arXiv subjects

P. Weber

Publications and source records attributed to P. Weber.

At least 37 records · Page 2Linked to original sources

arXiv2023

VLBI Scrutiny of a New Neutrino-Blazar Multiwavelength-Flare Coincidence

In the past years, evidence has started piling up that some high-energy cosmic neutrinos can be associated with blazars in flaring states. On February 26, 2022, a new blazar-neutrino coincidence has been reported: the track-like neutrino event IC220225A detected by IceCube is spatially coincident with the flat-spectrum radio quasar PKS 0215+015. Like previous associations, this source was found to be in a high optical and $γ$-ray state. Moreover, the source showed a bright radio outburst, which substantially increases the probability of a true physical association. We have performed six observations with the VLBA shortly after the neutrino event with a monthly cadence and are monitoring the source with the Effelsberg 100m-Telescope, and with the Australia Compact Telescope Array. Here, we present first results on the contemporary parsec-scale jet structure of PKS 0215+015 in total intensity and polarization to constrain possible physical processes leading to neutrino emission in blazars.

astro-ph.HE↗

arXiv2023

The ALMA view of MP Mus (PDS 66): a protoplanetary disk with no visible gaps down to 4 au scales

We present ALMA multiwavelength observations of the protoplanetary disk around the nearby (d$\sim$100 pc) young solar analog MP Mus (PDS 66). These observations at 0.89 mm, 1.3 mm, and 2.2 mm have angular resolutions of $\sim$ 1", 0.05", and 0.25", respectively, and probe the dust and gas in the system with unprecedented detail and sensitivity. The disk appears smooth down to the 4 au resolution of the 1.3 mm observations, in contrast with most disks observed at comparable spatial scales. The dust disk has a radius of 60$\pm$5 au, a dust mass of $0.14_{-0.06}^{+0.11} M_{\rm Jup}$, and a mm spectral index $<2$ in the inner 30 au, suggesting optically thick emission from grains with high albedo in this region. Several molecular gas lines are also detected extending up to 130$\pm$15 au, similar to small grains traced by scattered light observations. Comparing the fluxes of different CO isotopologues with previous models yields a gas mass of $0.1-1 M_{\rm Jup}$, implying a gas to dust ratio of 1-10. We also measure a dynamical stellar mass of $M_{\rm dyn}$=1.30$\pm$0.08 $M_\odot$ and derive an age of 7-10 Myr for the system. The survival of large grains in an evolved disk without gaps/rings is surprising, and it is possible that existing substructures remain undetected due to optically thick emission at 1.3 mm. Alternatively, small structures may still remain unresolved with the current observations. Based on simple scaling relations for gap-opening planets and gap widths, this lack of substructures places upper limits to the masses of planets in the disk as low as 2 $M_\oplus$-0.06 $M_{\rm Jup}$ at $r > 40$ au. The lack of mm emission at radii $r > 60$ au also suggests that the gap in scattered light between 30-80 au is likely not a gap in the disk density, but a shadow cast by a puffed-up inner disk.

astro-ph.EP↗

arXiv2023

SRG/eROSITA-triggered XMM-Newton observations of three Be/X-ray binaries in the LMC: Discovery of X-ray pulsations

Using data from eROSITA, the soft X-ray instrument aboard Spectrum-Roentgen-Gamma (SRG), we report the discovery of two new hard transients, eRASSU J050810.4-660653 and eRASSt J044811.1-691318, in the Large Magellanic Cloud. We also report the detection of the Be/X-ray binary RX J0501.6-7034 in a bright state. We initiated follow-up observations to investigate the nature of the new transients and to search for X-ray pulsations coming from RX J0501.6-7034. We analysed the X-ray spectra and light curves from our XMM-Newton observations, obtained optical spectra using the South African Large Telescope to look for Balmer emission lines and utilised the archival data from the Optical Gravitational Lensing Experiment (OGLE) for the long-term monitoring of the optical counterparts. We find X-ray pulsations for eRASSU J050810.4-660653, RX J0501.6-7034, and eRASSt J044811.1-691318 of 40.6 s, 17.3 s, and 784 s, respectively. The Halpha emission lines with equivalent widths of -10.4 A (eRASSU J050810.4-660653) and -43.9 A (eRASSt J044811.1-691318) were measured, characteristic for a circumstellar disc around Be stars. The OGLE I- and V-band light curves of all three systems exhibit strong variability. A regular pattern of deep dips in the light curves of RX J0501.6-7034 suggests an orbital period of ~451 days. We identify the two new hard eROSITA transients eRASSU J050810.4-660653 and eRASSt J044811.1-691318 and the known Be/X-ray binary RX J0501.6-7034 as Be/X-ray binary pulsars.

