Measurement of the ratio h/e with a photomultiplier tube and a set of LEDs

We propose a laboratory experience aimed at undergraduate physics students to understand the main features of the photoelectric effect and to perform a measurement of the ratio h/e, where h is the Planck's constant and e is the electron charge. The experience is based on the method developed by Millikan for his measurements on the photoelectric effect in the years from 1912 to 1915. The experimental setup consists of a photomultiplier tube (PMT) equipped with a voltage divider properly modified to set variable retarding potentials between the photocathode and the first dynode, and a set of LEDs emitting at different wavelengths. The photocathode is illuminated with the various LEDs and, for each wavelength of the incident light, the output anode current is measured as a function of the retarding potential applied between the cathode and the first dynode. From each measurement, a value of the stopping potential for the anode current is derived. Finally, the stopping potentials are plotted as a function of the frequency of the incident light, and a linear fit is performed. The slope and the intercept of the line allow respectively to evaluate the ratio h/e and the ratio W/e, where W is the work function of the photocathode.Comment: 14 pages, 4 figure

Exploring the bulk of the BL Lac object population. II. Gamma-ray properties

AIMS. We are studying an unbiased sample of 42 nearby (z < 0.2) BL Lacertae objects with a multi-wavelength approach. The results of VLBI observations were presented in the first paper of this series. In this paper, we study the $\gamma$-ray properties of the sample. METHODS. We analyse data collected by the Fermi LAT during its first 8.5 years of operation in the energy range 0.1-300 GeV. RESULTS. We reveal 23 sources with a test statistic greater than 25 (corresponding to $\sim$ 4.6-$\sigma$) out of 42, with 3 sources not detected in the 3LAC catalogue, and fluxes between $3.5\times 10^{-10}$ and $7.4\times10^{-8}$ ph cm$^{-2}$ s$^{-1}$. The majority of the sources have hard spectra ($\Gamma \leq 2$), with only four having values in the range 2.1-2.4. The three newly detected sources have fluxes in the range between $0.54\times10^{-9}$ and $1.35\times10^{-9}$ ph cm$^{-2}$ s$^{-1}$, and photon index 1.7-1.9. Among the 23 LAT-detected sources, 19 are included in the 3FHL, with a spectrum that connects relatively smoothly from 0.1 GeV to 2 TeV. LAT-detected BL Lacs are more luminous on parsec scales with respect to non-LAT-detected sources and have larger core dominance according to the unified models. CONCLUSIONS. The LAT-detected BL Lacs seem to be composed of a bulk of "classical" sources dominated by Doppler boosting and characterised by compact and bright radio emission as well as hard $\gamma$-ray spectra. Moreover, we have identified a possible population of low-luminosity BL Lacs not detected by LAT, lacking a VLBI core, and with a small Doppler factor. Furthermore, three LAT-detected sources show non-classical properties for $\gamma$-ray emitting BL Lacs (no evidence of relativistic jet, low Doppler factor in radio images, relatively low core dominance) and three other sources, while showing radio emission on parsec scales, are not detected in $\gamma$ rays so far.Comment: 10 pages, 12 figures, 4 tables. Astronomy & Astrophysics, 618A.17

Applicability of the kp method to modeling of InAs/GaSb short-period superlattices

We investigate the long-standing controversy surrounding modeling of the electronic spectra of InAs/GaSb short-period superlattices (SPSLs). Most commonly, such modeling for semiconductor heterostructures is based on the kp method. However, this method has so far failed to predict the band structure for type-II InAs/GaSb SPSLs. Instead, it has systematically overestimated the energy gap between the electron and heavy-hole minibands, which led to the suggestion that the kp method is inadequate for these heterostructures. Our results show that the physical origin of the discrepancy between modeling and experimental results may be the graded and asymmetric InAs/GaSb interface profile. We have performed band-structure modeling within the kp method using a realistic interface profile based on experimental observations. Our calculations show good agreement with experimental data, both from our own measurements and from the published literature. © 2009 The American Physical Society

