High Gamma-ray Flux Research Articles

An investigation of the state changes of PSR J2021+4026 and the Vela pulsar

ABSTRACT We report on long-term evolution of gamma-ray flux and spin-down rate of two bright gamma-ray pulsars, PSR J2021+4026 and Vela (PSR J0835−4510). PSR J2021+4026 shows repeated state changes in gamma-ray flux and spin-down rate. We report two new state changes, a first one from a low gamma-ray flux to a high flux that occurred around MJD 58910, and a second one from high to low flux that occurred around MJD 59510. We find that the flux changes associated with these two state changes are smaller than those determined in the previous events, and the waiting time of the new state change from the high gamma-ray flux to low gamma-ray flux is significantly shorter than previous events. Since the waiting time-scale of the quasi-periodic state changes of PSR J2021+4026 is similar to the waiting time-scale of the glitch events of the Vela pulsar, we search for the state change of the gamma-ray emission of the Vela pulsar to investigate the possibility that the glitching process is the trigger of the state change of PSR J2021+4026. For the Vela pulsar, the flux of the radio pulses briefly decreased around the 2016 glitch, suggesting that the glitch may have affected the structure of the magnetosphere. Nevertheless, we could not find any significant change of the gamma-ray emission properties using 15 yr of Fermi-LAT data. Overall, it seems inconclusive that a glitch-like process similar to that occurred to the Vela pulsar triggers the structure change of the global magnetosphere and causes state changes of PSR J2021+4026. Further and deep investigations to clarify the mechanism of the mode change for PSR J2021+4026 are required.

H.E.S.S. and Fermi-LAT observations of PSR B1259–63/LS 2883 during its 2014 and 2017 periastron passages

Context. PSR B1259–63/LS 2883 is a gamma-ray binary system consisting of a pulsar in an eccentric orbit around a bright Oe stellar-type companion star that features a dense circumstellar disc. The bright broad-band emission observed at phases close to periastron offers a unique opportunity to study particle acceleration and radiation processes in binary systems. Observations at gamma-ray energies constrain these processes through variability and spectral characterisation studies. Aims. The high- and very-high-energy (HE, VHE) gamma-ray emission from PSR B1259–63/LS 2883 around the times of its periastron passage are characterised, in particular, at the time of the HE gamma-ray flares reported to have occurred in 2011, 2014, and 2017. Short-term and average emission characteristics of PSR B1259–63/LS 2883 are determined. Super-orbital variability is searched for in order to investigate possible cycle-to-cycle VHE flux changes due to different properties of the companion star’s circumstellar disc and/or the conditions under which the HE gamma-ray flares develop. Methods. Spectra and light curves were derived from observations conducted with the H.E.S.S-II array in 2014 and 2017. Phase-folded light curves are compared with the results obtained in 2004, 2007, and 2011. Fermi-LAT observations from 2010/11, 2014, and 2017 are analysed. Results. A local double-peak profile with asymmetric peaks in the VHE light curve is measured, with a flux minimum at the time of periastron tp and two peaks coinciding with the times at which the neutron star crosses the companion’s circumstellar disc (~tp ± 16 d). A high VHE gamma-ray flux is also observed at the times of the HE gamma-ray flares (~tp + 30 d) and at phases before the first disc crossing (~tp − 35 d). The spectral energy range now extends to below 200 GeV and up to ~45 TeV. Conclusions. PSR B1259–63/LS 2883 displays periodic flux variability at VHE gamma-rays without clear signatures of super-orbital modulation in the time span covered by the monitoring of the source with the H.E.S.S. telescopes. This flux variability is most probably caused by the changing environmental conditions, particularly at times close to periastron passage at which the neutron star is thought to cross the circumstellar disc of the companion star twice. In contrast, the photon index remains unchanged within uncertainties for about 200 d around periastron. At HE gamma-rays, PSR B1259–63/LS 2883 has now been detected also before and after periastron, close to the disc crossing times. Repetitive flares with distinct variability patterns are detected in this energy range. Such outbursts are not observed at VHEs, although a relatively high emission level is measured. The spectra obtained in both energy regimes displays a similar slope, although a common physical origin either in terms of a related particle population, emission mechanism, or emitter location is ruled out.

