Transient observationsin the VHE gamma-ray band byMAGIC/CTA and Cherenkov Telescopes(MAGIC/CTAなどのチェレンコフ望遠鏡による突発天体の高エネルギーガンマ線観測)

Masaaki Hayashida(Institute Cosmic-ray research,
the Univ. of Tokyo)

1Outline of this talk
1. Very-high-energy gamma-ray band
– Imaging Atmospheric Cherenkov Telescope

2. Neutrino event follow-up (all IACTs)
3. GRB observations (MAGIC)
4. AGN flare observations (MAGIC)
5. CTA status and plan for transient
observations

2

Very-high-energy gamma-ray bandradio

Energy (eV)

UV

IR

X-ray

gamma-ray

Imaging Atmospheric Cherenkov Telescopeenergy density
E2F(E)

dust

star
light
Synchotoronaccretion
disk

hadronic
originInverse-Compton(leptonic)

TeV (>100 GeV) gamma-ray sky

176 sources
http://tevcat.uchicago.edu

TeV (>100 GeV) gamma-ray sky
Galactic
- SNR

23

- PWN

33

- PSR

2

- Binary

6- Clusters

5

Extragal.

Gal/Extragal.

Extragalactic3- Blazer

63

Blazer

- FRI

4

PWN

- Starburst gal.

2

UnID
SNR

UnID

BinaryClusters

40% Extragal. 40% Galactic + 20% UNID

72

- LMC

Source Class

Galactic

69

UNID

35

Total

176 5

Detection of E>100 GeV g-rays
g ray
100 GeV

Gamma
ray
Particle
showerProton,300 GeV~ 10 km~1o~ 120 mg showers

hadronic showers• Narrow images• Spread images• Aligned towards
source direction• Isotropic arrival
direction• Cherenkov light is emitted byrelativistic particles in the shower• number of hadron/gamma
more than 1000 times-> need to reject hadron events
2017/02/23

Masaaki Hayashida (ICRR)

6

Stereo observations
２−３ns flash!!!

2017/02/23

g-ray(signal) /hadron (background) > 1000

7

event (real) display (MAGIC)

2017/02/23

Masaaki Hayashida (ICRR)

8

IACTs in currently operation
17 m dia. x 2(28d45'43"N,17d53'24"W)~6hrs
12 m dia. x 4~2hrs(23d16.28' S,16d30' E)(30d40'30.21"N, 110d57'07.77"W)

12 m dia. x 4 + 28 m dia.

The MAGIC telescopes• observational performance:(Aleksic+16, APh)

– Energy threshold (trigger): 50 GeV (LZA), 100 GeV (at 40°)
– Energy resolution: 15% - 23 %(LZA: low zenith angle)– Angular resolution: 0.05° (>1 TeV), 0.09° (> 90 GeV)
– Sensitivity : 0.67±0.04% C.U. (at 290 GeV LZA ) Crab.-10 Unit-2(@290GeV)-1[50 hr obs.]

~10

cm s

VERITASOVERVIEW
Camera Field of View (FoV)

Energetic Radiation Imaging Telescope Array System (VERITAS)
VERITAS: ~ 3.5deg
MAGIC:~3.5deguthernArizonacameras:12-m mirrors,
499 photomultip~3.5 deg field of~105 m2 effective area.
~0.08 deg angula@1TeV.
HESS: ~5degabAcceptance on camera (MAGIC)e.
J. Aleksić et al. / Astroparticle Physics 72 (2016) 76–94

smaller89

Fig. 21. Dashed line: dependence of the amount of background integrated up to a cut
determined by Eq. (2) as a function of the radius of the source, normalized to the background for a point like source. Dotted line: fraction of the total γ events contained
within the cut. Solid line: sensitivity for an extended source divided by sensitivity fora point like source. A flat surface profile of the emission is assumed in the calculations.

uniform

Fig. 20. Top panel: integral sensitivity, computed according to SNex/√Nbkg prescription

before the upgrade, due to the smaller trigger area of the old MAGIC-I
camera. Both of these effects result in a slight rotational asymmetry of the camera acceptance. The effect is minimized by wobbling
such that the source and background estimation positions in the camera are being swapped. Before the upgrade the systematic uncertainity of the background determination was ! 2% [15]. We performeda similar study on a data sample taken after the upgrade. We compare the background estimated in two reflected regions on the sky,without known γ -ray sources. In order to achieve the needed statistical accuracy we apply very loose cuts. In the lowest energy range
we obtain 56428 ± 238 events in one position versus 55940 ± 237,

off-axis

11

further to about 75 GeV at low zenith angles. The value of the energy
threshold doubles at zenith angle of 43°. In the investigated zenith
angle range the value of the threshold after all cuts can be approxi−2.3
GeV.
mated by an empirical formula: 74 × cos(Zenith_Angle)