The Influence of a Continuum Background on Carrier Relaxation in InAs/InGaAs Quantum Dot

We have investigated the ultra-fast carrier dynamics in Molecular Beam Epitaxy (MBE)-grown InAs/InGaAs/GaAs quantum dots (QDs) emitting at 1.3 μm by time resolved photoluminescence (TRPL) upconversion measurements with a time resolution of about 200 fs. Changing the detection energies in the spectral region from the energy of the quantum dots excitonic transition up to the barrier layer absorption edge, we have found that, under high excitation intensity, the intrinsic electronic states are populated mainly by carriers directly captured from the barrier

Search for extended gamma-ray emission from the Virgo galaxy cluster with Fermi-LAT

Galaxy clusters are one of the prime sites to search for dark matter (DM) annihilation signals. Depending on the substructure of the DM halo of a galaxy cluster and the cross sections for DM annihilation channels, these signals might be detectable by the latest generation of $\gamma$-ray telescopes. Here we use three years of Fermi Large Area Telescope (LAT) data, which are the most suitable for searching for very extended emission in the vicinity of nearby Virgo galaxy cluster. Our analysis reveals statistically significant extended emission which can be well characterized by a uniformly emitting disk profile with a radius of 3\deg that moreover is offset from the cluster center. We demonstrate that the significance of this extended emission strongly depends on the adopted interstellar emission model (IEM) and is most likely an artifact of our incomplete description of the IEM in this region. We also search for and find new point source candidates in the region. We then derive conservative upper limits on the velocity-averaged DM pair annihilation cross section from Virgo. We take into account the potential $\gamma$-ray flux enhancement due to DM sub-halos and its complex morphology as a merging cluster. For DM annihilating into $b\overline{b}$, assuming a conservative sub-halo model setup, we find limits that are between 1 and 1.5 orders of magnitude above the expectation from the thermal cross section for $m_{\mathrm{DM}}\lesssim100\,\mathrm{GeV}$. In a more optimistic scenario, we exclude $\langle \sigma v \rangle\sim3\times10^{-26}\,\mathrm{cm^{3}\,s^{-1}}$ for $m_{\mathrm{DM}}\lesssim40\,\mathrm{GeV}$ for the same channel. Finally, we derive upper limits on the $\gamma$-ray-flux produced by hadronic cosmic-ray interactions in the inter cluster medium. We find that the volume-averaged cosmic-ray-to-thermal pressure ratio is less than $\sim6\%$.Comment: 15 pages, 11 figures, 4 tables, accepted for publication in ApJ; corresponding authors: T. Jogler, S. Zimmer & A. Pinzk

Constraints on dark matter models from a Fermi LAT search for high-energy cosmic-ray electrons from the Sun

During its first year of data taking, the Large Area Telescope (LAT) onboard the Fermi Gamma-Ray Space Telescope has collected a large sample of high-energy cosmic-ray electrons and positrons (CREs). We present the results of a directional analysis of the CRE events, in which we searched for a flux excess correlated with the direction of the Sun. Two different and complementary analysis approaches were implemented, and neither yielded evidence of a significant CRE flux excess from the Sun. We derive upper limits on the CRE flux from the Sun's direction, and use these bounds to constrain two classes of dark matter models which predict a solar CRE flux: (1) models in which dark matter annihilates to CREs via a light intermediate state, and (2) inelastic dark matter models in which dark matter annihilates to CREs.Comment: 18 pages, 8 figures, accepted for publication in Physical Review D - contact authors: Francesco Loparco ([email protected]), M. Nicola Mazziotta ([email protected]) and Jennifer Siegal-Gaskins ([email protected]

Gamma-ray flaring activity from the gravitationally lensed blazar PKS 1830-211 observed by Fermi LAT

