Research

My research is centered on the study of Active Galactic Nuclei (AGN) from a multifrequency point of view, though mainly focusing on the high energy properties. In particular I have been investigating the properties of radio-loud AGNs.
Radio-loud AGNs, Jets and Black Holes
The formation and evolution of relativistic jets in Active Galactic Nuclei is one of the most fundamental open problems in astrophysics. According to the current AGN paradigm, such jets are common to all radio-loud AGN, and it now seems to be generally accepted that their power derives from stress applied on the black hole (BH) space time and the gas that moves through it. Thus relativistic jets offer a significant clue to the biggest mystery, the elusive BH power source.

Yet a great deal remains unknown about AGN jets, such as their energy content; the energy balance among particles, fields, and radiation; the transport of energy along the jet; and the connection between the jet and the accretion disk.

The study of the physical properties of radio-loud AGNs is one of the main avenues to pursue to investigate how jets form and grow, how the black hole powers them, and what it the connection between the observed jets and the -putative- accretion disk.

Blazars as laboratories for Jet Physics
Blazars comprise the most extreme sources amongst radio-loud AGN. Their extreme variability and luminosity are successfully interpreted in terms of non-thermal radiation from relativistic jets.
In fact, this interpretation constitutes the basis for the unified scheme of radio-loud AGN, which implies blazar-type jets exist in all radio-loud AGN.
The basic paradigm based on the existence of jets and accretion disk has been very successful in bringing coherence to the ``zoo'' of radio-loud AGN, linking radio galaxies to quasars and blazars through orientation angle.

In blazars, beaming enhances the jet emission, making it thousands to millions of times brighter than in unaligned AGN (radio galaxies). The jets greatly outshine the host galaxy, making blazars unique laboratories for studying jet physics. Thus, studies of the properties of blazar radiation provide exceptional tools for exploring the deepest parts of extragalactic jets, their structure, physics and origin.

My research tackles the problem of understanding the physical properties of blazars, from two complementary points of view.

Unified schemes try to account for the AGN phenomenology by hypothesizing that objects appear to be different only as a result of a small number of parameters, such as their orientation with respect to our line of sight, or some more physical, intrinsic, properties, such as the mass of the black hole.
We have developed a new physical models to explain the vast range of blazar properties, proposing that blazars in fact form a sequence where their overall properties are mostly determined by their power (luminosity).
We are testing this new paradigm by means of population synthesis models, i.e. trying to reproduce the observed samples starting from ours and alternative hypotheses for the relationship between different types of blazars.
Toward this goal on one hand we are working on computer models and simulations, but on the other hand we are "developing" a new blazar sample, designed as to minimize biases due to selection incompleteness.

We address the questions on the physical conditions in the emission region(s), and the characteristics of the acceleration and energy losses of relativistic electrons by means of intensive -multiwavelength- observations of bright "blue" blazars.
The synoptic study of the properties of the variability at different frequencies, and for a variety of spectral types, will provide the means to constrain physical models.
Blue blazars provide the most interesting targets because in their case the synchrotron emission peaks right in the X-ray band, and the IC peak occurs in the TeV band, that is observable thanks to ground based Atmospheric Cherenkov Telescopes (ACT).
By observing the variability at the synchrotron peak, we are directly investigating the behavior of the electrons right at the energy at which they are most sensitive to the details of the acceleration and cooling processes. These electrons are the most direct probe of the physical conditions of the emitting plasma.
We have lead some of the most successful campaigns performed in the last few years, involving the RossiXTE x-ray observatory and the ground based VERITAS and HEGRA ACTs. We have un-ambiguously set a stringent upper limit on the time lag between the variations observed in x-ray and gamma-ray, and "measured" the flux-flux correlation with unprecedented detail.
The details of the relationship between X-ray and TeV fluxes and spectra set severe constraints on the values of some fundamental physical parameters, and it is challenging us to review our standard picture.