New ALMA and Fermi/LAT Observations of the Large-scale Jet of PKS 0637−752 Strengthen the Case Against the IC/CMB Model

Abstract The Chandra X-ray observatory has discovered several dozen anomalously X-ray-bright jets associated with powerful quasars. A popular explanation for the X-ray flux from the knots in these jets is that relativistic synchrotron-emitting electrons inverse-Compton scatter cosmic microwave background (CMB) photons to X-ray energies (the IC/CMB model). This model predicts a high gamma-ray flux that should be detectable by the Fermi/Large Area Telescope (LAT) for many sources. GeV-band upper limits from Fermi/LAT for the well-known anomalous X-ray jet in PKS 0637−752 were previously shown in Meyer et al. to violate the predictions of the IC/CMB model. Previously, measurements of the jet synchrotron spectrum, important for accurately predicting the gamma-ray flux level, were lacking between radio and infrared wavelengths. Here, we present new Atacama Large Millimeter/submillimeter Array (ALMA) observations of the large-scale jet at 100, 233, and 319 GHz, which further constrain the synchrotron spectrum, supporting the previously published empirical model. We also present updated limits from the Fermi/LAT using the new “Pass 8” calibration and approximately 30% more time on source. With these deeper limits, we rule out the IC/CMB model at the 8.7σ level. Finally, we demonstrate that complete knowledge of the synchrotron SED is critical in evaluating the IC/CMB model.

Non-planar four-mirror optical cavity for high intensity gamma ray flux production by pulsed laser beam Compton scattering off GeV-electrons

As part of the R&D toward the production of high flux of polarised Gamma-rays we have designed and built a non-planar four-mirror optical cavity with a high finesse and operated it at a particle accelerator. We report on the main challenges of such cavity, such as the design of a suitable laser based on fiber technology, the mechanical difficulties of having a high tunability and a high mechanical stability in an accelerator environment and the active stabilization of such cavity by implementing a double feedback loop in a FPGA.

PARSEC-SCALE JET BEHAVIOR OF THE QUASAR 3C273 DURING A HIGH GAMMA-RAY STATE IN 2009–2010

We analyze total and polarized intensity images of the quasar 3C273 obtained at 43 GHz monthly during 2008-2010 with the VLBA. We find a strong increase of activity in the parsec-scale jet during a high γ-ray state lasting from 2009 June to 2010 May featuring an increase of the total and polarized flux in the mm-wave core and the appearance of superluminal knots in the jets. Although the optical variability is low, owing to strong contamination by the big blue bump, we observe an increase in optical polarization by a factor of 3 during a gamma-ray flare in 2010 April. We discuss the locations in the jet where high gamma-ray fluxes occur. These include stationary features in the jet flow that we identify with standing shocks.

The γ-ray brightest days of the blazar 3C 454.3

In the first week of December 2009, the blazar 3C 454.3 became the brightest high energy source in the sky. Its photon flux reached and surpassed the level of 1e-5 ph/cm2/s above 100 MeV. The Swift satellite observed the source several times during the period of high gamma-ray flux, and we can construct really simultaneous spectral energy distributions (SED) before, during, and after the luminosity peak. Our main findings are: i) the optical, X-ray and gamma-ray fluxes correlate; ii) the gamma-ray flux varies quadratically (or even more) with the optical flux; iii) a simple one-zone synchrotron inverse Compton model can account for all the considered SED; iv) in this framework the gamma-ray vs optical flux correlation can be explained if the magnetic field is slightly fainter when the overall jet luminosity is stronger; v) the power that the jet spent to produce the peak gamma-ray luminosity is of the same order, or larger, than the accretion disk luminosity. During the flare, the total jet power surely surpassed the accretion power.

