We find an explicit formula for the gamma vector in terms of the input polynomial in a way that extends it to arbitrary polynomials. More specifically, we find explicit linear combination in terms of coefficients of the input polynomial (using Catalan numbers and binomial coefficients) and an expression involving the derivative of the input polynomial. The first expression suggests connections to common Coxeter group/noncrossing partition structures in existing gamma positivity examples. In the case where the input is the $h$-polynomial of a simplicial complex, this gives an interpretation of the gamma vector as a measure of differences in local and global contributions. We also apply them to connect signs/inequalities of (shifts of) the gamma vector to upper/lower bound conditions on coefficients of the input polynomial. Finally, we make use of the shape of the sums used to make these estimates and connections with intersection numbers to relate these properties of the gamma vector to algebraic structures (e.g. characteristic classes involved in existing log concavity and Schur positivity properties).
Indirect detection of gamma rays with ground-based observatories is currently the most sensitive experimental approach to characterize the gamma-ray sky at energies $>0.1$\,TeV. Ground-based detection of gamma-rays relies on the electromagnetic showers that gamma rays initiate in the Earth's atmosphere. In this chapter we will review the properties of electromagnetic air showers as well as the differences with respect to cosmic-ray showers that enable the rejection of the cosmic ray background. The experimental techniques that have been developed for ground-based detection of gamma rays will be introduced. These fall onto three main categories: air shower particle detectors, sampling Cherenkov arrays, and imaging atmospheric Cherenkov telescopes. Hybrid concepts as well as other experimental approaches are also discussed.
Detectors for gamma-ray astronomy are complex: they often comprise multiple sub-systems and utilize new and/or custom-developed detector components and readout electronics. Gamma rays are typically not detected directly: ground-based detectors measure extensive air showers of charged particles initiated by cosmic gamma-rays, and even so-called "direct detection" experiments on balloons or satellites usually reconstruct the incoming gamma-ray photons' properties from the secondary particles produced in the detector. At the same time, there are few "standard candles" and no feasible terrestrial sources of high-energy and very-high-energy gamma rays that could be used to calibrate the detectors. Simulations of particles interacting in the atmosphere and/or with the instrument are thus ubiquitous in gamma-ray astronomy. These simulations are used in event reconstruction and data analysis, to characterize detector performance, and to optimize detector design. In this chapter, we give an overview of how and why simulations are used in gamma-ray astronomy, as well as their limitations. We discuss extensive air shower simulations, simulations of gamma rays and secondary particles interacti
The MAGIC telescopes, located at Observatorio El Roque de los Muchachos (La Palma, Spain) are two Imaging Air Cherenkov Telescopes observing the Very High Energy (VHE) gamma rays. They are run by an international collaboration composed of over 40 institutions from 12 countries. The first telescope was inaugurated in October 2003. The commissioning of the second finished in 2008. The MAGIC telescopes were designed to lower the energies to which ground based telescopes had access as well as to be able to point to any direction in the sky in less than 25 seconds. The former required the large reflective surface of 17 meters as well as an effort to optimise the mirror reflectivity and photo sensor sensitivity. The latter was achieved by minimising the weight of the full instrument using for instance carbon fibre reinforced plastic tubes for the mirror frame. The sensitivity of the MAGIC telescopes have been improving over the years thanks to hardware upgrades as well as new analysis techniques, which allowed the collaboration to keep a rich scientific program. The discovery of VHE emission from Gamma Ray Bursts and pulsars have called for a revision of the models that explain the produ
If gamma ray bursts are highly collimated, radiating into only a small fraction of the sky, the energy requirements of each event may be reduced by several (up to 4 - 6) orders of magnitude, and the event rate increased correspondingly. The large Lorentz factors (Gamma > 100) inferred from GRB spectra imply relativistic beaming of the gamma rays into an angle 1/Gamma. We are at present ignorant of whether there are ejecta outside this narrow cone. Afterglows allow empirical tests of whether GRBs are well-collimated jets or spherical fireballs. The bulk Lorentz factor decreases and radiation is beamed into an ever increasing solid angle as the burst remnant expands. It follows that if gamma ray bursts are highly collimated, many more optical and radio transients should be observed without associated gamma rays than with them. In addition, a burst whose ejecta are beamed into angle zeta undergoes a qualitative change in evolution when Gamma < 1/zeta: Before this, Gamma ~ r^{-3/2}, while afterwards, Gamma decays exponentially with r. This change results in a potentially observable break in the afterglow light curve. Successful application of either test would eliminate the large