"SMARTS Optical and infrared monitoring of 12 gamma-ray bright blazars", 
Bonning, E.W., Urry, C.M., Bailyn, C.D., Buxton, M., Chatterjee, R., Coppi, P., Fossati, G., Isler, J., Maraschi, L.
2012, The Astrophysical Journal, 756, 13

"Similarity of the Optical-IR and -ray Variability of Fermi Blazars", 
Chatterjee, R., Bailyn, C.D., Bonning, E.W., Buxton, M., Coppi, P., Fossati, G., Isler, J., Maraschi, L., Urry, C.M.
2012, The Astrophysical Journal, 749, 191

"An optical–near IR outburst with no accompanying gamma-rays in the blazar PKS 0208-512",
Chatterjee, R., Fossati, G., Urry, C.M., Bailyn, Maraschi, L., Buxton, M., Bonning, E.W., Isler, J., Coppi, P.
2013, The Astrophysical Journal Letters, 763, L11

"Time-dependent simulations of emission from the FSRQ PKS 1510-089: multiwavelength variability of external Compton and synchrotron self-Compton models",
Chen, X., Fossati, G., Boettcher, M., Liang, E.P.
2012, Monthly Notices of the R.A.S., 424, 789

"Are the physical properties of extragalactic relativistic jets mainly a function of their kinetic power?"
Georganopolous, M., Meyer, E., Fossati, G. & Lister, M.
2012, The Astrophysical Journal, submitted

"Collective Evidence for Inverse Compton Emission from External Photons in High-Power Blazars",
Meyer, E. T., Fossati, G., Georganopoulos, M., Lister, M. L.
2012, The Astrophysical Journal Letters, 752, L4

"Time dependent simulation of blazar Mrk 421 using a Monte Carlo multizone code", 
Chen X., Fossati G., Liang E., Boettcher M., 
2011, Monthly Notices of the R.A.S., 416, 2368

"Correlated Variability in the Blazar 3C 454.3",
Bonning E. W., Bailyn C., Urry C. M., Buxton M., Fossati, G., Maraschi L., Coppi P., Scalzo R., Isler, J., Kaptur A.
2009, The Astrophysical Journal Letters, 697, L81

"Multiwavelength Observations of Markarian 421 in 2001 March: An Unprecedented View on the X-Ray/
TeV Correlated Variability",
Fossati, G., Buckley J. H., et al.
2008, The Astrophysical Journal, 677, 906

"Correlation between the TeV and X–ray emission in high-energy peaked BL Lac Objects",
Katarzynski, K., Ghisellini, G., Tavecchio, F., Maraschi, L., Fossati, G., & Mastichiadis, A.
2005, Astronomy & Astrophysics, 433, 479

"Coordinated multiwavelength observations of 3C 66A during the WEBT campaign of 2003–2004",
Böttcher, M., ..., Fossati, G., et al.
2005, The Astrophysical Journal, 631, 169

"Correlation between the TeV and X–ray emission in high-energy peaked BL Lac Objects",
Katarzyski, K., Ghisellini, G., Tavecchio, F., Maraschi, L., Fossati, G., & Mastichiadis, A.
2005, Astronomy & Astrophysics, 433, 479

"Correlated X-ray and TeV variability in Markarian 421",
Fossati, G., Buckley, J., Edelson, R.A., Horns, D., & Jordan, M.
2004, New Astronomy Reviews, 48, 419

"Multiwavelength Observations of Strong Flares From the TeV-Blazar 1ES 1959+650",
Krawczynski H., et al.
2004, The Astrophysical Journal, 601, 151

"Spectral energy distributions of flat-spectrum radio quasars observed with BeppoSAX",
Tavecchio, F., Maraschi, L., Ghisellini, G., Celotti, A., Chiappetti, L., Comastri, A., Fossati, G., et al.
2002, The Astrophysical Journal, 575, 137 

"Spectral energy distributions of 3C 279 revisited: BeppoSAX observations and variability models",
Ballo, B., Maraschi, L., Tavecchio, F., Celotti, A., Fossati, G., et al.
2002, The Astrophysical Journal, 567, 50 

