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The GLAST-LAT Project

The GLAST (Gamma-ray Large Area Space Telescope) is an international mission that will be launched in the last quarter of 2007. Its main instrument, the LAT (Large Area Telescope), will detect gamma rays with energies between 30 MeV and 300 GeV. Thanks to its large effective area (10000 cm2 at 1 GeV) and large field of view, FOV, (2.4 sr), the 1-year LAT sensitivity will be 4 10-9 ph (E> 100 MeV) cm-2s-1, a factor 25 better than the Third EGRET Catalog sensitivity. The satellite will orbit the Earth at an altitude of 565 km with an inclination of 28.5 deg. In the first year, it will operate in survey mode, by rocking alternatively by +/-35 deg with respect  to the direction opposite to the Earth every second orbit. This operating mode will provide a very uniform coverage of the sky. The mission expected livetime is 5 years. More details can be found on the LAT performance page.

Blazar Populations

With high confidence detections of more than 60 AGN, almost all of them identified with BL Lacs or FSRQs (Hartman et al. 1999), EGRET has established blazars as a class of powerful but highly variable gamma-ray emitters, in accord with the unified model of AGN as supermassive black holes with accretion disks and jets.  Although blazars comprise only several per cent of the overall AGN population, they largely dominate the high-energy extragalactic sky.  This is because most of the non-thermal power, which arises from relativistic jets that are narrowly beamed and boosted in the forward direction, is emitted in the gamma-ray band, whereas the presumably nearly-isotropic emission from the accretion disk is most luminous at optical, UV, and X-ray energies.   Most extragalactic sources detected by the LAT are therefore expected to be blazar AGN, in stark contrast with the situation at X-ray frequencies, where most of the detected extragalactic sources are radio-quiet AGN.

The estimated number of blazars that GLAST will detect ranges from a thousand (Dermer 2006) to several thousand (Stecker & Salamon, 1996; Chiang & Mukherjee 1998; Mücke & Pohl 2000).  Such a large and homogeneous sample will greatly improve our understanding of blazars and radio galaxies and will be used to perform detailed population studies and to carry out spectral and temporal analyses on a large number of bright objects. In particular, the very good statistics will allow us to a) extend the LogN-LogS curve to fluxes about 25 times fainter than EGRET, b) estimate the luminosity function and its cosmological evolution, and c) calculate the contribution of blazars and radio galaxies to the extragalactic gamma-ray background.  These observations will chart the evolution and growth of supermassive black holes from high-redshifts to the present epoch, probe the evolutionary connection between BL Lacs and FSRQs, verify the unified model for radio galaxies and blazars (Urry and Padovani 1995), and test the "blazar sequence" (Fossati et al. 1998). LAT blazar detections will be essential in  determining if a truly diffuse component of extragalactic gamma-ray emission is required,  or if such background can be accounted for by a superposition of various classes of discrete objects.

The Physics of Gamma-ray Emitting AGNs

The LAT's wide field of view will allow AGN variability to be monitored on a wide range of time scales. Rapid flares as bright as those observed by EGRET from 3C 279 S (E>100MeV) =10-5 ph cm-2s-1 (Kniffen et al. 1993) and by Swift from 3C454.3 (Giommi et al. 2006) will be measurable with GLAST at gamma-ray energies on time scales of hours.  In addition, the duty cycle of flaring of a large number of blazars will be determined with good accuracy.  The short variability time scale and luminous gamma-ray emission will place lower limits on the Doppler factor of the jet plasma.  The values of the Doppler factor can be correlated with gamma-ray intensity states for a specific blazar and correlated with membership in different subclasses for many blazars.  The Doppler factors can also be compared with values obtained from superluminal motion radio observations in order to infer the location of the gamma-ray emission site, with the goal to study the evolution of the jet Lorentz factor with the distance from the black hole.

