Using X-ray to TeV Instruments to Probe
Blazars, GRBs, and Cosmological
Parameters
Abe Falcone
(Penn State University)
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
Outline of Talk
• Ground based gamma-ray astronomy described
(briefly)
• VHE blazars & GRBs:
– Observed multiwavelength characteristics
– Source studies
– Implications for Universe
• Present status
• The future
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
Imaging Air Cherenkov Technique
Source emits g-ray
g-ray interacts
Effective area is size
of light pool ~ 105
m2
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
Cosmic Ray Rejection Technique
g-ray
proton
 g-ray images:
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- narrow, short, smooth
Hadronic images:
- broad, long
- local muons, patchy
hadron rejection: 99.7% (10-3)
5o
Crab Nebula with Whipple 10 m
~7 s in 1hour
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
Stereo Reconstruction
• Exclude muon background
80 m
• Determine source from intersection of
shower axis
• Crab nebula ~35σ in 1 hour with VERITAS
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
GeV/TeV Observatories
HEGRA
CAT
MAGIC
Whipple 10m
CANGAROO
STACEE
H.E.S.S.
CELESTE
Milagro
VERITAS
Tibet AS
Tibet
MAGIC
Solar Two
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
Blazar Categories
• FSRQ Vs. BL Lac
• Low Peaked Vs. High Peaked
Fossati et al. 1998, Ghisselini et al. 1998
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
Broadband Coverage
Swift
GLAST
VERITAS
X-ray Spectrum:
Swift,...
0.2 keV – 150 keV
Gamma ray:
GLAST, AGILE,...
30 MeV – 300 GeV
all sky (~103 AGNs)
VHE:
VERITAS, HESS,...
50 GeV – 50 TeV
(5 mCrab, 50 hours)
Pointed (~ 50 AGNs)
Mrk501 SED taken from Catanese & Weekes 1999
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
Why Study Blazars at VHE?
Figure from J.Buckley 1998
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Need to understand acceleration mechanisms capable of producing large luminosity at
very high energies:
– SSC? (Maraschi et al. 92, Tavecchio et al 98, …)
– External IC? (Dermer & Schlickeiser 2002, …)
– Proton cascades? (Mannheim 93, …)
– Proton synchrotron? (Muecke & Protheroe 2000, Aharonian 2000, …)
Constrain local environment characteristics: Doppler factor, seed populations, photon
vs. magnetic energy density, accel. and cooling timescales, …
Potential sources of cosmic ray acceleration
Need to understand blazar development and evolution
Constrain models of extragalactic infrared background
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
TeV Blazars - RXTE ASM Overview
96
97
98
99
00
01
02
03
Mrk421
Mrk501
1ES2344
1ES1959
PKS2155
H1426
2-12 keV
Krawczynski et al.
2003
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
Mrk 421 Spectral Variability
• Power law spectral index
varies from 1.89 ± 0.04 in the
high flux state to 2.72 ± 0.11
in the low flux state based on
2001/02 Whipple data
(Krennrich et al. 2002)
HEGRA sees of ~0.75 during
Mrk421 flaring (Aharonian et al.
2002)
(0.75-1.5 TeV) /
(1.5-4 TeV)
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
Correlated Lightcurves
Multiwavelength lightcurve of Mrk501 from 2-20 Apr 1999 (Catanese et al. 1997)
>350 GeV
50-150 keV
2-10 keV
Optical (U)
2-10 keV
>350 GeV
Correlated TeV - X-ray emission of a bright Mrk 421 flare on 19 Mar 2001
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
1ES1959: Overall Lightcurves & Orphan Flare
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
Doppler Factors of TeV Emitters
• SSC mechanisms require a large Doppler factor in the blazar jet
• Edwards and Piner (and independently, Marscher) have used the
VLBA to show that the Doppler factors are surprisingly low for TeV
gamma-ray emitting blazars (however, there is wiggle room since TeV
emission may originate much deeper in jet than ~5 pc)
• Furthermore, they find no new components emerging after periods of
high energy flaring, in contrast to observations of GeV blazars
Edwards and Piner
2004
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
Mrk 421
• Exhibited shortest observed TeV flaring timescale for any
blazar at <15 minutes (Gaidos et al. 1996, Nature)
• VHE emission seems to be dominated by flaring episodes
Mrk421 lightcurve from 28 Oct 2002 to 11 Mar 2003 based on 28.4 hours of
Whipple data
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
Blazars: Short Timescale Flaring
•Lower flux sources and more sources will enhance characterization of catalog
•Improved Spectra between ~0.1-10 TeV
•Well sampled lightcurves
From Gaidos et al. 1996, Nature
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
PKS 2155 on 2006 Jul 27
HESS
(>200GeV)
• A previously low flux (~0.05
Crab) source
• HESS observes:
• >10 Crab flux!!!
