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Title
Observation of the Crab Nebula with the HAWC Gamma-Ray Observatory
Permalink
https://escholarship.org/uc/item/0f97m85c
Journal
Astrophysical Journal, 843(1)
ISSN
0004-637X
Authors
Abeysekara, AU
Albert, A
Alfaro, R
et al.
Publication Date
2017-07-01
DOI
10.3847/1538-4357/aa7555
Copyright Information
This work is made available under the terms of a Creative Commons Attribution License,
availalbe at https://creativecommons.org/licenses/by/4.0/
Peer reviewed
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arXiv:1701.01778v1 [astro-ph.HE] 6 Jan 2017
Draft version January 10, 2017
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OBSERVATION OF THE CRAB NEB ULA WITH THE HAWC GAMMA-RAY OBSERVATORY
A.U. Abeysekara,
1
A. Albert,
2
R. Alfaro,
3
C. Alvarez,
4
J.D.
´
Alvarez,
5
R. Arceo,
4
J.C. Arteaga-Vel
´
azquez,
5
H.A. Ayala Solares,
6
A.S. Barber,
1
N. Bautista-Elivar,
7
A. Becerril,
3
E. Belmont-Moreno,
3
S.Y. BenZvi,
8
D. Berley,
9
J. Braun,
10
C. Brisbois,
6
K.S. Caballero-Mora,
4
T. Capistr
´
an,
11
A. Carrami
˜
nana,
11
S. Casanova,
12
M. Castillo,
5
U. Cotti,
5
J. Cotzomi,
13
S. Couti
˜
no de Le
´
on,
11
E. de la Fuente,
14
C. De Le
´
on,
13
T. DeYoung,
15
B.L. Dingus,
2
M.A. DuVernois,
10
J.C. D
´
ıaz-V
´
elez,
14
R.W. Ellsworth,
16
D.W. Fiorino,
9
N. Fraija,
17
J.A. Garc
´
ıa-Gonz
´
alez,
3
M. Gerhardt,
6
A. Gonz
´
alez Mu
¨
noz,
3
M.M. Gonz
´
alez,
17
J.A. Goodman,
9
Z. Hampel-Arias,
10
J.P. Harding,
2
S. Hernandez,
3
A. Hernandez-Almada,
3
J. Hinton,
18
C.M. Hui,
19
P. H
¨
untemeyer,
6
A. Iriarte ,
17
A. Jardin-Blicq,
18
V. Joshi,
18
S. Kaufmann,
4
D. Kieda,
1
A. Lara,
20
R.J. Lauer,
21
W.H. Lee,
17
D. Lennarz,
22
H. Le
´
on Vargas,
3
J.T. Linnemann ,
15
A.L. Longinotti,
11
G. Luis Raya,
7
R. Lu na-Garc
´
ıa,
23
R. L
´
opez-Coto,
18
K. Malone,
24
S.S. Marinelli,
15
O. Martinez,
13
I. Martinez-Castellanos,
9
J. Mart
´
ınez-Castro,
23
H. Mart
´
ınez-Huerta,
25
J.A. Matthews,
21
P. Miranda-Romagnoli,
26
E. Moreno,
13
M. Mostaf
´
a,
24
L. Nellen,
27
M. Newbold,
1
M.U. Nisa,
8
R. Noriega-Papaqui,
26
R. Pelayo,
23
J. Pretz,
24
E.G. P
´
erez-P
´
erez,
7
Z. Ren,
21
C.D. Rho,
8
C. Rivi
`
ere,
9
D. Rosa-Gonz
´
alez,
11
M. Rosenberg,
24
E. Ruiz-Velasco,
3
H. Salazar,
13
F. Salesa Greus,
12
A. Sandoval,
3
M. Schneider,
28
H. Schoorlemmer,
18
G. Sinnis,
2
A.J. Smith,
9
R.W. Springer,
1
P. Surajbali,
18
I. Taboada,
22
O. Tibolla,
4
K. Tollefson,
15
I. Torres,
11
T.N. Ukwatta,
2
L. Villase
˜
nor,
5
T. Weisgarber,
10
S. Westerhoff,
10
I.G. Wisher,
10
J. Wood,
10
T. Yapici,
15
G.B. Yodh,
29
P.W. Younk,
2
A. Zepeda,
25
and H. Zhou
2
1
Department of Physics and Astronomy, University of Utah, Salt Lake City, UT, USA
2
Physics Division, Los Alamos National Laboratory, Los Alamos, NM, USA
3
Instituto de F´ısica, Universidad Nacional Aut´onoma de M´exico, Ciudad de M´exico, M´exico
4
Universidad Aut´onoma de Chiapas, Tuxtla Guti´errez, Chiapas, M´exico
5
Universidad Michoacana de San Nicol´as de H idalgo, Morelia, M´exico
6
Department of Physics, Michigan Technological University, Houghton, MI, USA
7
Universidad Politecnica de Pachuca, Pachuca, Hgo, M´exico
8
Department of Physics & Astronomy, University of Rochester, Roc hest er, NY , USA
9
Department of Physics, University of Maryland, College Park, MD, USA