Most viable current models of formation and structure of relativistic jets involve conversion of the gravitational energy of matter flowing onto a central supermassive black hole. Gamma-ray flares are most likely related to the dissipation of magnetic accretion energy or extraction of energy from rotating black holes (c.f. Blandford and Znajek 1977).  However, the conversion process itselfis not well understood, and many questions remain about the jets, such as:  how are theycollimated and confined?  What is the composition of the jet, both in the initial and in the radiative phase?Where does the conversion between the kinetic power of the jet into radiation take place, and how?  What role is played by relativistic hadrons?  If hadrons play a significant role, this will require careful calculations ofparticle-particle and particle-field interactions in the rather extreme range of particle energies inferred for blazar jets. There are also questions about the role of the magnetic field, such as whether the total kinetic energy of the jet is, at least initially, dominated by Poynting flux.

The first step in answering these questions is to determine the emission mechanisms in order to infer the content of the luminous portions of jets. This understanding should, in turn, shed light on the jet formation process and its connection to the accreting black hole. Determining the emission mechanisms, whether dominated by synchrotron self-Compton, external Compton, or hadronic processes, will require sensitive, simultaneous multiwavelength observations. Such observations can uncover the causal relationships between the variable emissions in different spectral bands and provide detailed modeling of the time-resolved, broadband spectra. The sensitivity and wide bandpass of the LAT, coupled with well-coordinated multiwavelength campaigns, will be essential. Broadband campaigns will measure the total jet power as compared with accretion power, and the spectra from these observations should revealwhether a single zone structure is sufficient or whether multiple zones are required. Furthermore, the content of the inner part of the jet will be tightly constrained by broadband X-ray spectra and by temporal correlations between the X-ray and gamma-ray variability; this is because the radiative energy density in the vicinity of black holes in AGN can be reliably estimated from contemporaneous broadband data, and this circumnuclear radiation must Compton-scatter with all "cold" charged particles contained in the jet (e.g. Sikora and Madejski 2000;  Moderski et al. 2004).  Finally, the detection of anomalous gamma-ray spectral features will indicate the importance of hadronic processes, with significant implications for the origin of ultra-high-energy cosmic rays.
 Science goal table 

This table summarizes the issues to be addressed with the LAT data.. These Science Goals will drive the multi-wavelength observations performed in contemporaneous/ simultaneous campaigns, which are already being actively prepared (see the page of the LAT Multiwavelength Group).A more detailed document on these Science Goals is posted here.  

Extragalactic Background Light

The Extragalactic Background Light (EBL) carries unique information regarding the galaxy formation and evolution history. The LAT should be able to measure the EBL redshift evolution in the optical/UV band via the attenuation in the high-energy flux from high-redshift blazars (Chen et al. 2004). Thanks to the large population of such blazars that should be detected by the LAT, one expects to be able to disentangle the attenuation due to the EBL from intrinsic effects. However, since the cutoff energy will lie in the 50 GeV range, a long integration time (~ 1 year)  will be necessary. 

References

Blandford, R. D.  and Znajek R.L. , 1977, MNRAS, 179, 433.
Chiang J. and Mukherjee R., 1998, ApJ 496, 752.
Chen, A., Reyes L.C., and  Ritz S., 2004, ApJ 608, 686.
Dermer C. D. , 2006, ApJ, submitted (astro-ph/0605402).
Fossati G. et al., 1998, MNRAS, 299, 433.
Giommi P. et al. ,2006, A&A, 456, 911.
Hartman R.C. et al., 1999, ApJS, 123, 79.
Healley 2006
Kniffen D. A. et al., 1993, ApJ 411, 133.
Moderski R. et al. 2004, ApJ, 611, 770.
Mücke A. and Pohl M., 2000, MNRAS, 312, 177.
Sikora M. and Madejski G.,  2000, ApJ, 534, 109.
Stecker F.W. and Salamon M.H., 1996, ApJ, 464, 600.
Urry C.M.  and Padovani P., 1995, PASP, 107, 803.

Last updated 24 November 2006
B. Lott