Gamma-sky.net is a novel interactive website designed for exploring the gamma-ray sky. The Map View portion of the site is powered by the Aladin Lite sky atlas, providing a scalable survey image tesselated onto a three-dimensional sphere. The map allows for interactive pan and zoom navigation as well as search queries by sky position or object name. The default image overlay shows the gamma-ray sky observed by the Fermi-LAT gamma-ray space telescope. Other survey images (e.g. Planck microwave images in low/high frequency bands, ROSAT X-ray image) are available for comparison with the gamma-ray data. Sources from major gamma-ray source catalogs of interest (Fermi-LAT 2FHL, 3FGL and a TeV source catalog) are overlaid over the sky map as markers. Clicking on a given source shows basic information in a popup, and detailed pages for every source are available via the Catalog View component of the website, including information such as source classification, spectrum and light-curve plots, and literature references. We intend for gamma-sky.net to be applicable for both professional astronomers as well as the general public. The website started in early June 2016 and is being developed as
A search has been carried out for events in the channel p-barp --> gamma gamma jet jet. Such a signature can characterize the production of a non-standard Higgs boson together with a W or Z boson. We refer to this non-standard Higgs, having standard model couplings to vector bosons but no coupling to fermions, as a "bosonic Higgs." With the requirement of two high transverse energy photons and two jets, the diphoton mass (m(gamma gamma)) distribution is consistent with expected background. A 90(95)% C.L. upper limit on the cross section as a function of mass is calculated, ranging from 0.60(0.80) pb for m(gamma gamma) = 65 GeV/c^2 to 0.26(0.34) pb for m(gamma gamma) = 150 GeV/c^2, corresponding to a 95% C.L. lower limit on the mass of a bosonic Higgs of 78.5 GeV/c^2.
Very high energy gamma-rays(E>20GeV) from blazars traversing cosmological distances through the metagalactic radiation field can convert to electron-positron pairs in photon-photon collisions. The converted gamma-rays initiate electromagnetic cascades driven by inverse-Compton scattering off the microwave background photons. The cascades shift the injected gamma ray spectrum to MeV-GeV energies. Randomly oriented magnetic fields rapidly isotropize the secondary electron-positron beams resulting from the beamed blazar gamma ray emission, leading o faint gamma-ray halo. Using a model for the time-dependent metagalactic radiation field consistent with all currently available far-infrared-to-optical data, we compute (i) the expected gamma-ray attenuation in blazar spectra, and (ii) the cascade contribution from faint, unresolved blazars to the extragalactic gamma-ray background as measured by EGRET, assuming a generic emitted spectrum extending to an energy of 10 TeV. The latter cascade contribution to the EGRET background is fed by the assumed >20 GeV emission from the hitherto undiscovered sources, and we estimate their dN-dz distribution taking into account that the nearby (z&
This year, 2015, marks the centenary of the publication of Einsteins Theory of General Relativity and it has been named the International Year of Light and light-based technologies by the UN General Assembly. It is thus timely to discuss the possibility of broadening the present CERN research program by including a new component based on a novel concept of the light source which could pave a way towards a multipurpose Gamma Factory. The proposed light source could be realized at CERN by using the infrastructure of the existing accelerators. It could push the intensity limits of the presently operating light-sources by at least 7 orders of magnitude, reaching the flux of the order of 10^17 photons/s, in the particularly interesting gamma-ray energy domain of 1 < Ephoton < 400 MeV. This domain is out of reach for the FEL-based light sources. The energy-tuned, quasi-monochromatic gamma beams, together with the gamma-beam-driven, high intensity secondary beams of polarized positrons, polarized muons, neutrons and radioactive ions would constitute the basic research tools of the proposed Gamma Factory. The Gamma Factory could open new research opportunities in a vast domain of unc
We propose Gamma Conjectures for Fano manifolds which can be thought of as a square root of the index theorem. Studying the exponential asymptotics of solutions to the quantum differential equation, we associate a principal asymptotic class A_F to a Fano manifold F. We say that F satisfies Gamma Conjecture I if A_F equals the Gamma class G_F. When the quantum cohomology of F is semisimple, we say that F satisfies Gamma Conjecture II if the columns of the central connection matrix of the quantum cohomology are formed by G_F Ch(E_i) for an exceptional collection {E_i} in the derived category of coherent sheaves D^b_coh(F). Gamma Conjecture II refines part (3) of Dubrovin's conjecture. We prove Gamma Conjectures for projective spaces and Grassmannians.