4.3. Relative light scale between both telescopes

Zenith angle dependences

4.2. Effective collection area

For large arrays of IACTs the collection area well above the energy
threshold for low zenith angle observations is approximately equal tothe physical size of the array [25]. On the other hand for a single telescope or small arrays such as the MAGIC telescopes, the collection
area is mainly determined by the size of the Cherenkov light pool(radius of ∼ 120 m). We compute the collection area as the function of the energy E following the standard definition of Aeff (E ) =For observations at low zenith angles the density of Cherenkov
light photons on the ground produced by a VHE γ -ray shower depends mostly on its energy and its impact parameter. Except fora small dependence on the relative position of the shower axis
with respect to the Geomagnetic field, due to the geomagnetic
field effect (mostly pronounced at lowest energies, see e.g. [27]),
the density is radially symmetric.
th Thus, it is possible to compare
the light scale of both telescopes by selecting γ -like events fromdata in which the reconstructed impact parameter is similar inboth telescopes [36]. In the case of hadronic background events,
such a correlation is much weaker due to the strong internal fluctuations and poor estimation of the impact parameter. In order• observing energy range (energy threshold: E ) has
dependency on zenith angle (zd)higher Etheffective area

Fig. 6. Threshold of the MAGIC telescopes as a function of the zenith angle of the observations. The energy threshold is defined as the peak energy in the differential rate
plot for a source with −2.6 spectral index. Dotted curve: threshold at the trigger level.
Solid line: only events with images that survived image cleaning in each telescope with
2
at least 50 phe. Dashed line: with additional cuts of Hadronness < 0.5 and θ 2 < 0.03°
applied.

higher zenith angle

low Zd (10-30deg)
high Zd (58-70deg)
Fig. 7. Collection area of the MAGIC telescopes after the upgrade at the trigger level(dashed lines) and after all cuts (solid lines). Thick lines show the collection area forlow zenith angle observations, while thin lines correspond to medium zenith angle.
For comparison, the corresponding pre-upgrade collection areas are shown with graylines.

12

Gamma rays absorbed by EBL
Extragalactic Background Light (EBL)

blazar

IACT

gVHE

gEBL

Costamante+ 03

e+e-

g + gEBL → e+ + ethreshold condition: E e (1-cos q )>2 me2c4
Max cross-section:
λ ~ 1.24(Eγ /TeV) μm (at 90o interaction)e.g. TeV + IR
100 GeV + UV

(z=0.212)(MAGIC coll. A&A, 16)

g-ray attenuation
Effect of EBL on the spectrum of blazars

EBL spectrum

Franceschini & Rodighiero
A&A submitted

redshiftdN/dFobs = dN/dFint e-𝝉(z,E)
Intrinsic
Observed

log dN/dE

Energy [TeV]

Optical depth

z=0
z=0.6

z=0.2

log E
6

14

Observation strategy• Fixed (normal) observations– scheduled in period by period– time allocation (source selection) once a year
– time critical observations (MWL campaign, phase)• ToO observations– multi-messenger (n, GW) alerts– GRB alerts– multi-wavelength alerts (known sources)15

n event observations by MAGIC
3 ”archival” events + 2 real time alters

16Archival HESE events by MAGIC

Satalecka@AMON16

17

(archival) multi-PeV track (Atel#7856)
detected on 2014 June 11
2.6+/-0.3 PeV
reported on 2015 July 29

Satalecka@AMON16

18

Archival HESE events by H.E.S.S.
• Track like events• angular uncertainty < FOV• southern sourceno significant excess
Schuessler@AMON16z

19

f electromagnetic follow-ups to transient sources. We present preliminary results fromprompt follow-up of a “realtime" alert in Subsection 2.2.

Archival n events by VERITAS

ation of archival neutrino positions

neutrino positions observable with VERITAS were selected from three IceCube dataon events from a sample of high-energy starting events (HESE) collected by IceCubears [4], 21 events from a sample of through-going Northern-sky muon tracks collectedars [5], and 29 through-going tracks with high-astrophysical purity collected over six
ith some overlap between the different data sets. The map in Fig. 1 shows the position
events selected for observation.