10
Department of Physics, University of Wisconsin-Madison, Madison, WI, USA
11
Instituto Nacional de Astrof´ısica,
´
Optica y Electr´onica, Puebla, M´exico
12
Instytut Fizyki Jadrowej im Henryka Niewodniczanskiego Polskiej Akademii Nauk, IFJ-PAN, Krakow, Poland
13
Facultad de Ciencias F´ısico Matem´aticas, Benemrita Universidad Aut´onoma de Puebla, Puebla, M´e xico
14
Departamento de F´ısica, Centro Universitario de Ciencias Exactas e Ingenierias, Universidad de Guadalajara, Guadalajara, M´exico
15
Department of Physics and Astronomy, Michigan State Uni versity, East Lansing, MI, USA
16
School of Physics, Astronomy, and Computational Sciences, George Mason University, Fairfax, VA , USA
17
Instituto de Astronom´ıa, Universidad Nacional Aut´onoma de M´exico, Ciudad de M´exico, M´exico
18
Max-Planck Institute for Nuclear Physics, 69117 Heidelberg, Germany
19
NASA Marshall Space Flight Center, Astrophysics Office, Huntsville, AL 35812, USA
20
Instituto de Geof´ısica, Universidad Nacional Aut´onoma de M´exico, Ciudad de M´exico, M´exico
21
Dept of Physics and Astronomy, University of New Mexico, Albuquerq ue, NM, USA
22
School of Physics and Center for Relativistic Astrophysics - Georgia Institute of Technology, Atlanta, GA, USA 30332
23
Centro de Investigaci´on en Computaci´on, Instituto Polit´ecnico Nacional, Ciudad de M´exico, M´exico
24
Department of Physics, Pennsylvania State University, University Park, PA, USA
25
Physics Department, Centro de Investigaci´on y de Estudios Avanzados del IPN, Ciudad de M´exi co, DF, M´exico
26
Universidad Aut´onoma del Estado de Hi dalgo, Pachuca, M´exico
27
Instituto de Ciencias Nucleares, Universidad Nacional Aut´onoma de M´exico, Ciudad de M´exico, M´exico
Corresponding author: John Pretz
john.pretz@gmail.com
2 Albert et al.
28
Santa Cruz Institute for Particle Physics, U niversity of California, Santa Cruz, Santa Cruz, CA, USA
29
Department of Physics and Astronomy, University of California, Irvine, Irvine, CA, USA
(Received January 6, 2017; Revised; Accepted)
Submitted to ApJ
ABSTRACT
The Crab Nebula is the brightest TeV gamma-ray source in the sky and has been used for the past 25 years as
a reference source in TeV astronomy, for calibration and verification of new TeV instruments. The High Altitude
Water C he renkov Observatory (HAWC), completed in early 2015, has be en used to observe the Crab Nebula at high
significance across nearly the full spectrum of energies to which HAWC is sensitive. HAWC is unique for its wide
field-of-view, nearly 2 sr at any instant, and its high-energy reach, up to 100 TeV. HAWC’s sensitivity improves with
the gamma-ray energy. Above ∼1 TeV the sensitivity is driven by the best background rejection and angular resolution
ever achieved for a wide-field ground a rray.
We pres ent a time-integrated analysis of the Crab using 507 live days of HAWC data from 2014 November to 2016
June. The spec trum of the Crab is fit to a function of the form φ(E) = φ
0
(E/E
0
)
−α−β·ln(E/E
0
)
. The data is well-fit
with values of α = 2.63 ± 0.03, β = 0.15 ± 0.03, and log
10
(φ
0
cm
2
s TeV) = −12 .60 ± 0.02 when E
0
is fixed at 7 TeV
and the fit applies between 1 and 37 TeV. Study of the systematic errors in this HAWC measurement is discussed and
estimated to be ± 50% in the photon flux between 1 and 37 TeV.