"Four years of monitoring blazar PKS 2155-304 with BeppoSAX: Probing the dynamics of the jet",
Zhang, Y.H., Treves, A., Celotti, A., Chiappetti, L., Fossati, G., et al.
2002, The Astrophysical Journal, 572, 762 

"Hard X-ray properties of blazars",
Donato, D., Ghisellini, G., Tagliaferri, G., Fossati, G.
2001, Astronomy & Astrophysics, 375, 739 

"Extreme synchrotron BL Lac objects - Stretching the blazar sequence",
Costamante, L., Ghisellini, G., Giommi, P., Tagliaferri, G., Celotti, A., Chiaberge, M., Fossati, G., et al.
2001, Astronomy & Astrophysics, 371, 512

"Gamma-loud quasars: A view with BeppoSAX",
Tavecchio, F., Maraschi, L., Ghisellini, G., Celotti, A., Chiappetti, L., Comastri, A., Fossati, G., et al.
2000, The Astrophysical Journal, 543, 535 

"X-ray emission of Markarian 421: New clues from its spectral evolution. I. Temporal analysis",
Fossati, G. et al.
2000, The Astrophysical Journal, 541, 153 

"X-ray emission of Markarian 421: New clues from its spectral evolution. II. Spectral analysis and physical constraints",
Fossati, G. et al.
2000, The Astrophysical Journal, 541, 166

"The concave X-ray spectrum of the blazar ON 231: the signature of intermediate BL Lacertae objects",
Tagliaferri, G., Ghisellini, G., Giommi, P., Chiappetti, L., Maraschi, L., Celotti, A., Chiaberge, M., Fossati, G., et al.
2000, Astronomy & Astrophysics, 354, 431 

"Complex spectral variability from intensive multiwavelength monitoring of Markarian 421 in 1998",
Takahashi, T., ..., Fossati, G., et al.
2000, The Astrophysical Journal, 542, L105 

"Simultaneous X-ray and TeV observations of a rapid flare from markarian 421",
Maraschi L., Fossati G., et al.
1999, The Astrophysical Journal, 526, L81

"A unifying view of the spectral energy distributions of blazars",
Fossati G., Maraschi L., Celotti A., Comastri A., & Ghisellini G.
1998, Monthly Notices of the Royal Astronomical Society, 299, 433 

"A theoretical unifying scheme for gamma-ray bright blazars",
Ghisellini G., Celotti A., Fossati G., Maraschi L., & Comastri A.
1998, Monthly Notices of the Royal Astronomical Society, 301, 451

"Breaking the Blazar Sequence: A New View of Radio Loud AGN Unification",
Meyer, E. T., Fossati, G., Georganopoulos, M., Lister, M. L.
2012, Fermi & Jansky proceedings - eConf C1111101 [arXiv:1205.0794], Proc. of "Fermi &
Jansky Symposium"

"X-ray Time Lags in TeV Blazars", 
Chen, X., Fossati, G., Liang, E.P, Boettcher, M., 
2011, Journal of Astronomy & Astrophysics, 32, 185

"Time-Dependent Multi-Zone Radiation Transfer Modeling of Fast Blazar Variability", 
Fossati, G., Chen. X., 
2011, "IAU Symposium 275: Jets at all Scales", Cambridge Univ. Press Conf. Series vol. 275, 136, eds. G. E. Romero, R. A. Sunyaev & T. Belloni (Cambridge: CUP) 

"Toward a New Paradigm for the Unification of Radio Loud AGN and its Connection to Accretion",
Georganopoulos, M., Meyer, E. T., Fossati, G., Lister, M. L.
2011, eConf C110509 [arXiv:1111.4711], Proc. of “2011 Fermi Symposium”

"Are relativistic jets monoparametric engines?",
Georganopoulos, M., Meyer, E. T., Fossati, G., Lister, M. L.
2011, Mem. S. A. It., vol. 82, 192, eds. M. Massi & A. Lobanov