57 hours for 18 events

Santander+16 (arXIv:1612.0430)(3 HESE, 15 through-going track)

kymap in equatorial coordinates showing the positions of HESE (‘C’, circles) and throughsquares) muon events compared to the maximum elevation these sources reach when observed
AS. Typical observations are performed when sources go above 50 elevation. White markers
e positions where VERITAS has collected observations.

no significant excessr, VERITAS has collected a total of 57 hours of good-quality data on 18 muon neutrino
he observations have been performed using the standard wobble observation strategy
lescopes are offset from the position of the potential source to allow for a simultanenation of the background. Offsets of 0.5 and 0.7 with respect to the best fit neutrino
re used to provide better coverage of the neutrino error circle. During the analysis ofata, cuts are introduced to separate gamma-ray shower candidate events from a domiound of hadronic cosmic-ray showers. In this work, we have used soft cuts optimized

3-10 hours for each HESE (C) eventsand UL of 1-2 % Crab (>100 GeV)
20

Alert-1 : 1st HESE
2016-04-27 05:52:32 UTC (the first GCN event: HESE)
1st notice: 05:53:53 UT••

RA=239.66deg, Dec=+6.85deg (8.9deg@90%, 1.6@deg50%)
VERITAS observed it from 05:55:45 UTC for 71 min

2nd notice: 23:24:24 UT, a refined position (offline analysis)•••

RA=240.57deg, Dec=+9.34deg (0.6deg@90%)
VERITAS observed it on the next night (28th) for 118 min
MAGIC observed for ~2 h with ~42 hrs delay.
VERITAS

GCN#19377
Santander+16 (arXIv:1612.0430)

MAGIC

no excess..

UL: 5.7e-11(cm-2 s-1 TeV-1)
at 0.2- 0.28 TeV
at position of therefined n event

Zd: 18-26deg
Eth: ~ 120 GeV
21

Alert-2: 2nd HESE/1st EHE
2016-07-31:
01:55:58 UT (the 2nd GCN alert)
IC-EHE/HESE-20160731
from both HESE and EHE (the 1st EHE alert)

C. Hoischen et al., Baikal 2016

•
•
•
•

Initial HESE pos. = 215.11deg, Dec = -0.46deg
visible pos.
at the timeof the alert
Initial= 215.09deg,
Dec = -0.42deg▪ not EHE
visible at+ thetime of alert.
in thenextnight.
■Not
2016-08-01
2016-08-02:
visibleObservedfor ~1h above
45deg
elevation
H.E.S.S. for ~1 hr, MAGIC: 1.3 hrs (started 21:25 UT.),•

Refined postion: RA=214.544deg, Dec:-0.335deg (@Aug 1, 02:35:54 UT)■ 2016-07-31 (1:55 UT): IceCube/AMON alert on a EHE/HESE event■ OnSite analysis▪ no significant emission detected2016-08-02:Atel#9301
ATEL #9301▪
H.E.S.S.

Schuessler@AMON16

Zd>45deg

MAGIC

Atel#9315• Zd:45-65deg• Eth: ~ 600 GeV• no significant excess

no excess..
preliminary(VERITAS
preliminary can not be operational during summer)
22

GRB observations with MAGIC• Dedicated alert system from GCN• Automatic response<Galactic coordinate>(upgraded in 2013)– Zenith < 60°– Moon distance > 30°– Sun zenith > 103°

• up to 4h of observations 90 GRBs were observed
after prompt emission(since 2005)• Fast movement of the telescopes

23

GRBs observed with MAGICNo significant detection until now,,,

redshift(color: after 2013)

delay

100s
more desirable
condition

44 GRBs with d<10min
24

GRB observation status
Moretti@Yachay16

25

GRB 090102 limits from MAGIC
GRB090102 (z=1.547), afterglow observations
3106

J. Aleksić et al.

ULs both LAT and MAGIC MA
Table 1. MAGIC-I 95 per cent confidence level ULs
for the afterglow emission of GRB 090102. The values correspond to the first 5919 s of observation from
03:14:52 to 04:53:32 UT. α Bins central energy wasevaluated applying all analysis cuts to MC simulations.
β Statistical significance of the excess events observedby MAGIC.

Figure 1. Light curve for GRB 090102. Data in the R band (red points)
were taken from the TAROT, REM, NOT, GROND and Palomar telescopes.
The early near-infrared H band (blue points) are from the observations of
the REM telescope. All magnitudes are expressed in the Vega system. X-ray
data are the unbinned Swift/XRT and BAT data in the 0.5–10 keV (green
and magenta point, respectively). The MAGIC observation window is alsoplotted. R and H data from Gendre et al. (2010). XRT and BAT data retrieved
from http://www.swift.ac.uk/burst_analyser (Evans et al. 2010).