The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope routinely detects the highly dust-absorbed, reddened, and MeV-peaked flat spectrum radio quasar PKS 1830-211 (z=2.507). Its apparent isotropic gamma-ray luminosity (E>100 MeV) averaged over $\sim$ 3 years of observations and peaking on 2010 October 14/15 at 2.9 X 10^{50} erg s^{-1}, makes it among the brightest high-redshift Fermi blazars. No published model with a single lens can account for all of the observed characteristics of this complex system. Based on radio observations, one expects time delayed variability to follow about 25 days after a primary flare, with flux about a factor 1.5 less. Two large gamma-ray flares of PKS 1830-211 have been detected by the LAT in the considered period and no substantial evidence for such a delayed activity was found. This allows us to place a lower limit of about 6 on the gamma rays flux ratio between the two lensed images. Swift XRT observations from a dedicated Target of Opportunity program indicate a hard spectrum and with no significant correlation of X-ray flux with the gamma-ray variability. The spectral energy distribution can be modeled with inverse Compton scattering of thermal photons from the dusty torus. The implications of the LAT data in terms of variability, the lack of evident delayed flare events, and different radio and gamma-ray flux ratios are discussed. Microlensing effects, absorption, size and location of the emitting regions, the complex mass distribution of the system, an energy-dependent inner structure of the source, and flux suppression by the lens galaxy for one image path may be considered as hypotheses for understanding our results.Comment: 14 pages, 6 figures, 2 tables. Accepted by the The Astrophysical Journal. Corresponding authors: S. Ciprini (ASI ASDC & INAF OAR, Rome, Italy), S. Buson (INAF Padova & Univ. of Padova, Padova, Italy), J. Finke (NRL, Washington, DC, USA), F. D'Ammando (INAF IRA, Bologna, Italy

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 $\Gamma=$3.8$\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 $\tau=2$, 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.Comment: ApJL accepted March 17, 201

Search for Early Gamma-ray Production in Supernovae Located in a Dense Circumstellar Medium with the Fermi LAT

Supernovae (SNe) exploding in a dense circumstellar medium (CSM) are hypothesized to accelerate cosmic rays in collisionless shocks and emit GeV gamma rays and TeV neutrinos on a time scale of several months. We perform the first systematic search for gamma-ray emission in Fermi LAT data in the energy range from 100 MeV to 300 GeV from the ensemble of 147 SNe Type IIn exploding in dense CSM. We search for a gamma-ray excess at each SNe location in a one year time window. In order to enhance a possible weak signal, we simultaneously study the closest and optically brightest sources of our sample in a joint-likelihood analysis in three different time windows (1 year, 6 months and 3 months). For the most promising source of the sample, SN 2010jl (PTF10aaxf), we repeat the analysis with an extended time window lasting 4.5 years. We do not find a significant excess in gamma rays for any individual source nor for the combined sources and provide model-independent flux upper limits for both cases. In addition, we derive limits on the gamma-ray luminosity and the ratio of gamma-ray-to-optical luminosity ratio as a function of the index of the proton injection spectrum assuming a generic gamma-ray production model. Furthermore, we present detailed flux predictions based on multi-wavelength observations and the corresponding flux upper limit at 95% confidence level (CL) for the source SN 2010jl (PTF10aaxf).Comment: Accepted for publication in ApJ. Corresponding author: A. Franckowiak ([email protected]), updated author list and acknowledgement

Observation of the Shadowing of Cosmic Rays by the Moon using a Deep Underground Detector

Using data collected by the MACRO experiment during the years 1989-1996, we show evidence for the shadow of the moon in the underground cosmic ray flux with a significance of 3.6 sigma. This detection of the shadowing effect is the first by an underground detector. A maximum-likelihood analysis is used to determine that the angular resolution of the apparatus is 0.9+/-0.3 degrees. These results demonstrate MACRO's capabilities as a muon telescope by confirming its absolute pointing ability and quantifying its angular resolution.Comment: 14 pages, 8 figures Submitted to Phys. Rev.