrays from August 1994 to March 2000. The list of sought sources includes supernova remnants, pulsars, AGNs and binary systems. The energy threshold is around 15 TeV. For the Crab Nebula, a modest excess of 2.5 standard deviations above the cosmic ray background has been observed. Flux upper limits (at 90% c.l.) of around 1.3 times the flux of the Crab Nebula are obtained, in average, for the candidate sources. A different search procedure has been used for an all-sky search which yields absolute flux upper limits between 4 and 9 crabs depending on declination, in the band from δ = 0 to δ = 60 • .
Astronomy & Astrophysics, 2001
Data taken with the HEGRA system of imaging atmospheric Cherenkov telescopes during the 1997 flar... more Data taken with the HEGRA system of imaging atmospheric Cherenkov telescopes during the 1997 flares of Markarian 501 (Mkn 501) are reanalyzed using an algorithm providing improved energy resolution. A resolution of 10% to 12% is obtained by accounting for the variation of the Cherenkov light yield with the height of the shower maximum in the atmosphere. The improved energy resolution is particularly relevant for the study of the high-energy cutoff in the spectrum, which might be caused by interactions with the intergalactic infrared background radiation. The reanalysis presented here confirms the results obtained in the previous analysis, but hints a steeper slope of the spectrum in the region around 20 TeV.
Astronomy & Astrophysics, 2001
For distant extragalactic sources of gamma-rays in the PeV (10 15 eV) energy range, interactions ... more For distant extragalactic sources of gamma-rays in the PeV (10 15 eV) energy range, interactions of the gamma rays with intergalactic diffuse radiation fields will initiate a pair cascade. Depending on the magnetic fields in the vicinity of the source, the cascade can either result in an isotropic halo around an initially beamed source, or remain more or less collimated. Data recorded by the HEGRA system of imaging atmospheric Cherenkov telescopes are used to derive limits on the halo flux from the AGN Mrk 501. This is achieved by comparing the angular distribution of TeV gamma-rays during the 1997 burst phase-where direct photons should dominatewith the distribution during the 1998/99 quiescent state, where a steady-state halo contribution should be most pronounced. The results depend on the assumptions concerning the angular distribution of the halo; limits on the halo flux within 0.5 • to 1 • from the source range between 0.1% and 1% of the peak burst flux.
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Papers by Andreas Heusler