HEPL/Stanford group:

Introductory topics for rotation students.

Recommended Textbooks:

The GLAST gamma-ray observatory will be launched soon. The research of the HEPL/Stanford group centers on the data to be produced by its LAT instrument. Astrophysics with high-energy gamma rays involves some of the most spectacular events in the universe: flaring jets of active galactic nuclei, bursts of gamma-rays that may outshine a whole galaxy for a few seconds, Dark Matter that is dominating the visible matter in the universe, accreting black holes, pulsating neutron stars, stellar explosions and their remnants, etc. Even the Milky Way that we can sometimes see with our naked eyes is bright at gamma-ray energies as well. If you don't want to travel too far away into space and time, just look at the gamma-ray bright Sun, the Moon, or even our Earth! Understanding the observations of these phenomena requires knowledge in high-energy and particle astrophysics, astronomy of various source populations, data analysis methods, a broad multifrequency view, and a lot of imagination!

This is what you can learn about and research in our group:


HIGH-ENERGY/PARTICLE ASTROPHYSICS

Basic processes: You will study these processes in various astrophysical environments.
Leptonic model of emission in supernova remnants

The evidence for particle acceleration in supernova shells comes from electrons whose synchrotron emission is observed in radio and X-rays. Recent observations by the HESS Cherenkov telescopes reveal that supernova remnants also emit TeV gamma rays, long-awaited experimental evidence that supernova remnants can accelerate cosmic rays up to the "knee" energies (~10^16 eV). Still, uncertainty exists whether these gamma rays are produced by electrons via inverse Compton scattering or by protons via neutral pion decay. The multiwavelength spectra of supernova remnants can be fitted with both mechanisms, although a preference is often given to neutral pion decay due to the spectral shape at very high energies. The topic would be to build a model of leptonic emission (X-rays to TeV gamma rays) and to fit to observed SNR spectrum. This task will involve calculation of synchrotron emission and ICS.

References:

Contact persons: Igor Moskalenko & Stefan Funk (SLAC)
Hadronic model of TeV emission in supernova remnants

The evidence for particle acceleration in supernova shells comes from electrons whose synchrotron emission is observed in radio and X-rays. Recent observations by the HESS instrument reveal that supernova remnants also emit TeV gamma rays, long-awaited experimental evidence that supernova remnants can accelerate cosmic rays up to the "knee" energies. Still, uncertainty exists whether these gamma rays are produced by electrons via inverse Compton scattering or by protons via neutral pion decay. The multiwavelength spectra of supernova remnants can be fitted with both mechanisms, although a preference is often given to neutral pion decay due to the spectral shape at very high energies. The topic would be to build a model of leptonic emission for X-rays and hadronic emission for TeV gamma rays and to fit to observed SNR spectrum. This task will involve calculation of synchrotron emission and neutral pion production.

References:

Contact persons: Igor Moskalenko & Stefan Funk (SLAC)
Attenuation of  >TeV gamma rays by pair production in various environments

Pair production plays an important role in different objects and its observation allows us to probe the physical conditions in the object or between the object and the observer. The topic would be to study the pair production in different environments: extragalactic space, interstellar radiation field of the Milky Way, close binary systems.

References:

Contact persons: Igor Moskalenko & Anita Reimer
The structure of pair halos from jets of active galactic nuclei (AGN)

Pair production plays an important role in different objects and its observation allows us to probe the physical conditions in the object or between the object and the observer. Here we concentrate on gamma-ray absorption in the vicinity of VHE emitting AGN jets and the eventual formation of pair cascades. What can we learn from its spectral and spatial distribution about the environment and the jet itself?

References:

Contact persons: Anita Reimer & Olaf Reimer
Electron synchrotron and synchrotron-self-Compton emission in various environments

Synchrotron emission from different objects testifies the presence of high-energy electrons there. Under certain assumptions it may be used to conclude about the electron spectrum or magnetic field in the object, as well as about particle acceleration processes. The topic would be to study the synchrotron emission in different environments: from the halo of a normal galaxy, synchrotron-self-Compton emission from active galactic nuclei, radio lobes, and hot spots. This task will involve calculation of synchrotron emission, ICS, synchrotron and inverse Compton energy losses.

