Research Interests

Searching for Dark Matter Signals in Cosmic-Rays, Gamma-Rays, Microwaves, Radio, Neutrinos

Mountain View     The nature of dark matter remains to be a puzz- ling question in physics. Through cosmological measurements the dark matter abundance has been determined to be about 23% of the energy density in the Universe. It is distributed in clusters of gala- xies, in individual galaxies as well as in less massi- ve (dark matter) substructures. For the case where dark matter is composed of particles, one can use what is referred to in the literature as indirect pro- bes. That is the case when one can use the assump- tion that particle dark matter candidates annihilate or decay into Standard Model particles and produ- ce large amounts of high energyantimatter cosmic   Mountain View
ray particles. If these cosmic rays, influence the locally measured antimatter to matter ratios at high energies; they then become a probe for new physical phenomena. In addition one can search for signals in high energy neutrinos, in microwaves and in the diffuse gamma-ray radiation. Over the last decade measurements of cosmic-rays, neutrinos, in microwaves and in gamma-rays have been performed by a sequence of experiments including PAMELA, AMS-02, IceCube, WMAP, Planck, Fermi Large Area Telescope and HESS. My work has been using that information making connections and probing the pro- perties of particle dark matter.
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Selected Work in Dark Matter and its Indirect Signals

Studying Cosmic-Ray Propagation and Sources

Mountain View     With data in cosmic-rays and combined with gamma-rays, x-rays and microwaves, we can make connections between different wavelengths pro- bing the same astrophysical processes and sources. The allows us to study galactic and (at even higher energies) extragalactic high energy astrophysics. These include environments as are those at or close to supernova, pulsars, black holes and the galactic and extragalactic medium. With current and future detectors, we are able to even study new energy scales not directly accessible to laboratories. In the process we also advancing our understanding and modeling of cosmic-ray propagation in the Interstellar Medium.   Mountain View

Selected Work in Cosmic Ray Astrophysics

Analyzing Gamma-Ray data and studying their Astrophysical Implications

Mountain View     With the launching of the Fermi Large Area Te- lescope in 2008 that observed the sky at energies between ~ 50 MeV and up to 1 TeV, gamma-ray astronomy and astrophysics has entered a new era. To study the gamma-ray sky, new techniques have been developed including template analyses and analyses using wavelets. Some results, include the discovery of the Fermi Bubbles and the gamma-ray excess towards the galactic center, in both of which I have been involved. To study these gamma-ray diffuse emissions that are in the galaxy, one needs to work both on their characterization through data   Mountain View
analyses and also on their interpretation by making connections with cosmic-ray physics and physics of extreme bodies as are pulsars but also potentially with particle physics.
  The Fermi Bubbles, (initially Fermi Haze) are the gamma-ray counter part signal (throughout the up-scattering of CMB, IR and starlight by cosmic-ray electrons) of the microwave Haze that comes form the synchrotron radiation of the same cosmic-ray electrons. That cosmic-ray electron population extends up to ~10 kpc and it is suggested to have been originally injected into the interstellar medium a few million years ago.
  Through a different set of analyses a spatially extended component has also been revealed toward the galactic center, peaking at energies of ~2-5 GeV. The study of the extension of the excess emission, (in terms of spectrum and morphology) to higher latitudes is a key ingredient in both testing its validity and also in terms of its interpretation. Additionally, the galactic diffuse background form the Milky Way (that is dominant at low latitudes) and the identifi- cation and characterization of its uncertainties is necessary in properly subtracting its contribution from the gamma-ray data; which in turn allows us to cor- rectly interpret the excess emission. Millisecond pulsars has been suggested as a possible source of the gamma-ray excess towards Galactic Center. Yet one can use gamma-ray observations of other individually identified millisecond pulsars and also X-ray observations of bright low-mass X-ray binaries (proge- nitors of millisecond pulsars) to estimate that the number of millisecond pulsars in the Inner Galaxy can only explain ~ 10% of the total excess. Alternative- ly, a series of leptonic cosmic-ray outbursts could generate the observed gamma-ray excess towards the Galactic Center. That would be through a small se- ries of outbursts, taking place the last 100s of thousand to a million years ago. In fact, a connection to the Fermi Bubbles (originating form strong outflow activity a few million years ago) can be made.
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Selected Work in Gamma-Ray Data Studies and Astrophysics

Study of High Energy Neutrino Astrophysics

  A recently raised astrophysics question has been the interpretation of the PeV-scale neutrino obser- vations from IceCube. There are a number of possible sources and mechanisms that could produce the- se PeV neutrinos. Photo-meson interactions of 10-100 PeV protons is a very promising mechanism. This could be realized in a variety of astrophysical sources, including gamma-ray bursts (GRBs), acti- ve galactic nuclei (AGN) and starburst galaxies. Alternatively, starburst galaxies can explain such eve- nts. Among the galactic sources of high energy neutrinos, one can also study the possibility of detect- ing neutrino signals from either dark matter annihilations in the galactic halo, or from the Fermi-Bub- bles (in the case their origin is hadronic). Both would be toward the inner part of the Milky Way and could be probed by both current (IceCube) and future (KM3NeT) neutrino telescopes.   Mountain View

Selected Work in High Energy Neutrino Astrophysics

Study of Solar Astrophysics

  Cosmic-rays that reach detectors at Earth or satellites orbiting it, are affected by the solar wind and its embedded magnetic field, traveling outwards from the position of the Sun. The effect of the helio- spheric forces on cosmic-rays is referred to as solar modulation. It affects the locally observed cosmic-ray spectra in an evident manner bellow 10 GeV and has time, charge and cosmic-ray rigidity depend- ence. Due to the time variation of the solar magnetic field and the various time-scales associated with the propagation of cosmic-rays through the heliosphere, this has been a long standing problem in space physics. However, in the last years there has been a wealth of data from various directions. Those inc- lure satellite experiments orbiting the Earth, as are PAMELA and AMS-02, balloon experiments as the BESS missions, BESS-Polar I & II and archival cosmic-ray data from balloon missions in the 1990s. More importantly, they include measurements from Voyager 1 that has exited the heliopause where cosmic-rays are not influenced by the solar wind. With those data, we can model the time, charge and rigidity dependence of solar modulation in an analytic and predictive manner. To that end we also use measurements monitoring the solar wind and the properties of solar magnetic field.   Mountain View

Selected Work in Solar Astrophysics

Gravitational Waves

Mountain View     The first ever detection of gravitational waves from the coalescence of two black holes, by the LIGO Collaboration in September 2015, has open- ed a new window to astrophysics and cosmology. Together with collaborators from Johns Hopkins and the University of Minnesota, we have worked on various questions raised from the first observa- tions of gravitational waves. Those include, conne- ctions with dark matter and primordial black holes produced in the early stages of the Universe, future cross-correlations of gravitational wave data with data from galaxy surveys, studies for rare but cha- racteristic coalescence events where multiple gra-   Mountain View
vitational wave modes would be observable from inspiraling binaries, studies of the stochastic gravitational wave background and work on how future observations can increase our understanding on the properties of black hole binaries, that may lead to answers on their progenitors.

Selected Work in Gravitational Waves