E bin
(GeV)

⟨E⟩ α
(GeV)

σβ

Average flux limits(erg cm−2 s−1 )

25–50
50–80
80–125
125–175
175–300
300–1000

43.9
57.3
90.2
137.2
209.4
437.6

0.83
− 0.30
1.09
0.51
0.90
− 0.48

8.7 × 10−10
1.5 × 10−10
3.1 × 10−10
2.2 × 10−10
1.6 × 10−10
0.3 × 10−10

Figure 2. SSC modelled emission during the afterglow of GRB 090102.
analysisthresholdaround30 ULs
GeVderived
is achieved,
whichforis low-energy
evaluatedBlue
triangles
are 95ofper
cent CLby MAGIC
fromanalysis.
Monte Carlo(MC) simulations.
In orderestimate(LE)
The relatively
more constraining
ULto
inaccurately
the 50–80 GeV
is duethebackgroundfromhadronic
atmospheric
showers,
an
OFF
data
to a negative significance energy bin. For comparison, the regular energy
samplewas taken
one night
later withthereported
telescope
pointing
close
range
MAGIC
ULs (Gauget al. 2009a)
are also
in light
grey. Thered
triangles
reportthe
Fermi-LAT
95
per
cent
CL
ULs.
The
purpleandblack
to the burst location and in the same observational conditions and26
curvesdepictsetup.
the expectedenergyflux according
to MAGIC
the GRB Analysis
afterglow
instrument
Data wereanalysedusing themodel described in Sections 6 and 5. Physical parameters are ϵ = 0.1,

(J. Aleksic et al. (MAGIC Collaboration), 2014, MNRAS, 437, 3103)

GRB: Late time observations
GRB130427A (Ackerman+14)
VERITAS observed the source+20h (70ks), +2 days, + 3 days

The
AstrophysicalJournalLetters, 795:L3 (6pp), 2014 November 1
The Astrophysical Journal Letters, 795:L3(6pp),
2014 November
1(no detection)

Table 1
VERITAS Observations of GRB 130427A
Date

tstart(UTC)

tendDate(UTC)

2013 Apr 28
2013 Apr 29
2013 Apr 30

03:32:35
03:32:59
03:22:02

2013 Apr 28
04:31:16
2013 Apr 29
05:33:39
2013 Apr 30
06:05:40

Total

Total

Aliu et a

Table 1
VERITAS Observations of GRB 130427A

Exposure tstart non
(UTC)
(s)

tendnoff(UTC)

Exposure
αa
(s)

b
non
Significance(σ )2925 03:32:35 165
5746 03:32:59 322
7814 03:22:02 402

04:31:16
1164
05:33:39
2120
06:05:40
2820

2925
0.125
5746
0.143
7814
0.147

6104

16485
0.141

165
1.3
322
1.1
402
−0.5

16485

889

889
0.9

noff

Flux ULαc a−121164 9.4 × 100.125−122120 6.6 × 100.143−122820 2.7 × 100.147−126104 3.3 × 100.141

Notes.
Notes.
a Ratio of the signal region to background region.
a Ratio of the signal region to background region.
b Significanceb Significance calculated using Equation (17) of
calculated using Equation (17) of Li & Ma (1983).
Li & Ma (1983).
c 99%−1 Rolkec 99% confidence-level upper limit on νF in erg
upper
on using
νFν inthe
ergmethodcm−2 sof.The upper
is derived
methodof Rolke e
cm−2confidence-levels−1 .The upper limit
is limit
derivedet al. limit(2005),
quoted using
at 100theGeV,
and calculatedν
assumingan (asintrinsic
GRBby
spectrumofabsorbed(dN/dE)using∝ E −2.0
assuming an intrinsic GRB spectrum of (dN/dE)∝ E −2.0(as measured
the LAT)absorbedmeasuredthe LAT)the EBLmodel of by
Gilmoreet al.
(2009). using the EBL m(Aliu+14)