"Gamma-ray Clues to the Relativistic Jet Dichotomy",
Meyer, E. T., Fossati, G., Georganopoulos, M., Lister, M. L.
2011, eConf C110509 [arXiv:1111.6490], Proc. of “2011 Fermi Symposium”

"Clues on Blazars Phenomenology from the FIRST Flat Spectrum Sample",
Fossati, G., Meyer, E.T.
2010, A.S.P. Conf. Series vol. 427, “Accretion and Ejection in AGN: a Global View”, 141, eds.
Maraschi, G. Ghisellini, R. Della Ceca & F. Tavecchio (San Francisco: ASP)

"The FIRST Flat Spectrum Sample",
Fossati, G.
2005, Rev. Mex. A & A, 23, 68–76, Proc. of  "The Ninth Texas-Mexico Conference on Astrophysics",
eds. S. Torres-Peimbert & G. MacAlpine

"Demographics of Blazars",
Fossati, G.
2001, A.S.P. Conf. Series, vol. 227, “Blazars Demographics and Physics", 218, eds. P. Padovani, C.M. Urry (San Francisco: ASP)

"The FIRST Flat Spectrum Sample",
Fossati, G.
2008, A.S.P. Conf. Series vol. 386, "Extragalactic Jets: Theory and Observation from Radio to Gamma-Ray", 533, eds. T. Rector, S. De Young (San Francisco: ASP)

"The FIRST Flat Spectrum Sample",
Fossati, G.
2007, AIP Conf. Series vol. 921, "The First GLAST Symposium", 29, eds. S. Ritz, P. Michelson, &
C. A. Meegan, (Melville: New York)

"Blazar Demographics with the FIRST Flat Spectrum Sample",
 Fossati, G.,
2004, in Proc. of "The Composite Nature of Active Galactic Nuclei (AGN6)", held in Volterra (Italy), May 2004

"Investigating the nature of relativistic jets by studying the demographics of Blazars",
Fossati, G.
2004, In Proc. of "Growing Black Holes: Accretion in a Cosmological Context", eds. A. Merloni, S. Nayakshin, R. Sunyaev, (ESO Astrophysics Symposia, Springer)

"The FIRST Flat Spectrum Sample",
Fossati, G.
2004, In Proc. of "Multiwavelength AGN Surveys", eds. R. Mujica, R. Maiolino (World Scientific, Singapore)

"Strong and weak jets and a paradigm shift in the unification of radio-loud AGNs." Guillermo Haro 2011 Workshop: Surveying the Gamma-ray Sky, INAOE, Puebla, Mexico . (July 2011)

"Multifrequency Variability Studies of Mrk421." Frascati Workshop 2009, "Multifrequency Behavior of High Energy Cosmic Sources'' , Vulcano, Italy. (May 2009)

"Multiwavelength Observations of Markarian 421: X-Ray/TeV Correlated Variability." 213rd Meeting of the American Astronomical Society, Long Beach, CA. (January 2009)

Presenter.  "Blazar Sequence, Demographics and GLAST." 4th Heidelberg International Symposium on High Energy Gamma Ray Astronomy, Heidelberg, Germany. (July 7-11)

Presenter.  "Radio-Loud AGN Unification: Demographics and Blazar Sequence." Third Coast Astronomical Society Meeting / Milos Milosavljevic, Austin, TX. (May, 8-10)

"Challenges to the Models for Blazar Emission from X-ray/TeV Simultanoeus Observations." Challenges of Relativistic Jets, Crakow, Poland. (June 2006)

"Radio-loud AGNs unification issues: power, colors and evolution." The Ninth Texas-Mexico Conference on Astrophysics, San Antonio, TX. (April 2005) With S. Laurent-Muehleisen, C.M. Urry

"Correlated X-ray and TeV variability in Markarian 421." Second VERITAS Symposium on the TeV Astrophysics of Extragalactic Sources,, Chicago. (April 2003)

"The most successful X-ray/TeV campaign on the TeV blazar Mrk421." Laboratory for High Energy Astrophysics, NASA Goddard Space Flight Center, Greenbelt, MD. (November 6th 2003)