astro-ph.HE↗

arXiv2022

eRASSt J040515.6-745202, an X-ray burster in the Magellanic Bridge

During the third all-sky survey (eRASS3), eROSITA, the soft X-ray instrument aboard Spectrum-Roentgen-Gamma, detected a new hard X-ray transient, eRASSt J040515.6-745202, in the direction of the Magellanic Bridge. We arranged follow-up observations and searched for archival data to reveal the nature of the transient. Using X-ray observations with XMM-Newton, NICER, and Swift, we investigated the temporal and spectral behaviour of the source for over about 10 days. The X-ray light curve obtained from the XMM-Newton observation with an 28 ks exposure revealed a type-I X-ray burst with a peak bolometric luminosity of at least 1.4e37 erg/s. The burst energetics are consistent with a location of the burster at the distance of the Magellanic Bridge. The relatively long exponential decay time of the burst of 70 s indicates that it ignited in a H-rich environment. The non-detection of the source during the other eROSITA surveys, twelve and six months before and six months after eRASS3, suggests that the burst was discovered during a moderate outburst which reached 2.6e36 erg/s in persistent emission. During the NICER observations, the source showed alternating flux states with the high level at a similar brightness as during the XMM-Newton observation. This behaviour is likely caused by dips as also seen during the last hour of the XMM-Newton observation. Evidence for a recurrence of the dips with a period of 21.8 hr suggests eRASSt J040515.6-745202 is a low-mass X-ray binary (LMXB) system with an accretion disk seen nearly edge on. We identify a multi-wavelength counterpart to the X-ray source in UVW1 and g, r, i, and z images obtained by the optical/UV monitor on XMM-Newton and the Dark Energy Camera at the Cerro Tololo Inter-American Observatory. (abbreviated)

astro-ph.HE↗

arXiv2021

A multiwavelength analysis of the spiral arms in the protoplanetary disk around WaOph 6

[Full abstract in the paper] In recent years, protoplanetary disks with spiral structures have been detected in scattered light, millimeter continuum, and CO gas emission. The mechanisms causing these structures are still under debate. A popular scenario to drive the spiral arms is the one of a planet perturbing the material in the disk. However, if the disk is massive, gravitational instability is usually the favored explanation. Multiwavelength studies could be helpful to distinguish between the two scenarios. So far, only a handful of disks with spiral arms have been observed in both scattered light and millimeter continuum. We aim to perform an in-depth characterization of the protoplanetary disk morphology around WaOph 6 analyzing data obtained at different wavelengths, as well as to investigate the origin of the spiral features in the disk. We present the first near-infrared polarimetric observations of WaOph 6 obtained with SPHERE at the VLT and compare them to archival millimeter continuum ALMA observations. We traced the spiral features in both data sets and estimated the respective pitch angles. We discuss the different scenarios that can give rise to the spiral arms in WaOph 6. We tested the planetary perturber hypothesis by performing hydrodynamical and radiative transfer simulations to compare them with scattered light and millimeter continuum observations.

astro-ph.EP↗

arXiv2020

Extreme ultra-soft X-ray variability in an eROSITA observation of the Narrow-Line Seyfert 1 Galaxy 1H 0707-495

The ultra-soft narrow-line Seyfert 1 galaxy 1H 0707-495 is a well-known and highly variable active galactic nucleus (AGN), with a complex, steep X-ray spectrum, and has been studied extensively with XMM-Newton. 1H 0707-495 was observed with the extended ROentgen Survey with an Imaging Telescope Array (eROSITA) aboard the Spectrum-Roentgen-Gamma (SRG) mission on October 11, 2019, for about 60,000 seconds as one of the first calibration and pointed verification phase (CalPV) observations. The eROSITA light curves show significant variability in the form of a flux decrease by a factor of 58 with a 1 sigma error confidence interval between 31 and 235. This variability is primarily in the soft band, and is much less extreme in the hard band. No strong ultraviolet variability has been detected in simultaneous XMM-Newton Optical Monitor observations. The UV emission is about 10^44 erg s^-1, close to the Eddington limit. 1H 0707-495 entered the lowest hard flux state seen in 20 years of XMM-Newton observations. In the eROSITA All-Sky Survey (eRASS) observations taken in April 2020, the X-ray light curve is still more variable in the ultra-soft band, but with increased soft and hard band count rates more similar to previously observed flux states. A model including relativistic reflection and a variable partial covering absorber is able to fit the spectra and provides a possible explanation for the extreme light-curve behaviour. The absorber is probably ionised and therefore more transparent to soft X-rays. This leaks soft X-rays in varying amounts, leading to large-amplitude soft-X-ray variability.