program insince
it beganoperationsin 2007and onhasthe VHEWe
program since it began full array operations
2007
and hasfull array We
derive upper
derive upper
limitsgamma-rayflux limfromoreofthan
100 follow-up
observationsof GRBs
performed more than 100 follow-upperformed
observations
GRBs
The assu
GRB 130427A.
The assumed
spectral GRB
shape130427A.
is extrapolated
fro
byal.
space-based(Acciari
et al. 2011).
The a power-law
detected by space-based instruments detected(Acciari et2011). Theinstruments
the LAT spectrum
observations,the LATobservations,
namelywith
VERITAS
trigger
in 2011ofandthe camera
VERITAS trigger system was upgradedin 2011 andthe systemcamerawas upgradedof 2 withphoton index
2 withno intrinsic photon
cutoff. index
The upperlim
wasyear later, resulting
in improved
sensitivity
was upgraded one year later, resulting
in upgraded
improved one
sensitivity
calculated
for 1.
each time
calculated
for each timeinterval are given
in Table
lower Itenergy
threshold (Kieda et al. 2013). It is estimatedand a lower energy threshold (Kieda etandal.a2013).
is estimated
VERITASshouldthat VERITAS should be sensitive thatenoughto detectbrightbe sensitive enough to detect bright3.2. Fermi-LAT
and/or nearby GRBs.
and/or nearby GRBs.
the (07:47
time ofUTC),the Fermi-GBM
trigger the(07:47UTC),
At the time of the Fermi-GBM At
trigger
We analyzed
We analyzed
LAT data
using an unbinned
maximumlik
27 the LAT
GRB 130427A
wasofat52a◦ relatively
favorable
elevationof52◦ in lihood
GRB 130427A was at a relatively favorable
elevation
method(as implihood method (as implemented
the Fermi
ScienceToo
35
35
for VERITAS.
Unfortunately,bright moonlight
conditions (97%for VERITAS. Unfortunately, bright moonlight
conditions(97%• Many LAT GRBs show late time (delayed) emission inthe (early) afterglow phase• GRB observation strategy of MAGIC has been
updated to observe events even +2-3 days.

AGN (blazar) flare(MH+16)
250
200
150

1day binned light curve(>100 MeV) highly variable

100
50
0
0

500

1000

1500

2000

[MJD - 55000](MH+12, ApJ)

PKS1424-418 (possible n associations?)(Kalder+16, Nat.Phys)

wide energy range
à multi-band correction
28

Flare alert system
1. VHE self-trigger– Under “MAGIC/HESS/VERITAS/HAWC/FACT flaring AGN agreement”• When one find a flare, one inform others by e-mail.

– Usually, onsite analysis can shows a results in a few 10 minutes

2. HE (Fermi-LAT) trigger– Self monitoring. Communication between LAT and IACTs– About 6 - 8 hour delay at fastest (until LAT data becomes available)

3. X-ray/optical trigger– many “monitoring” programs are on going• On-site optical telescope in each group– The Astronomer's Telegram– informed by individuals

29

FSRQ PKS1441+25 (z=0.94)
Initiated by high flux with a hard-index in Fermi-LAT (e.g., Atel7402)• 25.5s from periods A,B• higher IC peak E during
the flare.
• VHE flare from outside
BLR. (while inside BLR
in steady state?)
Gobs=4.62±0.11
Gsrc= 3.18±0.15

The Astrophysical Journal Letters, 815:L23 (8pp), 2015 December 20(Ahnen+15 ApJL)

30

QSO B0218+357 (z=0.944)••

QSO B0218+357 is a gravitationallylensed blazar at z=0.944 with a spiralgalaxy B0218+357G at z=0.68
11-days is the time-delay• In 2014, Fermi got the first flare, and11 days after, MAGIC detection• First gravitationally lensed VHE g
rays ever observed• – 2hours, 6 sigma significance(Ahnen+16, A&A)

31

Hadronic origin g-ray scenario: 3C 279(Petropoulou, Nalewajko, MH+17, MNRAS)• Base code: Petropoulou & Dermer 16, ApJL• GeV g-ray origin: proton synchrotron•(need less power but higher proton energies
100PeV
than photomeson (pg) origin)(à PeV n)
Data : Orbit-D (red: simultaneous UV, X-ray and g-ray data)
d=50, B=0.8kG, Pjet=1e48 erg/s(LAT collab.16, MH+16)

internal gg absorption(EM cascade)target g: BLR photons
32Towards
the next
generationof IACTs IACTs
Towards
thenext generation
The Cherenkov Telescope Array (CTA)

Artistic impression of the CTA, image courtesy G. Perez, SMM, IAC
33
E. Bissaldi

SciNeGHE • Pisa • 20 Oct ober 2016

4

大口径
low energy threshold(~20 GeV)à high-z, pulsar

中口径
deep observations
小口径
highest energies
à PeVatron

CTA Consortium
CTA-Japan
30 institutes、
118 members

全体：
32 countries
209 institutes
1346 members (456 FTE)

35

CTA : Two Array Sites
Science Management
CenterNorth (IAC):
agreement signed

South (ESO):
Negotiation processing

Headquarters• Improved sensitivity

CTA performance– Access the entire Galaxy– Instantaneous sensitivity (AGN, GRBs)

energy‘reach’ telescopes)(compared• toExcellent
the current
Cherenkov• Field-of-view > 8°

1. energy– range
is extended:
20 GeV to >100 TeV
Efficient survey
instrument– Measure diffuse emission
2. increased
FOV• Arcminute angular resolution–(5°-8°–[CTA]vs 3°-5°[now]) sources
Resolveextended