We obtained new upper limits on the diffuse gamma rays from the inner Galactic (IG) and outer Galactic (OG) planes in 3.10 TeV region, using the Tibet air shower data and new Monte Carlo simulation results. A difference of the effective area of the air-shower array for observing gamma rays and cosmic rays was carefully taken into account in this analysis, resulting in that the flux upper limits of the diffuse TeV gamma rays were reduced by factors of 4.0.3.7 for 3.10 TeV than those in our previous results (Amenomori, M., Ayabe, S., Cui, S.W., et al. Observation of multi-TeV diffuse gamma rays from the Galactic plane with the Tibet air shower array. Astrophys. J. 580, 887.895, 2002.). This new result suggests that the inverse power index of the energy spectrum of source electrons responsible for generating diffuse TeV gamma rays through inverse Compton effect should be steeper than 2.2 and 2.1 for IG and OG planes, respectively, with 99%C.L.
We consider the effect of new physics on the branching ratio of B_s--> l+ l- gamma where l=e,mu. If the new physics is of the form scalar/pseudoscalar, then it makes no contribution to B_s--> l+ l- gamma, unlike in the case of B_s--> l+ l-, where it can potentially make a very large contribution. If the new physics is in the form of vector/axial-vector operators, then present data on B-->(K,K^*)l+ l-, does not allow a large enhancement for B_s--> l+ l- gamma. If the new physics is in the form of tensor/pseudotensor operators, then the data on B-->(K,K^*)l+ l- gives no useful constraint but the data on B-->K^* gamma does. Here again, a large enhancement of B(B_s--> l+ l- gamma), much beyond the Standard Model expectation, is not possible. Hence, we conclude that the present data on b-->s transitions allow a large boost in B_s--> l+ l- but not in B_s--> l+ l- gamma.
High-energy gamma rays are a valuable tool for studying particle acceleration and radiation in the magnetospheres of energetic pulsars. The seven or more pulsars seen by instruments on the Compton Gamma Ray Observatory (CGRO) show that: the light curves usually have double-peak structures (suggesting a broad cone of emission); gamma rays are frequently the dominant component of the radiated power; and all the spectra show evidence of a high-energy turnover. For all the known gamma-ray pulsars, multiwavelength observations and theoretical models based on such observations offer the prospect of gaining a broad understanding of these rotating neutron stars. The Gamma-ray Large Area Space Telescope (GLAST), now in planning for a launch in 2007, will provide a major advance in sensitivity, energy range, and sky coverage.
We study several properties of blazars detected in the gamma-ray energy range by comparing the EGRET sources with a sample of radio blazars which can be considered possible gamma-ray candidates. We define three classes: non-gamma-ray blazars, blazars with quasi-steady gamma-ray emission, and gamma-ray blazars with substantial activity level. By combining the information of detected and candidate AGNs, we characterise the blazar activity, including the discovery of a region of consistency between the gamma-ray flaring duty-cycle and the recurrence time between flares. We also find a possible relation between the activity index of FSRQs and their black hole mass.