References:

Contact persons: Anita Reimer & Igor Moskalenko
Inverse Compton scattering in different environments

ICS is one of the major processes to produce gamma rays. The topic would be to calculate gamma-ray production on stellar photons in the vicinity of a star or star association, on solar photons in the heliosphere, from the accreting disk in a binary system, in a AGN jet, Comptonization of the microwave background by the galaxy clusters (Sunyaev-Zeldovich effect) etc.

References:

Contact persons: Igor Moskalenko
Pion production, gamma rays, and neutrinos

Very-high energy gamma rays and neutrinos are produced via pion production in proton-proton and proton-photon interactions. The topic would be to calculate the spectra of gamma rays and neutrinos for different injection spectra of protons, to find the neutrino/photon ratio, and to apply it to different objects to predict the neutrino flux. You may also study the Greisen-Zatsepin-Kuzmin (GZK) effect at Ultra High Energies.

References:

Contact persons: Anita Reimer & Igor Moskalenko

POPULATION STUDIES


Blazar evolution and contribution to extragalactic diffuse emission: evolutionary uncertainties

Here you study the impact of different models for the cosmological blazar evolution on the source contribution to the extragalactic diffuse gamma-ray background. For this undertaking a template blazar spectrum for each source class compiled from published blazar spectral energy distributions (SEDs) will be used.

References:

Contact persons: Anita Reimer and Igor Moskalenko
How many FR-I type radio galaxies will GLAST detect?

Based on the unification scheme of radio-loud AGN (where the viewing angle is the main driver for transition from blazars to radio galaxies), the Synchrotron-Self-Compton model for gamma-ray production in BL Lac and FR-I type AGN and their proposed luminosity functions together with the capabilities of GLAST-LAT we will be estimating how many misaligned BL Lac objects GLAST will be able to detect.

References:

Contact persons: Anita Reimer and Olaf Reimer
Contribution of the normal galaxies to the extragalactic gamma-ray background

The diffuse extragalactic gamma-ray background includes contribution from unresolved sources such as AGNs, galaxy clusters, large scale shocks, normal galaxies and may include exotic physics (such as dark matter annihilation, black hole evaporation etc.). It is, therefore, important to understand the composition of the extragalactic background. The galactic diffuse gamma-ray emission is dominating the overall galactic luminosity especially at high energies. This galactic diffuse emission is produced in energetic interactions of cosmic ray particles (mainly protons & electrons) with the interstellar gas and the radiation field. The major processes are the decay of neutral pions produced in proton-proton interactions, inverse Compton scattering, and bremsstrahlung. Only one normal galaxy, the Large Magellanic Cloud, has been detected by EGRET. Another object whose emission has been studied in many details is the Milky Way galaxy. The topic will address the contribution of normal galaxies into the extragalactic background by assuming the diffuse emission of the Milky Way galaxy as a template. This work will include (i) gathering the statistics of the spiral galaxies such as their mass distribution, gas contents, supernova (SN) rate from the literature; (ii) the gamma-ray luminosity of the Milky Way will be scaled with the mass and the SN rate; (iii) calculation of the contribution of the spiral galaxies into the extragalactic background under the assumption of different cosmological evolution models. The calculation is to be made with two template spectra: with and without the so-called "GeV excess." These calculations will also give us an estimate of the systematic uncertainties involved.

References:

Contact persons: Igor Moskalenko and Seth Digel (SLAC)
Statistics of starburst galaxies and evaluation of their gamma-ray luminosities

The starburst galaxies are the normal galaxies with the high star formation rate. They are the potential sources to be detected by GLAST. There is a handful of these galaxies with their emission well measured in radio, optics, and X-rays. The topic will include, in the first part, gathering the properties of the starburst galaxies such as their gas contents, intensity of the radiation field, and luminosities and making a statistical analysis of the correlations. The second part will include a choice of a template gamma-ray spectrum (e.g. Arp 220) and application, under the reasonable assumptions, the correlations found in the first part with a goal to produce the list of the starburst galaxies with predicted gamma-ray fluxes.