3. better angular resolution–(0.05° [CTA] vs 0.1° [now])

4. better energy resolution–(10-20%[CTA] vs 15-30% [now])DM
CTA(IACTs)Fermi�
11/24/2016
fromaeV tovs
ZeV

for transients/variables�
9
Funk & Hinton 2013!

more feasible than Fermi-LAT
for short time observations

Sensitivity>4km2

J.$Hinton

LST (Large Size Telescope)
•
•
•
•
•
•
•
•
•
•
•
•

Diameter : 23 m
Dish Area: 368 m2
Focal Length : 28 m
F/D :
1.2
Dish profile : Parabolic(<0.6nsec)
segmented mirror : 198
Camera FOV: 4.5°
1855 pixel (0.1°/50mm/pix)
dish structure : CFRP
total weight : ~ 120 ton
re-pointing : < 20 sec
Active Mirror Control (<14’’)

28 m
28 m

23 m23 m

Science Working Group coordinators

Stefano Vercellone
Stefan FunkSix Science WG’s

Key Science
Program (KSP)Transient KSP
Transients KSP!
• GRBs-> Tam- GRBs!
• HE neutrinotransients- Galactic transients!
(microquasars,
PWN flares, novae, etc)! -> Saito
• Galactictransients- X-ray/optical/radiotransients!
– microquasars,
PWN flares,
novae, etc(TDEs, SN shock breakout
events, FRBs, new transients)!
• X-ray/optical/radiotransients-> Aoki, Totani
->
YoshidaHE
neutrinotransients!
– TDEs, SN shock breakout events, FRBs, new transients- gravitational
wavetransientstransients! -> Kisaka• gravitational
wave
Wanted:!
- serendipitous
VHE
transients!
• serendipitous
VHE
transients- VHE transient survey via divergent pointing! - gamblers!
• VHE transient survey via divergent pointing- fortune!
Related KSPs:!
seekers!
- thrill hounds!
- AGN (flares), LIV studies!

Related KSPs:• AGN (flares), LIV studies?42

GRB light curve: Fermi vs. CTA
GRB light curve: Fermi vs CTA!
LAT!
>100!
MeV�

based
on GRB 080916C
GRB 080916C

Abdo+ 09�

>1GeV�

SI+ 2013!
Astropart. Phys.!
43, 252!

CTA!
>30GeV, 0.1 sec bin�inc.!
Yamamoto!
Y. Inoue!
Yamazaki!
(for CTA)!

Clarify physics of emission
Test UHECR origin, LIV

expected detection rate of GRB ~0.5 GRB yr-1
(very model dependent, though,,)43

For CTA, an improved Galactic plane survey should be a major
objective and it will also be capable of performing an all-sky survey
in unprecedentedly short time at high sensitivity; the scientificrationale and feasibility of both survey types are thoroughly discussed in [13]. As also discussed in [13], such surveys can be performed in various modes of observation, in particular, large
number of high-performance IACTs allows for using non-parallelmodes with an enlarged FOV. The proper adaptation of such a
mode
for CTAa specific telescope array can be a non-trivial task. The
Divergent
pointing
with
Lucie Gérard
ergentpointing
with
CTA
Lucie Gérard
optimization of the pointing strategy, taking into account numerous characteristics of an array, e.g. distance between telescopes,
FOV, energy threshold etc, can significantly reduce the observation
Normalmodepointing
mode
pointing modetimetoachievea given sensitivity.
Normalneeded
pointing
Divergent Divergent
pointing mode
In this work we consider the array of15Middle15Sized Telescopes
15
15
300
300
(MST) working in various, paralleland
800 non-parallel, modes. By per800
forming high-statistics Monte Carlo (MC) simulationsof the sky10
10
10
10
700
700
250
250
survey observations, we derive for each mode the basic performance parameters at both trigger
and
600 analysis levels, which then
600
5
5
5
5
200
200
allow us to compare efficiencies of the modes. Our study is a part500
500
of an intensive work within the CTA
Monte Carlo Work Package
0
0 scheme.
0
0
150
150
aimed at optimizing the CTA
Whereas we
400 observation
400
consider in detail different modes with the MST array, independent
300 for-5-5 the divergent
investigations are currently300
performed
mode of
-5
-5
100
100
Large Sized Telescopes (LST)200
sub-array
200 and the full CTA array working in divergent modes.
-10
-10
50
-10
-10
50

for transients/variables
Divergent
pointing�

Wide FoV at expense
Szanecki+ 2015
ofsensitivity
Wider FoV at[degree][degree][degree][degree]divergent pointing� for MSTs/SSTs