Confirmation of the Crab flux se rves to establish the HAWC instrument’s sensitivity for surveys of the sky. The
HAWC survey will exceed s e nsitivity of current-generation observatories and open a ne w view of 2/3 of the sky above
10 TeV.
Keywords: gamma rays: observations — surveys — acceleration of particles — pulsars: indiv idual
(Sn-1054) — I SM: individua l (Crab Nebula)
Observation of the Crab N ebul a with HAWC 3
1. INTRODUCTION
The Crab Pulsar Wind Nebula (the Crab Nebula or the Crab) occupies a place of special distinction in the history
of high-energy astr ophysics. It was the first high-confidence TeV detection in 1 989 using the Whipple telescope
(
Weekes et al. 1 989) and is the brightest stea dy s ource in the Northern sky a bove 1 TeV. It has been observed with
imaging atmospheric Cherenkov telescopes (IACTs) sinc e (
Ta nimori et al. 1998; Aharonian et al. 2004, 2006; Celik
2008
; Aleksi´c et al. 2015). The first observation using a ground array was the 2003 Milagro detection (Atkins et al.
2003
), and the signal was subsequently seen in other ground arrays (Amenomori et al. 2009; Bartoli et al. 2015).
The TeV emission arises from inverse-Compton (IC) up-sca ttering of low-energy ambient photons by energetic
electrons accelerated in shocks surrounding the central pulsa r (
Atoyan & Ahar onian 1996). Photons from synchrotron
emission of the electrons themselves are likely the dominant IC target with sub-dominant contributions fro m the cosmic
microwave background and the extragalactic background light (
Mart´ın et al. 2012). Despite rare flaring emission below
1 TeV (
Tavani et al. 2011; Abdo et al. 2011), and a potential TeV flare (Aielli et al. 2 010), the Crab is generally believed
to be steady at higher energies (
Abramowski et al. 2014; Aliu et al. 2014; Bartoli et a l. 2015). Co nsequently, the Crab
Nebula has been adopted as the reference source in TeV astro nomy and is a re liable be am of high-energy photons to
use for calibrating and understanding new TeV gamma-ray instruments.
The High Altitude Water Cherenkov (HAWC) obser vatory is a new ins trument sensitive to multi-TeV hadron and
gamma-ray a ir showers, operating at latitude of +19
◦
N at an altitude of 4,100 meters in the Sierra Negra, Mexico.
HAWC consists of a large 22,000 m
2
area densely covered with 300 Water Cherenkov Detecto rs (WCDs), of which
294 have been instrumented. Each WCD consists of a 7.3-meter diameter, 5-meter tall steel tank lined with a plastic
bladder and filled with purified water. Figure
1 shows a schematic of the WCD and an overhead view of the full
instrument. At the bottom of each WCD, three 8-inch Hamamatsu R5912 photomultiplier tubes (PMTs) are anchored
in an equilateral triangle of side length 3.2 meters, with one 10-inch high-quantum efficiency Hamamatsu R7081 PMT
anchored at the center.
A high-energy photon impinging on the atmosphere ab ove HAWC initiates an extensive electromagnetic air shower.
The resulting mix of relativistic electrons, pos itrons and gamma rays propagates to the ground in a thin tortilla of
particles at nearly the speed of light. Energetic particles that reach the instrument can interact in the water and
produce optical light via Cherenkov radiation. The high altitude of HAWC sets the scale for the photo n energy that
can be detected. At HAWC’s altitude, the shower from a 1 TeV photon from directly overhead will have about 7% of
the original photon energy left when the shower reaches the ground. The fra ction of energy reaching the ground rises
to ∼28% at 100 TeV. The detector is fully efficient to g amma rays with a primary ener gy above ∼1 TeV. Lower-energy
photons can be detected when they fluctuate to interact deeper in the atmos phere tha n typical.
The voltages on the HAWC PMTs are chosen to match the PMT gains across the array. PMT pulses are amplified,
shaped, and passed through two discriminators at approximately 1/4 and 4 PEs (
Abeyseka ra et al. 2016) and digitized.