"Physics with Broad Band Spectral Variability of Mrk 421." 34th COSPAR Scientific Assembly - New X-Ray Results from Clusters of Galaxies and Black Holes, Houston, TX. (October 2002)

"Variability Properties of Mkn 421 across 6 orders of magnitude in timescales, and 3 in brightness." AAS/HEAD Meeting, joint with APS, Albuquerque, NM. (April 2002)

"Multiwavelength variability of blazars for external Compton and SSC models by time-dependent multi-zone simulations [contributed talk]." Heidelberg Int’l Symposium on High Energy Gamma-Ray Astronomy (Gamma2012), Heidelberg (Germany). (July 2012) With Chen, X.

"Strong and Weak Jets and a Paradigm Shift in the Unification of Radio-loud AGNs [contributed talk]." IAU 290: Feeding Compact Objects: Accretion on All Scales, Beijing (China). (Augut 2012)

"The Accretion Mode - Morphology Link in Radio-Loud AGN jets: A More Complete Unification Scheme? [contributed talk / co-author]." Black Holes by the Black Sea, Istanbul, Turkey. (June 2012) With Meyer, E.

"The broken sequence and the torn blazar envelope [contributed talk]." Fourth International Fermi Symposium, Monterey (CA). (October 2012)

co-author.  "Breaking the Blazar Sequence: A new view of Radio Loud AGN Unification [contributed talk]." Fermi and Jansky: Our Evolving Understanding of AGN, St.Michaels, MD. (November 2011) With E.T. Meyer (GF's graduate student)

"From Blazar Sequence to Blazar Envelope: Strong and Weak Jets and a Paradigm Shift in the Unification of Radio-loud AGNs [contributed talk]." Fermi Symposium 2011, Roma, Italy. (May 2011)

"Strong and Weak Jets and a Paradigm Shift in the Unification of Radio-loud AGNs [contributed talk]." AAS/HEAD Meeting, Newport, RI. (September 2011)

"Time-Dependent Multi-Zone Radiation Transfer Modeling of Fast Blazar Variability [contributed talk]
." IAU Symposium 275: Jets at all scales, Buenos Aires, Argentina. (September 2010) With Xuhui Chen

"Time-Dependent Multi-Zone Radiation Transfer Modeling of Fast Blazar Variability. [contributed talk]
." Multiwavelength Variability of Blazars, Guangzhou, PRC. (September) With Xuhui Chen

"Blazar Demographics and Sequence with Fermi - [contributed talk]." Accretion and Ejection in AGN: a Global View, Como, Italy. (June 2009)

[contributed talk].  "The FIRST Flat Spectrum Sample: Demographics and Blazar Sequence." AAS/High Energy Astrophysics Division Meeting, Los Angeles, CA. (March 31 - April 3)

[contributed talk].  "Challenges to the Model for Blazar Emission from X-ray/Tev Simultaneous Observations." High Energy Phenomena in Relativistic Outflows, Dublin, Ireland. (September 2007)

[contributed talk].  "Towards an improved census of radio-loud AGNs: the FIRST Flat Spectrum Sample. ." First GLAST Symposium, Stanford University, Palo Alto, CA. (February 2007)

[contributed talk].  "Investigating the nature of relativistic jets by studying the demographics of Blazars." AAS 205th Meeting, San Diego, CA. (January 2005)

[contributed talk].  "Blazar Demographics with the FIRST Flat Blazar Demographics with the FIRST Flat Spectrum Sample Spectrum Sample." AGN6, Volterra (Italy). (May 2004)

[contributed talk].  "The X-ray/TeV correlated variability of Mrk 421." Seventh Meeting of the AAS High Energy Astrophysics Division, Mt. Tremblant, Quebec, Canada. (March 2003)

"From blazar sequence to blazar envelope: a new view of the relativistic jet dichotomy in radio-loud AGN." Understanding Relativistic Jets, Krakow, Poland. (July 2011)