normal
Divergent
pointing with 100
CTA100(only MST/SST?)
expenseof angular/energy resolution

divergent

Lucie GérardToO:
FoV ~20via
deg dia.
• of events
fast scanningof large
5.
transientsurveydivergent
pointing!
Gérard+2015
ureFigure
4: Gamma-ray
acceptanceafter directionand energy
reconstruction
cuts. The
total
number4: Gamma-ray
acceptanceafter directionand energy
reconstruction
cuts.
The
total
numberof
eventsfast
scanning of large
singthose cuts
469are051
for051
thefornormalmode and447
918447
for918the fordivergentmode. mode. - possibly effective for surveys!
passing
thosearecuts
469
the normalmode
andthe divergent
e.g., GW follow up
regions, e.g.region,
GW follow-up
of persistent point sources!
Survey:divergent 2hPreliminary studies:
-15
-15

-15
-10
-15

0

-52.
5 0 modes
10 5
1510-10Sky0 -5
15
survey[degree] [degree]

0

-15
-15

-15
-10
-15

-5-10

0 -5

Fig. 1. Three modes of configuration of the telescope system used in the sky-survey
scans: (a) normal (parallel) mode; (b) divergent mode; (c) convergent mode.
0
0
5 0
10 5
15 10
15
[degree] [degree]

Differential Flux Sensitivity [C.U.]t

Integral Flux Sensitivity (E>E ) [C.U.]tnormal 8h

Integral Flux Sensitivity (E>E ) [C.U.]t

Integral Flux Sensitivity (E>E ) [C.U.]t

Integral Flux Sensitivity (E>E ) [C.U.]