The length of time that PMT pulses spend above these thresholds (time-over-threshold o r ToT) is used to estimate
the total amount of charge collected in the PMT. Noise arises from a number of sources including PMT afterpulsing,
fragments of sub-threshold air showers, PMT dark no ise, and other sources. The 8-inch PMTs have a hit rate (a hit
being each time the PMT signal c rosses the 1/4 PE threshold) due to the combined effect of these sources of 20–30
kHz a nd the 10-inch PMTs have a hit rate of 40 –50 kHz.
The data from the front-end electronics is digitized with commercial time-to-digital converters (TDCs) and passed
to a farm of computers for real-time triggering and processing. Events are preserved by the computer farm if they pass
the trigger condition: a simple multiplicity trigger, requiring some number, N
thresh
, PMTs hit within 150 ns. Hits 500
ns prior to a trigger and up to 1000 ns after a trigger are also saved for reconstruction. During the operation of HAWC,
N
thresh
has varied between 20–50. The trigger rate at the time of writing, due primarily to hadronic cosmic-ray air
showers, is ∼24 kHz with N
thresh
= 28.
The reconstruction process involves determining the direction, the likelihood for the event to be a photon, and
the e vent’s size. A first-look r econstruction is applied at the HAWC site. In this analysis, all the data has been
reconstructed again (the fourth revisio n, or Pass 4, of the reco nstruction process) off-site in orde r to have a unifo rm
dataset and the best calibrations available. The chief background to gamma-ray observation is the abundant hadronic
cosmic-ray populatio n. Individual gamma-ray-induced air s howers can be disting uished from cosmic-ray showers by
their topology and the pr e sence of deeply penetrating particles at the ground.
The strength of HAWC over the IACT technique is that photon showers may be detected acros s the entire ∼ 2 sr
field-of-view of the instrument, day or night, regardless of weather conditions. As such, HAWC is uniquely suited to
4 Albert et al.
−50 0 50 100
x [meter]
150
200
250
300
350
y [meter]
Figure 1. The left pan el shows a schematic of a single HAWC WCD including the steel tank, the covering roof, the th ree 8-inch
Hamamatsu R5912 PMTs, and one 10-inch Hamamatsu R7081-MOD PMT. The tanks are 7.5 meters diameter and 5 meters
high. Water is filled to a d epth of 4.5 meters with 4.0 meters of water above each PMT. The right p an el shows the layout of
the completed HAWC instrument, covering 22,000 m
2
. The location of each WCD is indicated by a large circle and PMTs are
indicated with smaller circles. The gap in the center hosts a building with the data acquisition system.
study the long-duratio n light curve of objects and to search for flaring sources in real time. Additionally, since sources
are obser ved on every transit, HAWC obtains thousands of hours of exposure on each source, greatly improving the
sensitivity to the highest-energ y photons.
Section
2 outlines the algorithms by which the direction, size, and type (photon or hadron) of each shower is
determined. Section
3 describes the identification of the gamma-ray signal from the Crab Nebula. T he fit to the Crab
energy spectrum, inc luding a treatment of systematic errors, is described in Section
4. Finally, a discussion of the result
is presented in Section
5, including a compariso n to prior spec tra measured by peer experiments and a computation of
the sensitivity of the HAWC instrument, anchored in the agree ment of the HAWC measurement to other experiments.
2. AIR SHOWER RECO NSTRUCTION
Events from the detector are reconstruc ted to determine the arrival dir ection of the primary particle and the size
of the resulting air shower on the ground, a proxy for the primary particle’s energy. Table
1 summarizes the steps in
reconstruction of HAWC events as explained below.
To illustrate the event reconstruction, Figure
2 shows a s trong gamma-ray candidate from the Crab Nebula. In
Section
2.1, the simulation is briefly described as it is key to evaluating the recons truction proc ess. Section 2.2
describes the calibra tion, by which the time and light level in individual PMTs are determined. Section 2.3 discusses
the selection of PMT signals for reconstr uc tion and the event size measurement. The directio n reconstruction occurs
in two steps, first the core recons truction, described in Sectio n
2.4, and then the direction determination, described
in Section 2.5. The air shower core, the de nse concentration of particles a long the direction of the original primary, is
needed to ma ke the best reconstruction of the air shower’s direction since the air shower arrival front is delayed from