• < 5minute doubling time!!!
time bins = 2min
Costamante et al. 2006, Aharonian et al.
2007, Falcone et al. 2007
(analysis ongoing)
• During huge TeV flares, the Xray flux was also variable, but to a
significantly lower degree
• ~2x flux variability
• little/no shifting of 1st Epeak
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
H1426+428
•Extreme BL-Lac type active galactic nuclei (AGN)
•Known TeV emitter
•Displays characteristic double humped spectral energy
distribution (SED) in a F representation
•High peaked: Perhaps the highest peak of any known AGN.
Its first peak in the SED is in excess of 100 keV during a
quiescent state!
•Relatively distant TeV emitting object at a redshift of z=0.129
 excellent target for studies of IR background
•Steep measured TeV spectrum
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
H1426: X-ray Variability
TeV Flux
(Whipple)
X-Ray Flux
X-Ray
Index
X-ray Spectral variability is evident and it does not directly track the flux level.
No TeV flaring evident.
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
Blazar H 1426+428: HIDs
• X-ray HID diagrams from different
observations exhibit varying
orientation
Falcone, Cui, Finley 2003, ApJ
SCIPP seminar (26 Jan 2007)
• We need high-statistics hardnessintensity diagrams at TeV energies,
contemporaneous with wavelengths
spanning the first peak in the SED.
New instruments should achieve
this, thus constraining acceleration
and cooling timescales at different
regions of spectra.
Abe Falcone, Penn State
Extragalactic Background
• Broad multiwavelength spectra are required to ascertain the existence of a
VHE cutoff in the observed blazar spectrum
• The following factors will contribute to the ability of this generation of
VHE instruments to measure the EBL:
– Increased source count by more than an order of magnitude will
improve statistics and knowledge of intrinsic source spectrum
– Increased sensitivity results in sources at higher redshifts, thus
allowing us to study more severely attenuated sources
– Improved spectral resolution will allow for a more accurate
determination of cutoff energy
• Aharonian et al. (2006) have used new distant blazar 1ES 1101 (along with
a small handful of others) and an assumed intrinsic spectrum to constrain
EBL lower limits to values close to the minimum predicted values
(Primack et al. 2004)
• Contemporaneous multiwavelength campaigns are crucial. GLAST and
X-ray instruments, such as Swift and RXTE are needed to measure full
SED!
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
Potential New Source Types
• LBLs
• FSRQs
(Falcone et al. 2004, and
Perlman et al.)
Fossati et al. 1998, Ghisselini et al.
1998
Use VHE along with longer wavelengths to characterize complete blazar
main sequence and/or to characterize blazar evolution
 Explore potential of AGN acceleration mechanisms in the presence of
different ambient medium, including potentially higher electron densities
and increased scattering
 Search for cosmic ray acceleration signatures
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
VERITAS
• Array of f/1.0 imaging air Cherenkov telescopes with 12 m
diameters
• Located at Kitt Peak in Southern Arizona, USA
• Sensitivity: <0.005 Crab at 200 GeV (50hr, 5s)
• Slewing Speed: 1 deg/sec
• Angular Resolution: < 0.05o
• Energy Resolution: E/E = 0.15 to 0.20
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
Science with VERITAS
Active Galactic Nuclei
Extragalactic Background Light
Gamma Ray Bursts
Shell-type Supernova Remnants
Galactic Diffuse Emission
Gamma-ray Pulsars
Plerions
Unidentified Sources
Dark Matter (Neutralino Annihilation) *
Cosmic Ray Origin
Lorentz symmetry violation
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
VERITAS Camera and Electronics
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499 pixels per camera
Each pixel is a 28mm Photonis XP2970/02 PMT
Pixel spacing = 0.15o  FOV = 3.5o
Each PMT has a pre-amplifier located in the camera
Readout of each PMT through dual-gain 8-bit FADC boards
Trigger:
– CFD for single channel
– Pattern trigger for coincidence between multiplicities of neighboring
channels
– Array trigger for multiple telescopes operating stereoscopically
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
Shower Timing: The Movie
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
Expected Performance
Differential Flux Sensitivity
(3s, 50 hours)
Whipple 10-m
VERITAS - 4
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
Relative Sensitivity
GLAST and next generation VHE instruments complement one another well
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
VERITAS Status
• During Winter/Spring 2004, we completed the operation of
the VERITAS prototype.