parison of the performance of the three modes and a related issue,
Fig. 1 illustrates possible modes for a large telescope array used
i.e. an optimal value of the offset angle (giving the amount of thefor sky surveys. The parallel and divergent configurations weredifference of the pointing directions, as defined below), appears
considered before in [13]; below we introduce also a novel, conver1 the most efficient survey strategy.
crucial for planning1
gent
mode (note the difference between our terminology and thatof [13], were the◦ parallelmode is referred to as convergent).
◦
0 − 7 Divergent Pointing
3. MC simulations
The performanceof a telescope system operating in the sky sur0◦ − 7 ◦ Divergent Pointing0◦ − 3.5 ◦ Normal Pointing
vey mode depends
on the FOV of the system and the time of obser◦◦
0 − 3.5 Normal Pointing
For all three modes, we simulate the response of the telescope
vation needed to achieve a given significancelevel, i.e. its
11
11
array to the Extensive Air Showers (EAS) induced by gamma rayssensitivity.
M.
Szanecki et al. / Astroparticle Physics 67 (2015) 33–46
and proton background. To simulate the development of EAS weInthesimplest
approach,sky
surveysmay
beperformed
with
10-1
10-1code [14,15], used as a standard in CTA. We simuse CORSIKA 6.99
telescopes pointed parallely into the same direction of the sky[9–11], over a dozen new sources wereulated 2:1 ! 107 gamma rays and 3:8 ! 108 proton events2 – both(Fig. 1a), however, in such a case the FOV of the telescope system
is highly limited by the FOVs of individual telescopes. The FOV ofwith energies between 30 GeV and 10 TeV generated from differen-1
10-1
Galacticplane
survey ashould
be aarray
major
10-1
10-1 deviating◦
telescope
can0beenlarged 10
by slightly
tial spectra withindex C ¼ #2:0. However, in our anal− 1 ◦significantly◦0◦ − 1 ◦ the 0spectral0◦ − 1 ◦− 1◦◦
1◦ − 2 ◦e capable of performinganall-skysurvey1◦ − 2 ◦− 2 ◦ weights◦◦
1◦ − 2 ◦ corresponding to spectra with C ¼ #2:57
the pointing direction
ysis, we use 1event
2 − 3of each◦◦telescope. In the divergent mode,
◦
◦
2
−
3
◦
◦
2 −3
3◦ − 4 ◦◦2 −3
3− 4 ◦ outward direction, see Fig. 1b, by
time at high sensitivity;
the scientific◦◦telescopes
are inclined
thefor gamma rays
andC ¼ #2:73 for protons. Gamma rays are simu3◦ − 3.5
4◦ − 5into
3◦ − 3.5 ◦4◦ − 5 ◦5◦ − 6 ◦3.5 ◦ − 4 ◦◦− 6◦
3.5 ◦ −test4◦of both survey types arethoroughly
dis- with
an-2angle
increasingtelescopedistance
from
the
array
cenlated
fromapoint-likesource with the direction defined by◦◦ the 5
◦
◦
6 −7
◦
◦
4 − 10
4.5-2
6◦ − 7 ◦
10
◦
◦
4
− 4.5
$
7 −8
◦
◦
◦
◦
7
−
8
ter. -2As can
explained
below,
aperformance
improvement
for
such
a
the
Zenith
angle
Za
=
20
and
the -1Azimuth Az = 180$ measured with
◦
◦
4.5
−
5
cussed in [13], such surveys
be
per-1
2
-24.5 − 58◦ − 9 ◦
◦ 1 ◦
10
10
10
10
10
102
1
10
10
8 −9
-2
10-2 energy-2
configuration
can be expectedprimarily at highenergiesof prirespect to the magnetic
North. The background proton showers areenergy [TeV]
[TeV]
10-2
10E
es 10of10-2 observation,inparticular,
large
-1
2
-2
-1
2
t -210
10
10
10
-1 1
2
-1 1
2
10
10
$
10-2 10
10
10
10
10
10
1
10
1
10
simulated isotropicallywith directions within a 10 half-angle coneenergy E [TeV]
energy E [TeV]ance IACTs allows for maryusing photons.
non-parallel
energy E [TeV]
energy E [TeV]tttt(larger than the FOV of all considered modes) centered on the direcFor the
FOV. The proper adaptation
ofdivergent
such a configuration, images of gamma rays impingtion of the gamma-ray source. We set the maximum impact paraming close the array center are shifted toward the camera edge,
opeFigurearray
cana non-trivial
task.atTheure
5: Integrated
sensitivities
differentto the centerof the field
view.
Left:
hours8of
1 distances
5: beIntegrated
at different
distancesto event.
the centerof thetofeter
field
of gamma
view. 8rays
Left:
hours
of for protons to 1500 m. The
forto 1000m andwhich sensitivities
leads to a leakage
or complete
loss of an
While theing
strategy,takingintoaccount
numerervations
with thedivergentmode.
Right:
2 hoursofobservations
with thenormalmode.
detectorarray
is assumed to be located at the Namibian (H.E.S.S.)
largerloss
of events
is mostly
pronounced
forthe lower-energy
observations
withthedivergentmode.
Right:
2hours
ofobservations
withthenormalmode.
array, e.g. distance between
telescopes,
site at the altitude of 1800 m a.s.l.
gamma rays, the leakage effect concerns mainly events with higher, can significantly reduce
the observationenergies.
As a result even if an event is registered it is poorly reconThe response of the telescope array is simulated with the CTA
given sensitivity.
structed. On the other hand, orientation of telescopes in the diversim_telarray code [15,16]. We use the MST subarray of the CTA
der the array of Middle
Sized
Telescopes
gent
mode
is suitable for efficient detection of events with large
array E from the so-called production-1; the subarray includes 23
parallelnon-parallel,modes.
By perComparison
withthenormalpointing
mode
impactparameter
and/orarrivingfrom directions
telescopes with positions shown in Fig. 2. The direction of the cen3.2 andComparison
withthenormalpointing
mode further from the
FOV center(in skyboth cases mainly with high energies).
tral telescope No. 5 is always approximately in the center of theonte Carlo (MC) simulationsof theFOV of the array (a slight displacement may occur due to the presQualitatively, one can expect that those negative effects can be

GRBs
fromonset!
onset prompt
point source
surveyfrom-• GRBs
efficiency comparable?
emission
physics physics!
promptemission-> possibly interesting(crucial but poorly understood)(crucial
buttransient
poorly survey
understood)
for commensal
unbiased•unbiasedtransient- survey
Lorentz invariance violation!
e.g. FRBs

Figure 7: Left: Integrated sensitivities above the E . Right Differential sensitivities, within the energy range(5 bins per energy decade). The sensitivities are calculated within each mode effective fields of views, for 8hours for the divergent mode and 2 hours for the normal mode, the ratio of the observation times corresponds
to the ratio of the effective field of view areas.

(big improvement over Fermi)- unbiased transient survey!
44
e.g. FRBs

Summary• IACT trainset follow-up programs (ToO program)– can start observation in real time (auto) and the next night (manual)– duty cycle ~15% (~1200 hr) including moderate moon time.
• multi-messenger (n) alerts (auto/manual) à no detection, yet– archival HESE/track events, HESE/EHE alerts• GRB alerts (real time) à no detection, yet– MAGIC observed 90 GRBs (and several within prompt)• multi-wavelength alerts (manual)– successful for blazars à extended the g-ray horizon to z~1• CTA: `transients’ is one of the key sciences– pre-defined ToO program within the consortium– plan: 1st LST: 2017 Nov., Scientific operation starts: 2021?
45