astro-ph.HE↗

arXiv2020

The eROSITA X-ray telescope on SRG

eROSITA (extended ROentgen Survey with an Imaging Telescope Array) is the primary instrument on the Spectrum-Roentgen-Gamma (SRG) mission, which was successfully launched on July 13, 2019, from the Baikonour cosmodrome. After the commissioning of the instrument and a subsequent calibration and performance verification phase, eROSITA started a survey of the entire sky on December 13, 2019. By the end of 2023, eight complete scans of the celestial sphere will have been performed, each lasting six months. At the end of this program, the eROSITA all-sky survey in the soft X-ray band (0.2--2.3\,keV) will be about 25 times more sensitive than the ROSAT All-Sky Survey, while in the hard band (2.3--8\,keV) it will provide the first ever true imaging survey of the sky. The eROSITA design driving science is the detection of large samples of galaxy clusters up to redshifts $z>1$ in order to study the large-scale structure of the universe and test cosmological models including Dark Energy. In addition, eROSITA is expected to yield a sample of a few million AGNs, including obscured objects, revolutionizing our view of the evolution of supermassive black holes. The survey will also provide new insights into a wide range of astrophysical phenomena, including X-ray binaries, active stars, and diffuse emission within the Galaxy. Results from early observations, some of which are presented here, confirm that the performance of the instrument is able to fulfil its scientific promise. With this paper, we aim to give a concise description of the instrument, its performance as measured on ground, its operation in space, and also the first results from in-orbit measurements.

astro-ph.HE↗

arXiv2019

Structural and magnetic properties of 3d transition metal oxide chains on the (001) surfaces of Ir and Pt

We present a survey of the structural and magnetic properties of submonolayer transition metal dioxides on the (001) surfaces of the heavy face-centered cubic (fcc) noble metals Ir and Pt performed by spin-averaged scanning tunneling microscopy (STM) and spin-polarized (SP-)STM. Our STM results confirm that deposition of Co, Fe, Mn, and Cr on the (2 {\times} 1) oxygen-reconstructed Ir(001) surface leads to the formation of quasi one-dimensional chains with a (3 {\times} 1) unit cell. As recently predicted by density functional theory [Ferstl et al., Phys. Rev. Lett. 117, 046101 (2016)], our SP-STM images of FeO2 and MnO2 on Ir(001) show a two-fold periodicity along the chains which is characteristic for an antiferromagnetic coupling along the chains. In addition, these two materials also exhibit spontaneous, permanent, and long-range magnetic coupling across the chains. Whereas we find a ferromagnetic inter-chain coupling for FeO2/Ir(001), the magnetic coupling of MnO2 on Ir(001) appears to be a non-collinear 120° spin spiral, resulting in a (9 {\times} 2) magnetic unit cell. On Pt(001) patches of (3 {\times} 1)-reconstructed oxide chains could only be prepared by transition metal (Co, Fe, and Mn) deposition onto the cold substrate and subsequent annealing in an oxygen atmosphere. Again SP-STM on MnO2/Pt(001) reveals a very large, (15 {\times} 2) magnetic unit cell which can tentatively be explained by a commensurate 72° spin spiral. Large scale SP-STM images reveal a long wavelength spin rotation along the MnO2 chain.

cond-mat.mtrl-sci↗

arXiv2019

A volumetric Modulated Arc Therapy (VMAT) combined with deep inspiration breath hold (DIBH) technique for adjuvant irradiation for left sided breast cancer