• While not intended to provide competitive sensitivity, the
prototype was intended to provide a test bed for VERITAS
systems. Many important lessons were learned.
• The prototype was converted into a complete telescope, T1
of the array.
• VERITAS-4 construction is complete.
Engineering/comissioning data is being taken. Science
quality data has been obtained with 3 telescope array.
• Sources are being detected and studied (stay tuned)!
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
VERITAS Multiwavelength Observing Strategies
• ToO observations from initiated by space-based
instruments (ASM, Swift, GLAST, …)
• Scheduled multiwavelength campaigns (RXTE, Integral,
Swift, GLAST, …)
• Ground-based monitoring of VHE sources generating
ToO’s for satellite instruments
 It is very important to have:
1) All-sky X-ray and gamma-ray monitoring (and
notification) by space based instruments
2) ToO and monitoring programs in place at all
wavelengths from GeV down to radio
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
What Has Been Learned about blazars?
• Very short TeV emission timescales
 small regions for TeV gamma-ray acceleration
• One flare is not the same as another flare. Some TeV
flares have correlated X-ray emission, while others do not
(and vice versa).
 Simple one-component SSC does not explain all TeV emission,
while it seems to work for some cases
 Cooling electrons in the jet are certainly related to the TeV
emission at some times, but the coupling may be either directly or
indirectly
• Much work to be done by applying more robust and
diverse models and much work to be done to obtain full
contemporaneous multiwavelength coverage for more
flares!
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
Why Study GRBs at VHE?
• Need to understand acceleration mechanisms in jets, energetics, and therefore
constrain the progenitor and jet feeding mechanism
• Constrain local environment characteristics: Doppler factor, seed populations,
photon vs. magnetic energy density, accel. and cooling timescales, …
• Potential sources of UHE cosmic ray acceleration
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
VHE GRB Observations
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At this time, there are no firm detections of >100 GeV
photons from GRBs (There are a few low significance
potential detections at the ~3σ level; e.g. Atkins et al.
and Amenomori et al.)
There are several reported upper limits (e.g. SazParkinson et al. 2006, Atkins et al., Albert et al, Horan
et al. 2007)
This is not surprising since the predictions for
emission are just barely obtainable by the most
sensitive current instruments such as VERITAS
Detection of VHE photons from GRBs would be very
constraining to jet parameters. In particular, it could
help to determine the hadronic component of the jet
and the bulk Lorentz factor of jet plasma. (Could
solve mysery of UHECRs!)
X-ray flares may provide another mechanism for
detecting inverse Compton scattering from GRBs
Zhang & Meszaros 2001
At Least VERITAS/HESS/MAGIC-2 sensitivity is
needed, along with fast slewing OR all-sky coverage
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
Cosmic Ray Source
• While most GeV/TeV emission is expected to be IC,
there is some component from p+ synchrotron, p+γ
initiated cascades, and inelastic n+p initiated cascades.
The latter is thought to be dominant.
• If there is significant UHECR acceleration, then we
could detect these
• BUT, like blazars, it will be difficult to break degeneracy
between IC and hadronic
– Have the advantage of better constraints on Lorentz factor and
smaller timescales/regions
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
Cosmology with GRBs
Multiple Methods:
Use high redshift GRBs (4 < z < 20) to probe star formation history and epoch of
reionization (see Woosley 2006, Lamb & Reichart 2000)
(requires ability to obtain accurate redshifts from follow-up, which is tricky when redshifted into
deep IR)
Use GRBs as a back-illuminating light to map WHIM dark matter by means of its
absorption features on the GRB spectra (see Nicastro et al.)