"Time lags, flare shapes, multiwavelength correlated variability: results from time-dependent multi-zone radiative transfer simulations." Understanding Relativistic Jets, Krakow, Poland. (July 2011)

"Blazar Unification through Population Synthesis." HEAD Meeting, Kona, Hawaii. (March) With Eileen Meyer

"Time Dependent Multi Zone Modeling of X-ray and Gamma-ray Variability of the TeV Blazar Mrk 421." HEAD Meeting, Kona, Hawaii. (March) With Xuhui Chen

"Time dependent simulation of blazar Mrk421 using a Monte Carlo multizone code." 2009 Fermi Symposium, Washington, DC. (November 2009)

"Towards an Improved Census of Radio-Loud AGNs: the FIRST Flat Spectrum Sample." Extragalactic Jets | Theory and Observation from Radio to Gamma Ray, Girdwood, Alaska. (May 2007)

"Challenges to Models for Blazar Emission from X-ray and TeV Simultaneous Observations." AAS/High Energy Astrophysics Division Meeting, San Francisco. (October 4-7, 2006)

"Fast Variability in the Chandra long observations of the TeV blazar Mrk421." The X-ray Universe 2005, El Escorial, Madrid, Spain. (September 2005) With Brunner, G.

"Testing the blazar sequence with an improved census of radio-loud AGNs: The FIRST Flat Spectrum Sample." The X-ray Universe 2005, El Escorial, Madrid, Spain. (September 2006)

"Towards an improved census of radio-loud AGNs: the FIRST Flat Spectrum Sample." Isola d'Elba, Italy. (May 2005)

"Investigating the nature of relativistic jets by studying the demographics of Blazars." Growing Black Holes, Garching (Germany). (June 2004)

"Investigating the nature of relativistic jets by studying the demographics of Blazars." High Energy Astrophysics Division (HEAD) of the AAS meeting, New Orleans, LA. (September 2004)

"The FIRST Flat Spectrum Sample." Multiwavelength AGN Surveys, Cozumel, Mexico. (December 2003)

"From Snapshot to Movie: variability properties of Mrk 421." Active Galactic Nuclei: from Central Engine to Host Galaxy, Paris, France. (July 2002)

"Multiwavelength correlated variability of blue and red blazars: results from time-dependent multi-zone radiative transfer simulations." (Prof. Markos Georganopoulos), Univ. of Maryland, Baltimore County. (March 2012)

"A multipronged approach to the study of extragalactic relativistic jets." Rice University. (October 2011)

"A multipronged approach to the study of extragalactic relativistic jets." (Prof. Matthew Lister), Purdue University, West Lafayette, IN. (November 2011)

"Extragalactic relativistic jets: clues from single outbursts and population multiwavelength studies." (Prof. Markus Boettcher), Ohio University, Athens, OH. (April 2011)

"Extragalactic relativistic jets: clues from single outbursts and population multiwavelength studies." (Prof. Andres Meza), Universidad Andres Bello, Santiago, Chile. (March 2011)

"Strong and weak jets and a paradigm shift in the unification of radio-loud AGNs." (Prof. Greg Madejski), Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Palo Alto, CA. (October 2011)

Speaker.  "What do we know about radio-loud Active Galactic Nuclei at the beginning of the Fermi era." Pontificia Universidad Católica de Chile. (January)

"What do we know about radio-loud Active Galactic Nuclei at the beginning of the "Fermi era"?." Trinity University, San Antonio, TX. (February 2009)

"Challenges to the model for blazar emission from x-ray and tev observations." Krawczynski, Buckley, Washington University, St.Louis. (April 28)

"Challenges to the common-wisdom model for blazar emission from x-ray/tev simultaneous observations." Rene Ong, University of California, Los Angeles. (December 6)

"Challenges to the common-wisdom model for blazar emission from x-ray/tev simultaneous observations." Antonucci, University of California, Santa Barbara. (December 5)

"The most successful X-ray/TeV campaign on the TeV blazar Mrk421." University of British Columbia (host: Jeremy Heyl), Vancouver, BC. (October 2004)