References:

Contact persons: Igor Moskalenko and Olaf Reimer
Photon clustering above 10 GeV and their potential AGN origin

The photons detected by EGRET detected at highest energy (E>10 GeV) will be used to investigate the statistical significance of photon clustering at high galactic latitudes. Active Galactic Nuclei, whose high energy peak outside the nominal EGRET energies (and thus catalogs) are the prime candidates to be correlated with such photon clusters. Develop a simple clustering code to be able to evaluate the reported claims in the recent literature, and check on the presumably AGN origin of those clusters using the online data base http://nedwww.ipac.caltech.edu/

References:

Contact persons: Olaf Reimer and Eduardo do Couto e Silva (SLAC)
Dense ISM in correlation with SNR and VHE surveys as tracer of hadronic particle acceleration sites

The connection between three surveys of the Inner Galaxy will be correlated for spatial coincidence and intensity: high density ISM, supernova remnants, and very high energy gamma-ray emission. Known distances to radio or X-ray selected SNRs will be compared with the kinematically determined distances from a molecular cloud survey. Favorable sites for particle acceleration are studied for coincident VHE gamma-ray excesses emission. We will obtain an unbiased view on the connection between dense molecular material in the vicinity of SNR, and their suggested connection to gamma-ray astronomy.

References:

Contact persons: Olaf Reimer and Stefan Funk (SLAC)
AGN Content of a Galaxy Cluster

Both AGN and Galaxy Clusters are prime candidates to be detected by GLAST and thus contributors to the extragalactic diffuse gamma-ray background. Since gamma-ray emission scenarios in both types of astronomical objects are distinctively different, it is interesting to investigate which galaxy clusters have an AGN contribution and which have not. Are both samples identical in terms of their physical parameters like mass, distance, size, radio or X-ray flux? The non-thermal flux from a prominent cluster containing a radio-loud AGN (M87) is to be modeled with a simple emission scenario for dominant AGN jet emission, and a diffuse emission scenario for cosmic ray interactions in the intercluster medium. Will the gamma-ray waveband distinguish both emission scenarios?

Basic references:

Contact persons: Olaf Reimer and Anita Reimer
Contribution of not-yet-detected but potential GLAST-source populations to the diffuse gamma-ray background

Many astronomical objects have been proposed to emit MeV-GeV photons, but on a flux level that is below the sensitivity abilities of EGRET, LAT's predecessor. These will contribute to the diffuse gamma-ray background. In this project we have a closer look at these objects and calculate their contribution to the diffuse background, with implications for their detectability with the LAT.

Contact persons: Anita Reimer &


DATA ANALYSIS


The effect of the energy response of an instrument on point sources and diffuse emission spectra

The measurements of the spectra of point sources and diffuse emission (produced by energetic protons and electrons in the insterstellar medium) depend very much on the understanding of the instrument. This topic will study the effect of uncertainty in the energy measurement on the derived spectra as applied to Crab pulsar (a standard candle) and diffuse gamma-ray emission. If the dispersion can be ignored in some cases, the required computation will be greatly reduced. This project can be done only with simulated data, so it can be done before the launch of GLAST.

References:

Contact persons: Pat Nolan & Igor Moskalenko
Study of log N - log S diagram for different source populations

When astronomical objects of a given class are detected in sufficient numbers, it is possible to study their distribution of brightness. For distant extragalactic sources like blazars, this can lead to information about their evolution over the history of the universe. For Galactic sources we can learn about the shape of their spatial distribution, and thus their correlations with other objects. We need to prepare our procedures for analyzing data and correcting for biases. This can be begun with simulated data.

References:

Contact persons: Pat Nolan and Olaf Reimer