HEASTRO VHE Astronomy with a 1km2 Cherenkov Array

1
HE-ASTROVHE Astronomy with a 1km2 Cherenkov Array

• Stephen FeganVladimir VassilievUCLA

2
Primary Science Goals

• Detect and measure VHE transients at cosmological
  distances
• Self triggering
• Measure lightcurve from Mrk-421 at z1 with few
  minute resolution
• Survey of VHE sky to level of 1-2 mCrab
• Detailed observations of Galactic sources in the
  energy range 20GeV to gt50 TeV

3
Requirements From Science Goals

• To resolve a few min variability time scale in
  the emission of aMrk 421-like AGN at z1 the
  collecting area must be 1km2.
• To survey the sky to 1-2 mCrab over a few years
  of operation, must have VERITAS sensitivity over
  full sky
• 1km2 collecting area ? Crab Nebula rate of ?
  1g/min gt10 TeV ? 2g/hr gt100 TeV

E Interval RateGeV min125-50 1.350-100
0.7100-200 0.3
4
GRB Prompt VHE VERITAS?

• Prompt emission can only be observed with a
  current generation IACT if the GRB happens to go
  off in the field of view
• Duty cycle 1200hrs/yr 0.137
• Field of view 6 6.9510-4
• Probability per GRB 9.410-5
• 50 probability after 7400 GRB
• 1 GRB/day gt20 years

5
GRB Prompt VHE VERITAS?
Wide FoV IACT

• Prompt emission can only be observed with a
  current generation IACT if the GRB happens to go
  off in the field of view
• Duty cycle 1200hrs/yr 0.137
• Field of view 6 6.9510-4 90 - 0.15
• Probability per GRB 9.410-5 0.02
• 50 probability after 7400 GRBs 34
• 1 GRB/day gt20 years 1 month

6
Other Physics

• Lower energy threshold
• VHE AGN population studies, evolution
• EBL / cosmological evolution
• Time resolution
• High energy properties of space-time
• Order of magnitude sensitivity increase
• Galactic transient phenomena mergers
• Dark matter annihilation
• Diffuse gamma-rays SFR, ULIRG, gal. clusters
• Pulsars, X-ray binaries,

7
Baseline Design

• Array
• 217 telescopes
• 8 hexagonal rings 1
• 80m separation
• Telescope and Detector
• ø10m equivalent
• QE 0.25 (Bialkali)
• 15º field of view
• Facts and Figures
• Outer radius 640m
• Single cell area 5543m2
• Total area 1.06km2

Distance From Center Of Array m
Distance From Center Of Array m
8
Collecting Area vs. Field Of View
Current IACTAs Narrow field of view lt0.01 km2 _at_
40 GeV 0.05-0.1 km2 _at_ 100 GeV 0.2-0.3 km2 _at_ 10 TeV
Field of view p sr
Field of view deg
Square KM Array Continuum of modes Trade area for
solid angle
Parallel mode Narrow field of view 1 km2 _at_ 40
GeV 2 km2 _at_ 100 GeV 4-5 km2 _at_ 10 TeV
Flys Eye mode Wide field of view 0.02-0.03 km2
_at_ 40 GeV 0.1-0.2 km2 _at_ 100 GeV 3-4 km2 _at_ 10 TeV
Collecting Area km2
9
All Sky Coverage Flys Eye Mode
Each telescope points in different direction. If
position of telescope n on ground is (xn,yn)
Zenith ? ( xn2 yn2 )1/2 Azimuth
tan-1( xn / yn )
10
Question(s)
How can an array of mid-sized telescopes operate
in the E30 GeV range?
OR
Why is the collecting area of instruments like
VERITAS so large at Egt100GeV?
11
Cell Effect Collecting Area
Gamma-rays INSIDE detector Instrument has
efficiency e(E) such that effective area
is AIe(E)pRI2 Gamma-rays OUTSIDE
detector Instrument detects ?s to radius RO(E)
such that effective area is AOe(E)pRO(E)(2RIRO(
E)) Energy Dependence e(E) 0.4 _at_ 20 GeV 0.8 _at_
40 GeV RO(E) lt80m _at_ 20 GeV 600m _at_ 10 TeV
OUTSIDE DETECTOR
INSIDEDETECTOR
RI
RO
12
Cell Effect Small Impact Parameter

• Infinite Array Of Telescopes
• 3500m ASL?RCherenk 85m
• DScopes 80m

Distance m

• Geometry Dictates That
• Impact point of every shower is in some cell
• BMax 47m
• At least 3 telescopes contained in Cherenkov
  light pool