Background: The deep inspiration breath hold technique (DIBH) is widely used for left-sided breast radiotherapy (RT), in order to reduce the dose to the heart and the risk of cardiotoxicity. The volumetric arc therapy (VMAT) technique increases the dose conformity to the planning target volume (PTV). We hereby describe the procedure of combining VMAT and DIBH without a dedicated equipment, and report a dosimetric study related on our implemented technique. Methods: The DIBH technique is based on voluntary breath hold of the patient which is controlled by a laser on tattoo marks. Patients were selected depending their ability to breath hold for 25s, and VMAT plans were optimized with an arrangement of 4 arcs with a maximum delivery time of 20s each. A retrospective dosimetric study was undertaken on 30 patients treated with this technique: 10 received local (whole breast) RT (L-RT) with a simultaneous integrated boost (SIB), 10 a locoregional (LR)RT of the chestwall, 10 a LR RT of the whole breast with SIB. For these patients, their DIBH-VMAT treatment plans were compared to free breathing (FB) VMAT plans, and dose characteristics of PTV and organs at risk (OAR) were evaluated. Results: The PTV coverage was identical among plans under DIBH or FB. The mean heart dose was reduced from 2.8+/-0.9Gy in FB to 1.8+/-0.6Gy in DIBH for local RT, and reduced from 3.4+/-1.3Gy in FB to 2.2+/-0.5Gy in DIBH for LR-RT. The ipsilateral mean lung dose was reduced by a factor of 12% for LR-RT, but remained unchanged for L-RT with DIBH-VMAT compared to FB treatment. No significant differences were observed in the mean dose of contra-lateral organs with either DIBH or FB VMAT techniques. Conclusion: The VMAT-DIBH technique performed without a dedicated breath hold equipment, provided important dose reductions at the heart, while preserving the ipsilateral lung and contralateral normal tissues.

physics.med-ph↗

arXiv2018

High quality factor graphene-based 2D heterostructure mechanical resonator

Ultralight mechanical resonators based on low-dimensional materials are well suited as exceptional transducers of minuscule forces or mass changes. However, the low dimensionality also provides a challenge to minimize resistive losses and heating. Here, we report on a novel approach that aims to combine different 2D materials to tackle this challenge. We fabricated a heterostructure mechanical resonator consisting of few layers of niobium diselenide (NbSe$_2$) encapsulated by two graphene sheets. The hybrid membrane shows high quality factors up to 245'000 at low temperatures, comparable to the best few-layer graphene mechanical resonators. In contrast to few-layer graphene resonators, the device shows reduced electrical losses attributed to the lower resistivity of the NbSe$_2$ layer. The peculiar low temperature dependence of the intrinsic quality factor points to dissipation over two-level systems which in turn relax over the electronic system. Our high sensitivity readout is enabled by coupling the membrane to a superconducting cavity which allows for the integration of the hybrid mechanical resonator as a sensitive and low loss transducer in future quantum circuits.

cond-mat.mes-hall↗

arXiv2016

Switching off energy decay channels in nanomechanical resonators

Energy decay plays a central role in a wide range of phenomena, such as optical emission, nuclear fission, and dissipation in quantum systems. Energy decay is usually described as a system leaking energy irreversibly into an environmental bath. Here, we report on energy decay measurements in nanomechanical systems based on multi-layer graphene that cannot be explained by the paradigm of a system directly coupled to a bath. As the energy of a vibrational mode freely decays, the rate of energy decay switches abruptly to a lower value. This finding can be explained by a model where the measured mode hybridizes with other modes of the resonator at high energy. Below a threshold energy, modes are decoupled, resulting in comparatively low decay rates and giant quality factors exceeding 1 million. Our work opens up new possibilities to manipulate vibrational states, engineer hybrid states with mechanical modes at completely different frequencies, and to study the collective motion of this highly tunable system.

cond-mat.mes-hall↗

arXiv2015

Switchable Coupling of Vibrations to Two-Electron Carbon-Nanotube Quantum Dot States

We report transport measurements on a quantum dot in a partly suspended carbon nanotube. Electrostatic tuning allows us to modify and even switch 'on' and 'off' the coupling to the quantized stretching vibration across several charge states. The magnetic-field dependence indicates that only the two-electron spin-triplet excited state couples to the mechanical motion, indicating mechanical coupling to both the valley degree of freedom and the exchange interaction, in contrast to standard models.

cond-mat.mes-hall↗

arXiv2014

Coupling graphene mechanical resonators to superconducting microwave cavities

Graphene is an attractive material for nanomechanical devices because it allows for exceptional properties, such as high frequencies and quality factors, and low mass. An outstanding challenge, however, has been to obtain large coupling between the motion and external systems for efficient readout and manipulation. Here, we report on a novel approach, in which we capacitively couple a high-Q graphene mechanical resonator ($Q \sim 10^5$) to a superconducting microwave cavity. The initial devices exhibit a large single-photon coupling of $\sim 10$ Hz. Remarkably, we can electrostatically change the graphene equilibrium position and thereby tune the single photon coupling, the mechanical resonance frequency and the sign and magnitude of the observed Duffing nonlinearity. The strong tunability opens up new possibilities, such as the tuning of the optomechanical coupling strength on a time scale faster than the inverse of the cavity linewidth. With realistic improvements, it should be possible to enter the regime of quantum optomechanics.

