Using X-ray to TeV Instruments to Probe Blazars, GRBs, and Cosmological Parameters

1
Using X-ray to TeV Instruments to Probe Blazars,
GRBs, and Cosmological Parameters

• Abe Falcone
• (Penn State University)

2
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

3

• Imaging Air Cherenkov Technique

Effective area is size of light pool 105 m2
4
Cosmic Ray Rejection Technique
g-ray
proton

• g-ray images
• - 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
5
Stereo Reconstruction

• Exclude muon background
• Determine source from intersection of shower
  axis
• Crab nebula 35s in 1 hour with VERITAS

80 m
6
GeV/TeV Observatories
HEGRA
CAT
MAGIC
Whipple 10m
CANGAROO
STACEE
H.E.S.S.
CELESTE
Milagro
VERITAS
Tibet AS
MAGIC
Tibet
Solar Two
7
Blazar Categories

• FSRQ Vs. BL Lac
• Low Peaked Vs. High Peaked

Fossati et al. 1998, Ghisselini et al. 1998
8
Broadband Coverage
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
9
Why Study Blazars at VHE?
Figure from J.Buckley 1998

• 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

10
TeV Blazars - RXTE ASM Overview
Mrk421
Mrk501
1ES2344
1ES1959
PKS2155
H1426
2-12 keV Krawczynski et al. 2003
11
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)
12
Correlated Lightcurves

• Multiwavelength lightcurve of Mrk501 from 2-20
  Apr 1999 (Catanese et al. 1997)

50-150 keV
2-10 keV
13
1ES1959 Overall Lightcurves Orphan Flare
14
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
15
Mrk 421

• Exhibited shortest observed TeV flaring timescale
  for any blazar at lt15 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
16
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
17
PKS 2155 on 2006 Jul 27

• A previously low flux (0.05 Crab) source
• HESS observes
• gt10 Crab flux!!!
• lt 5minute doubling time!!!
• During huge TeV flares, the X-ray flux was also
  variable, but to a significantly lower degree
• 2x flux variability
• little/no shifting of 1st Epeak

HESS (gt200GeV)
time bins 2min
Costamante et al. 2006, Aharonian et al. 2007,
Falcone et al. 2007 (analysis ongoing)
18
H1426428

• 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 z0.129 ? excellent target for
  studies of IR background
• Steep measured TeV spectrum

19
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.

20
Blazar H 1426428 HIDs

• X-ray HID diagrams from different observations
  exhibit varying orientation
• We need high-statistics hardness-intensity
  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.

Falcone, Cui, Finley 2003, ApJ
21
H1426 TeV Spectrum

• Very steep TeV spectrum found by both Whipple
  (Petry et al. 2002) and HEGRA (Aharonian et al.
  2002)
• HEGRA results coupled with measurements of the
  EBL lead to an intrinsic spectral index of 1.9

Figure from Aharonian et al. 2002
22
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!

23
Potential New Source Types

• LBLs
• FSRQs

(Falcone et al. 2004, and Perlman et al.)

• ?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

24
VERITAS

• Array of f/1.0 imaging air Cherenkov telescopes
  with 12 m diameters
• Located at Kitt Peak in Southern Arizona, USA
• Sensitivity lt0.005 Crab at 200 GeV (50hr, 5?)
• Slewing Speed 1 deg/sec
• Angular Resolution lt 0.05o
• Energy Resolution ?E/E 0.15 to 0.20

25
Science with VERITAS
26
VERITAS Camera and Electronics

• 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

27
Shower Timing The Movie
28
Expected Performance
(3?, 50 hours)
29
Relative Sensitivity
GLAST and next generation VHE instruments
complement one another well
30
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)!

31
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
  ToOs for satellite instruments

• It is very important to have
• All-sky X-ray and gamma-ray monitoring (and
  notification) by space based instruments
• ToO and monitoring programs in place at all
  wavelengths from GeV down to radio

32
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!

33
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

34
VHE GRB Observations

• At this time, there are no firm detections of
  gt100 GeV photons from GRBs (There are a few low
  significance potential detections at the 3s
  level e.g. Atkins et al. and Amenomori et al.)
• There are several reported upper limits (e.g.
  Saz-Parkinson 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
35
Cosmic Ray Source

• While most GeV/TeV emission is expected to be IC,
  there is some component from p synchrotron, p?
  initiated cascades, and inelastic np 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

36
Cosmology with GRBs

• Multiple Methods
• Use high redshift GRBs (4 lt z lt 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 E?) 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 Om vs O? and w0 vs
  w1 (see Firmani et al. 2006)
• (however, contraints may not compete with SNe at
  low redshifts without many more observations)

37
GRB 050904 z 6.29
High redshift, extremely bright (J17.5, possibly
as bright as 12 very early) gt 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
gt100Msolar stars that formed in the
early universe and collapsed to GRBs, as
predicted by Woosley et al. (2006). Eiso
1054 ergs, Ejet 1051 ergs gt very similar to
other bursts at lower z
38
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 0.661017 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 gt10x
  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 z0.5 !!!
• Need a very sensitive (gt10x VERITAS) instrument
  to create light curves

39
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!

40
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 (--gt
  addressing gt10 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

41
Conclusions What the future holds

• 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
• Better energy resolution and many sources, some
  at higher redshift ? Better EBL determination
• 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 dont 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
  ?-ray missions in space (although funding will
  certainly be tight)

42
The VERITAS Prototype

• 87 mirrors (1/4 full)
• 240 chs, using old recycled PMTs (1/2 full
  camera)
• VERITAS DAQ system

43
Detailed 1ES1959 Lightcurves
TeV
X
10 keV
X-ray index
optical
Orphan TeV Flare
44
Expected Performance of VERITAS-4 and VERITAS-7