Distance m
13
Cell Effect Cherenkov PE Density

• PE density after
• Atmosphere
• Mirror reflection
• Photocathode

Photoelectron density PE/m2
Cell Geometry Consider only the density within
80m of core
Midsized telescopes ø10m, A78m2 E32 GeV,
b80m ?nPE78
Distance from shower core m
14
Trigger Threshold vs. Pixel Size
Normalized trigger threshold for given QE, NSB
rate and FOV
QE0.25, FoV15o, Rnsb0.1 kHz
QE0.25, FoV10o, Rnsb0.1 kHz
(nth-Nnsb) / Q
QE0.25, FoV15o, Rnsb1.0 kHz
QE0.25, FoV10o, Rnsb1.0 kHz
Importance of effect 1) QE 2) Rate 3) FoV
QE0.5, FoV15o, Rnsb0.1 kHz
QE0.5, FoV10o, Rnsb0.1 kHz
QE0.5, FoV15o, Rnsb1.0 kHz
QE0.5, FoV10o, Rnsb1.0 kHz
Trigger Pixel Size degree
15
Trigger Efficiency vs Pixel Size I
(Central Telescope)
Parameters Eg42 GeV FoV15o Rnsb1kHz
QE 1.0, D7m QE 0.5, D10m
El 3.5 km
QE 0.5, D7m QE 0.25, D10m
Optimum trigger sensor pixel size is 0.07o-0.3o
Central Telescope Trigger Efficiency
Weak dependence on QE, D, El
Trigger Pixel Size degree
16
Trigger Efficiency vs Pixel Size II
(Full Array)
Array Trigger Three telescopes above
operational threshold
p0.05o
p0.08o
p0.10o
Array Trigger Efficiency
p0.13o
p0.16o
Array Parameters Elevation 3.5 km QE
0.25 Reflector 10 m FoV 15o
p0.20o
Photon Energy GeV
17
Trigger Peak Detection Energy
Diff. spectral index 2.5
12 GeV
Diff. Rate
15 GeV
Trigger Efficiency
20 GeV
El4.5km, QE 1.0, D7m El4.5km, QE 0.5,
D10m El3.5km, QE 1.0, D7m El3.5km, QE 0.5,
D10m El4.5km, QE 0.5, D7m El4.5km, QE 0.25,
D10m El3.5km, QE 0.5, D7m El3.5km, QE 0.25,
D10m
27 GeV
Photon Energy GeV
Photon Energy GeV
Parameters Trigger pixel size
0.146o Un-localized source (FoV15o) Rnsb 1kHz
Effects 1) Cell operation mode 2) Optimum
trigger pixel size 3) QE, Reflector Size 4)
Elevation 5) Rnsb
18
Trigger Proton Rate
19
Trigger Telescope Multiplicity
Average Number of Telescopes in Trigger
40 GeV g triggers 7 telescopes
Photon Energy GeV
20
Cleaning Sample Event
Photon direction deg
Photon direction deg
Photon direction deg
Event 1 (42 GeV)
Event 2 (42 GeV)
21
Cleaning Voronoi Diagram
0.0
0.3
-0.1
0.2
-0.2
0.1
Photon direction deg
-0.3
0.0
-0.4
-0.1
-0.5
-0.2
-0.6
-0.3
-0.7
-0.4
-0.8
-0.5
-0.9
-0.6
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
-0.1
0.0
0.1
0.2
0.3
0.5
0.5
0.6
0.7
0.8
Photon direction deg
Photon direction deg
Event 1 (42 GeV)
Event 2 (42 GeV)
22
Cleaning P.E. Separation Scales
g 21 GeV NSB 150 g/deg2
g 42 GeV NSB 150 g/deg2
Diff. density Arbitrary
P.E. separation deg
g 100 GeV NSB 150 g/deg2
QE 0.25 Reflector Diameter 10m Elevation 3.5
km Trigger pixel size 0.146o
Voronoi Diagram P.E.-P.E. separation scales in
Image 0.015o-0.045o
23
Reconstruction Angular Acceptance
Optimum cut 4 photons within circle of 0.02o
radius
21 GeV 42 GeV 100 GeV CR
Event containment fraction 1
q radius deg
24
Cleaning Sample Event
Single 42 GeV event View from 4 telescopes
Optimal cleaning (from consideration of angular
reconstruction) keeps only photons near
core Multiple cleaning schemes may be
appropriate. ? Shower axis ? Shape cut ? Energy
estimate
25
(No Transcript)
26
Background Rejection Shape Cut
Parameter ltdgt gives the width of the emission
region in space
21 GeV 42 GeV 100 GeV CR
NPE 1
Electromagnetic showers tightly confined along
shower axis, have small ltdgt
Hadronic events reconstructed cascade radius
is larger than for gamma-rays
Mean cascade radius ltdgt m
27
Background Rejection Shower Max
Parameter ltpgt gives the mean emission height of
the Cherenkov photons.
21 GeV 42 GeV 100 GeV CR
q lt 0.2o
No q cut
Ln(Ng) 1
1st interaction Proton 70g/cm2 Gamma 37g/cm2
Distribution of reconstructed ltpgt different for
each species.
Mean emission height above array m
28
Background Rejection Shower Max
29
Lessons