HIGH-ENERGY GAMMA-RAY AFTERGLOWS FROM LOW-LUMINOSITY GAMMA-RAY BURSTS

The observations of gamma-ray bursts (GRBs) such as 980425, 031203 and 060218, with luminosities much lower than those of other classic bursts, lead to the definition of a new class of GRBs -- low-luminosity GRBs. The nature of the outflow responsible for them is not clear yet. Two scenarios have been suggested: one is the conventional relativistic outflow with initial Lorentz factor of order of $\Gamma_0\ga 10$ and the other is a trans-relativistic outflow with $\Gamma_0\simeq 1-2$. Here we compare the high energy gamma-ray afterglow emission from these two different models, taking into account both synchrotron self inverse-Compton scattering (SSC) and the external inverse-Compton scattering due to photons from the cooling supernova or hypernova envelope (SNIC). We find that the conventional relativistic outflow model predicts a relatively high gamma-ray flux from SSC at early times ($<10^4 {\rm s}$ for typical parameters) with a rapidly decaying light curve, while in the trans-relativistic outflow model, one would expect a much flatter light curve of high-energy gamma-ray emission at early times, which could be dominated by both the SSC emission and SNIC emission, depending on the properties of the underlying supernova and the shock parameter $\epsilon_e$ and $\epsilon_B$. The Fermi Gamma-ray Space Telescope should be able to distinguish between the two models in the future.

SUZAKU OBSERVATIONS OF THE EXTREME MeV BLAZAR SWIFT J0746.3+2548

We report the Suzaku observations of the high luminosity blazar SWIFT J0746.3+2548 (J0746) conducted in November 2005. This object, with z = 2.979, is the highest redshift source observed in the Suzaku Guaranteed Time Observer (GTO) period, is likely to show high gamma-ray flux peaking in the MeV range. As a result of the good photon statistics and high signal-to-noise ratio spectrum, the $Suzaku$ observation clearly confirms that J0746 has an extremely hard spectrum in the energy range of 0.3-24 keV, which is well represented by a single power-law with a photon index of 1.17 and Galactic absorption. The multiwavelength spectral energy distribution of J0746 shows two continuum components, and is well modeled assuming that the high-energy spectral component results from Comptonization of the broad-line region photons. In this paper we search for the bulk Compton spectral features predicted to be produced in the soft X-ray band by scattering external optical/UV photons by cold electrons in a relativistic jet. We discuss and provide constraints on the pair content resulting from the apparent absence of such features.

Radiolysis of aqueous cesium iodide

In hypothetical accident scenarios for Light Water reactors, the extent of release of iodine upon irradiation needs to be assessed for the purpose of evaluation of the applicable source term. In this context, an understanding of the behaviour of aqueous cesium iodide solutions subjected to high gamma-ray fluxes acquires significant importance. In the present work, gamma radiolysis of a cesium iodide solution (10−2M I−) with and without boron additive is investigated by irradiating with60Co source at ambient temperature. Upon irradiation of the CsI solution, iodine is liberated, and the concentration of iodide in the KOH trap present in the radiolysis vessel increases with dose. The radiolytic products I 3 − , IO 3 − and H2O2 formed in the irradiated solution are also estimated and G values obtained are reported. G(I 3 − ) and G(IO 3 − ) are of the order of 10−3 and 10−4, respectively. G(H2O2) decreases with increase in dose. Addition of boron up to 200 ppm, does not appear to alter significantly the release fraction of iodine.

Pulsars and the Distribution of Celestial Gamma Rays

The flux of celestial gamma rays from discrete sources (for example, pulsars) is calculated, showing that these sources emit fluxes of several ${10}^{\ensuremath{-}6}$ quanta/${\mathrm{cm}}^{2}$ sec over regions of several tens of degrees (among them Vela X). Hence they should be observable. The calculations show further that the high gamma-ray flux values of OSO-III follow from the expected correlation of cosmic-ray sources with regions of large hydrogen density.

878. Vacuum diode detector for measuring high intensity gamma-ray flux: P J Ebert and A F Lauzon, Rev sci instrum, 38 (12), 1967, 1747–1752.

878. Vacuum diode detector for measuring high intensity gamma-ray flux: P J Ebert and A F Lauzon, Rev sci instrum, 38 (12), 1967, 1747–1752.