In the 1990s a comparison of sparse EGRET measurements with single-dish flux density monitoring from the Metsahovi and UMRAO programs established a temporal connection between the onset of flaring at radio band and the occurrence of gamma-ray activity. Correlations between the emergence of new VLBI components from the core, flares in linearly polarized radio flux, and gamma-ray activity in bright EGRET-detected blazars supported a picture in which the gamma-ray and the radio band emission arises in the same shocked region of the jet, with the high energy emission produced via inverse Compton scattering by the synchrotron-emitting electrons in the jet. Quantitative tests of this scenario, however, were hampered by insufficient temporal sampling of the data and the simple nature of the models adopted. The extensive data from Fermi coupled with the wealth of well-sampled radio band data from old as well as new programs such as the F-GAMMA project now permit statistical studies for large numbers of sources, including weak HBLs, and detailed analyses of individual highly-active class members. I summarize progress in understanding the origin of the gamma-ray emission using these new meas
High-energy gamma rays are a valuable tool for studying particle acceleration and radiation in the magnetospheres of energetic pulsars. The six or more pulsars seen by CGRO/EGRET show that: the light curves usually have double-peak structures (suggesting a broad cone of emission); gamma rays are frequently the dominant component of the radiated power; and all the spectra show evidence of a high-energy turnover. Unless a new pulsed component appears at higher energies, progress in gamma-ray pulsar studies will be greatest in the 1-20 GeV range. Ground-based telescopes whose energy ranges extend downward toward 10 GeV should make important measurements of the spectral cutoffs. The Gamma-ray Large Area Space Telescope (GLAST), now in planning for a launch in 2005, will provide a major advance in sensitivity, energy range, and sky coverage.
We describe \,$q$-hypergeometric solutions of the equivariant quantum differential equations and associated qKZ difference equations for the cotangent bundle $T^*F_λ$ of a partial flag variety \,$F_λ$\,. These \,$q$-hypergeometric solutions manifest a Landau-Ginzburg mirror symmetry for the cotangent bundle. We formulate and prove Pieri rules for quantum equivariant cohomology of the cotangent bundle. Our Gamma theorem for \,$T^*F_λ$ \,says that the leading term of the asymptotics of the \,$q$-hypergeometric solutions can be written as the equivariant Gamma class of the tangent bundle of $T^*F_λ$ multiplied by the exponentials of the equivariant first Chern classes of the associated vector bundles. That statement is analogous to the statement of the gamma conjecture by B.\,Dubrovin and by S.\,Galkin, V.\,Golyshev, and H.\,Iritani, see also the Gamma theorem for \,$F_λ$ \,in Appendix B.
Star-forming regions have been tentatively associated with gamma-ray sources since the early days of the COS B satellite. After the Compton Gamma-Ray Observatory, the statistical evidence for such an association has became overwhelming. Recent results from Cherenkov telescopes indicate that some high-energy sources are produced in regions of active star formation like Cygnus OB2 and Westerlund 2. In this paper I will briefly review what kind of stellar objects can produce gamma-ray emission in star-forming regions and I will suggest that the formation process of massive stars could in principle result in the production of observable gamma rays.
We report on a search for the lepton flavor violating tau- -> mu- gamma and tau- -> e- gamma decays based on 535/fb of data accumulated at the Belle experiment. No signal is found and we set 90% confidence level upper limits on the branching ratios Br(tau- -> mu- gamma) < 4.5x10^-8 and Br(tau- -> e- gamma) < 1.2x10^-7.
Pulsed emission from gamma-ray pulsars originates inside the magnetosphere, from radiation by charged particles accelerated near the magnetic poles or in the outer gaps. In polar cap models, the high energy spectrum is cut off by magnetic pair production above an energy that is dependent on the local magnetic field strength. While most young pulsars with surface fields in the range B = 10^{12} - 10^{13} G are expected to have high energy cutoffs around several GeV, the gamma-ray spectra of old pulsars having lower surface fields may extend to 50 GeV. Although the gamma-ray emission of older pulsars is weaker, detecting pulsed emission at high energies from nearby sources would be an important confirmation of polar cap models. Outer gap models predict more gradual high-energy turnovers at around 10 GeV, but also predict an inverse Compton component extending to TeV energies. Detection of pulsed TeV emission, which would not survive attenuation at the polar caps, is thus an important test of outer gap models. Next-generation gamma-ray telescopes sensitive to GeV-TeV emission will provide critical tests of pulsar acceleration and emission mechanisms.