cond-mat.mes-hall↗

arXiv2013

Low-frequency Noise in Individual Carbon Nanotube Field-Effect Transistors with Top, Side and Back Gate Configurations: Effect of Gamma Irradiation

We report on the influence of low gamma irradiation (10^4 Gy) on the noise properties of individual carbon nanotube (CNT) field-effect transistors (FETs) with different gate configurations and two different dielectric layers, SiO2 and Al2O3. Before treatment, strong generation-recombination (GR) noise components are observed. These data are used to identify several charge traps related to dielectric layers of the FETs by determining their activation energy. Investigation of samples with a single SiO2 dielectric layer as well as with two dielectric layers allows us to separate traps for each of the two dielectric layers. We reveal that each charge trap level observed in the side gate operation splits into two levels in top gate operation due to a different potential profile along the CNT channel. After gamma irradiation, only reduced flicker noise is registered in the noise spectra, which indicates a decrease of the number of charge traps. The mobility, which is estimated to be larger than 2x10^4 cm2V-1s-1 at room temperature, decreases only slightly after radiation treatment demonstrating high radiation hardness of the CNTs. Finally, we study the influence of Schottky barriers at the metal-nanotube interface on the transport properties of FETs analyzing the behavior of the flicker noise component.

cond-mat.mtrl-sci↗

arXiv2013

Transport in coupled graphene-nanotube quantum devices

We report on the fabrication and characterization of all-carbon hybrid quantum devices based on graphene and single-walled carbon nanotubes. We discuss both, carbon nanotube quantum dot devices with graphene charge detectors and nanotube quantum dots with graphene leads. The devices are fabricated by chemical vapor deposition growth of carbon nanotubes and subsequent structuring of mechanically exfoliated graphene. We study the detection of individual charging events in the carbon nanotube quantum dot by a nearby graphene nanoribbon and show that they lead to changes of up to 20% of the conductance maxima in the graphene nanoribbon acting as a good performing charge detector. Moreover, we discuss an electrically coupled graphene-nanotube junction, which exhibits a tunneling barrier with tunneling rates in the low GHz regime. This allows to observe Coulomb blockade on a carbon nanotube quantum dot with graphene source and drain leads.

cond-mat.mes-hall↗

arXiv2011

Simulation of energy barrier distributions using real particle parameters and comparison with experimental obtained results

In this work we compare previously measured energy barriers over the course of temperature with the results of simulations of the behaviour of the energy barriers. For the measurements the temperature dependent magnetorelaxation method (TMRX) was used. For the simulations of the energy barrier distribution we have used the real particles properties such as anisotropy and core size volume of the fractions of two magnetically fractionated ferrofluids. There is a good agreement between the simulated behaviour and the experimental obtained results. The influence of the particle volume concentration and agglomeration on the energy barrier distribution has been investigated. Finally the simulations confirm a previously published explanation for an experimentally obtained relaxation effect.

cond-mat.mes-hall↗

arXiv2008

Systematic study of trace radioactive impurities in candidate construction materials for EXO-200

The Enriched Xenon Observatory (EXO) will search for double beta decays of 136Xe. We report the results of a systematic study of trace concentrations of radioactive impurities in a wide range of raw materials and finished parts considered for use in the construction of EXO-200, the first stage of the EXO experimental program. Analysis techniques employed, and described here, include direct gamma counting, alpha counting, neutron activation analysis, and high-sensitivity mass spectrometry.

physics.ins-det↗

arXiv2007

Observation of single collisionally cooled trapped ions in a buffer gas

Individual Ba ions are trapped in a gas-filled linear ion trap and observed with a high signal-to-noise ratio by resonance fluorescence. Single-ion storage times of ~5 min (~1 min) are achieved using He (Ar) as a buffer gas at pressures in the range 8e-5 - 4e-3 torr. Trap dynamics in buffer gases are experimentally studied in the simple case of single ions. In particular, the cooling effects of light gases such as He and Ar and the destabilizing properties of heavier gases such as Xe are studied. A simple model is offered to explain the observed phenomenology.

physics.atom-ph↗