• Cell Effect operation in 40 GeV range possible
  with mid-sized telescopes
• Optimum trigger pixel size is 0.1º
• Optimum image pixel size is 0.01º
• Very hard cleaning required to optimize
  reconstruction of shower axis
• Multiple cleaning regimes is suggested
• Reconstructed emission height can be used to
  reject protons

30
Comparison Collecting Area
Collecting Area m2
Energy GeV
31
Comparison Differential Rate
Differential Rate arbitrary
Energy GeV
32
All Sky Survey One Year Sensitivity
Sensitivity mCrab
33
Summary Of Array Parameters

• Array of 217 telescopes
• Elevation 3.5km
• Telescopes coupling distance 80m
• Area 1.0km2 (1.6km2)
• Single Telescope Field of View 15º
• FoV area 177deg2
• Reflector Diameter 7m
• Reflector Area 40m2
• QE 50 (200-400nm)
• Trigger sensor pixel size 0.146º
• Trigger Sensor Size 31.2cm
• NSB rate per Trigger pixel 3.2pe / 20ns
• Single Telescope NSB Trigger Rate 1kHz
• Energy Range 20200GeV
• Differential Detection Rate Peak 30GeV
• Single Telescope CR trigger rate 30kHz

• Image pixel size 0.0146º
• Readout image 128 x 128 pixels
• Readout Image size 1.875º x 1.875º
• NSB per pixel 0.032 (20 nsec gate)
• ADC 8 bit (S/N improved, 10 gt8)
• Pixel dimension 12mm x 12mm
• Sensor area 12.3mm x 12.3mm
• Shutter exposure a few msec
• Image integration time - 20ns
• Optical system TBD
• Array trigger protocol TBD
• Data Rates 80 Mb/secper node
• Online data processing TBD

34
Constraints on Hardware I

• Large field of view 15º
• Small imaging pixels 0.01º
• gt 1.75M channels
• PMT based camera not an option.
• Image intensifier and CMOS/CCD sensor
• Silicon avalanche photo-diode

35
Constraints on Hardware II

• Large field of view 15º
• Small imaging pixels 0.01º
• Large diameter telescope 7-10m
• Low light loss
• Focal plane instrument size lt1m?
• gt Unique optical challenge

36
(No Transcript)
37
Two Mirror Designs RC
Fp
concave
Fs
s
convex
FFp Fs / (Fs s - Fp)
Traditional RC design is inconsistent with small
plate scale requirement
38
Two Mirror Designs Modified RC
Aplanatic Highly aspherical non-conic mirror
surfaces Astigmatism and high order Coma can be
contained within specs for FoV 15 deg. Focal
Plane Size, FPS, cannot be made arbitrary small
Fp
concave
Fs
s
concave
FFp Fs / (Fs - s Fp) F/Dp gt 1/2
39
Modified RC Best Design
Example of detailed ray tracing in modified RC
design Dp10m Ds4.1m Df1.6m A(0)0.81 x pi
D2/4 A(7.5)0.55 x pi D2/4 Spot size can be a
few arcmin at the edge of the FoV
40
Emerging Options
D lt 5 m
PMT
Telescopes could be deployed individually or
combined on a single mount
Combine electrical signals from all cameras
operating in single photon counting
mode Star-like approach
Combine optical signals (MMT, Keck,
SALT,) Ashra-like approach
41
Further Study

• Finish optical design
• Simulations with full optics
• Flys-eye mode
• Parallel mode
• Performance with
• Wider separation
• Smaller telescopes
• Smaller FoV
• Alternate camera designs