(requires XMM/Chandra level of spectral resolution)
Use GRB as a standard candle (after correcting Eiso to Eγ) and then measure
cosmological expansion parameters, similar to SNe methods (see Ghirlanda et al. 2005)
(even with the correction factors on Eiso,still unclear if GRBs can be treated as standard candles)
(need to measure Epeak and redshift)
Use above method to constrain Ωm vs ΩΛ and w0 vs w1 (see Firmani et al. 2006)
(however, contraints may not compete with SNe at low redshifts without many more observations)
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
GRB 050904: z = 6.29
High redshift, extremely bright (J=17.5, possibly as bright as 12 very early)
=> good cosmological probe; as predicted by Lamb and Reichart (2000)
GRB survey is very efficient at finding high z objects (1/14 Swift redshifts)
Near reionization time
Limits on earliest star / galaxy formation
Massive star implied by early collapse time; This GRB may be the first
of many that can be used to probe >100Msolar stars that formed in the early
universe and collapsed to GRBs, as predicted by Woosley et al. (2006).
Eiso ~ 1054 ergs,
SCIPP seminar (26 Jan 2007)
Ejet ~ 1051 ergs => very similar to other bursts at lower z
Abe Falcone, Penn State
Lorentz Invariance Violation
• Energy dependent delays of simultaneously emitted photons can limit (or
measure) Lorentz invariance
• Best lower limits to-date are from GRBs at keV/MeV energies;
– 0.0066Epl ~ 0.66×1017 GeV
• Our major disadvantage: we can't see the distant GRBs due to IR
absorption
• Our major advantage: High and broad energy range, especially if we
measure a delay between GLAST - TeV
• Everyone's disadvantage: Inherent energy dependent delays
• With a detection of ~1 TeV photons by a >10x V/H/M sensitivity
instrument and a detection by GLAST, the limit could be increased by
~100x (to ~Epl), asumming a GRB like 050502B at z=0.5 !!!
• Need a very sensitive (>10x VERITAS) instrument to create light curves
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
UnIdentified Objects (an aside)
• TeV counterparts to unidentified objects from surveys at other wavelengths
can provide strict constraints
• Superior sensitivity of VERITAS will define the high energy spectrum of
many EGRET UnIDs
• With the upcoming launch of GLAST, SWIFT, … many new
UnID objects are expected. TeV instruments are creating their
own UnID catalog, as well!
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
The Future
• On the Ground:
– There is an initiative within VERITAS to expand the current array with 3
enhanced telescopes
– There are multiple "Beyond VERITAS" ideas to be proposed. Short term
plans include MRI proposals to build small telescopes on a wide baseline
(--> addressing >10 TeV region very cheaply)
– Larger "Beyond VERITAS" plans are being developed (a white paper is
being written)
• From Space:
– There will be a paucity of X-ray telescopes in about 6 years; this void will
need to be filled
– In the long term, there are several large X-ray missions planned (Con-X,
EDGE, ...) to address blazars, GRBs, and cosmological questions
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
Conclusions: What the future holds
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Bigger source catalogs (diverse, deep, & distant) should be created by this generation of
IACTs, as well as GLAST and Swift-BAT
• Increased source count will allow population studies
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Better energy resolution and many sources, some at higher redshift  Better EBL
determination
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Better sensitivity  better time resolution
 Flaring timescales may further limit size of emission region
 More detailed correlation studies and more accurate time lag studies
Detailed HID diagrams can be created at VHE and compared to lower energies as a
function of time! More modeling will be necessary to interpret these data, and it may
severely constrain tcool & taccel
Well-defined campaigns with guaranteed contemporaneous multiwavelength data are
required! Plan now.
New source detections, particularly high-peaked FSRQs, may provide some of the most
exciting upcoming results
We don’t yet understand the acceleration mechanism for VHE gamma rays in blazars (in
spite of SSC popularity). The new data has the potential to put the nail in the coffin for
many models
At this time, it is unwise to rule out blazar models involving proton acceleration!
There is scientific motivation and room for future TeV instruments on the ground and
X-ray to γ-ray missions in space (although funding will certainly be tight)
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State
The VERITAS Prototype
• 87 mirrors (~1/4 full)
• 240 chs, using old recycled PMTs (~1/2 full camera)
• VERITAS DAQ system
SCIPP seminar (26 Jan 2007)
Abe Falcone, Penn State