DISCOVERY OF VERY HIGH ENERGY gamma-RAYS FROM THE BLAZAR S5 0716+714

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arXiv:0907.2386v2 [astro-ph.CO] 22 Sep 2009
Discovery of Very High Energy γ-rays from the blazar
S5 0716+714
H. Anderhuba, L. A. Antonellib, P. Antoranzc, M. Backesd, C. Baixerase, S. Balestrac,
J. A. Barrioc, D. Bastierif, J. Becerra Gonz´ alezg, J. K. Beckerd, W. Bednarekh,
A. Berdyuginu, K. Bergerh, E. Bernardinii, A. Bilanda, R. K. Bockj,f, G. Bonnolik,
P. Bordasl, D. Borla Tridonj, V. Bosch-Ramonl, D. Bosec, I. Brauna, T. Bretzm,
D. Britzgerj, M. Camarac, E. Carmonaj, A. Carosib, P. Colinj, S. Commichaua,
J. L. Contrerasc, J. Cortinan, M. T. Costadog,o, S. Covinob, F. Dazzip,∗, A. De Angelisp,
E. de Cea del Pozoq, R. De los Reyesc, B. De Lottop, M. De Mariap, F. De Sabatap,
C. Delgado Mendezg,∗∗, A. Dom´ ınguezr, D. Dominis Presters, D. Dornera, M. Dorof,
D. Elsaesserm, M. Errandon, D. Ferenct, E. Fern´ andezn, R. Firpon, M. V. Fonsecac,
L. Fonte, N. Galantej, R. J. Garc´ ıa L´ opezg,o, M. Garczarczykn, M. Gaugg, N. Godinovics,
F. Goebelj,∗∗∗, D. Hadasche, A. Herrerog,o, D. Hildebranda, D. H¨ ohne-M¨ onchm, J. Hosej,
D. Hrupecs, C. C. Hsuj, T. Joglerj, S. Klepsern, D. Kranicha, A. La Barberab, A. Laillet,
E. Leonardok, E. Lindforsu,∗∗∗∗, S. Lombardif, F. Longop, M. L´ opezf, E. Lorenza,j,
P. Majumdari, G. Manevav, N. Mankuzhiyilp, K. Mannheimm, L. Maraschib, M. Mariottif,
M. Mart´ ınezn, D. Mazinn,∗∗∗∗, M. Meuccik, J. M. Mirandac, R. Mirzoyanj, H. Miyamotoj,
J. Mold´ onl, M. Molesr, A. Moralejon, D. Nietoc, K. Nilssonu, J. Ninkovicj, R. Oritoj,
I. Oyac, R. Paolettik, J. M. Paredesl, M. Pasanenu, D. Pascolif, F. Paussa, R. G. Pegnak,
M. A. Perez-Torresr, M. Persicp,w, L. Peruzzof, F. Pradar, E. Prandinif, N. Puchadesn,
I. Puljaks, I. Reichardtn, W. Rhoded, M. Rib´ ol, J. Ricox,n, M. Rissia, A. Roberte,
S. R¨ ugamerm, A. Saggionf, J. Sainiou, T. Y. Saitoj, M. Salvatib, M. S´ anchez-Conder,
K. Sataleckai, V. Scalzottof, V. Scapinp, T. Schweizerj, M. Shaydukj, S. N. Shorey,
A. Sierpowska-Bartosikh, A. Sillanp¨ a¨ au, J. Sitarekj,h, D. Sobczynskah, F. Spanierm,
S. Spirob, A. Stamerrak, L. S. Starka, T. Surics, L. Takalou, F. Tavecchiob, P. Temnikovv,
D. Tescaron, M. Teshimaj, D. F. Torresx,q, N. Turinik, H. Vankovv, R. M. Wagnerj,
C. Villforthu, V. Zabalzal, F. Zandanelr, R. Zaninn, J. Zapateroe

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ABSTRACT
aETH Zurich, CH-8093 Switzerland
bINAF National Institute for Astrophysics, I-00136 Rome, Italy
cUniversidad Complutense, E-28040 Madrid, Spain
dTechnische Universit¨ at Dortmund, D-44221 Dortmund, Germany
eUniversitat Aut` onoma de Barcelona, E-08193 Bellaterra, Spain
fUniversit` a di Padova and INFN, I-35131 Padova, Italy
gInst. de Astrof´ ısica de Canarias, E-38200 La Laguna, Tenerife, Spain
hUniversity of ? L´ od´ z, PL-90236 Lodz, Poland
iDeutsches Elektronen-Synchrotron (DESY), D-15738 Zeuthen, Germany
jMax-Planck-Institut f¨ ur Physik, D-80805 M¨ unchen, Germany
kUniversit` a di Siena, and INFN Pisa, I-53100 Siena, Italy
lUniversitat de Barcelona (ICC/IEEC), E-08028 Barcelona, Spain
mUniversit¨ at W¨ urzburg, D-97074 W¨ urzburg, Germany
nIFAE, Edifici Cn., Campus UAB, E-08193 Bellaterra, Spain
oDepto. de Astrof´ ısica, Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain
pUniversit` a di Udine, and INFN Trieste, I-33100 Udine, Italy
qInstitut de Ci` encies de l’Espai (IEEC-CSIC), E-08193 Bellaterra, Spain
rInst. de Astrof´ ısica de Andaluc´ ıa (CSIC), E-18080 Granada, Spain
sRudjer Boskovic Institute, Bijenicka 54, HR-10000 Zagreb, Croatia
tUniversity of California, Davis, CA-95616-8677, USA
uTuorla Observatory, University of Turku, FI-21500 Piikki¨ o, Finland
vInst. for Nucl. Research and Nucl. Energy, BG-1784 Sofia, Bulgaria
wINAF/Osservatorio Astronomico and INFN, I-34143 Trieste, Italy
xICREA, E-08010 Barcelona, Spain
yUniversit` a di Pisa, and INFN Pisa, I-56126 Pisa, Italy
*supported by INFN Padova
**now at: Centro de Investigaciones Energ´ eticas, Medioambientales y Tecnol´ ogicas
***deceased
****Send off-print requests to Elina Lindfors elilin@utu.fi and Daniel Mazin mazin@ifae.es

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The MAGIC collaboration reports the detection of the blazar S5 0716+714
(z = 0.31 ± 0.08) in very high energy gamma-rays. The observations were per-
formed in November 2007 and in April 2008, and were triggered by the KVA
telescope due to the high optical state of the object. An overall significance
of the signal accounts to S = 5.8σ for 13.1 hours of data. Most of the signal
(S = 6.9σ) comes from the April 2008 data sample during a higher optical state
of the object suggesting a possible correlation between the VHE γ-ray and optical
emissions. The differential energy spectrum of the 2008 data sample follows a
power law with a photon index of Γ = 3.45±0.54stat±0.2syst, and the integral
flux above 400GeV is at the level of (7.5 ± 2.2stat± 2.3syst) × 10−12cm−2s−1,
corresponding to a 9% Crab Nebula flux. Modeling of the broad band spectral
energy distribution indicates that a structured jet model appears to be more
promising in describing the available data than a simple one zone synchrotron
self-Compton model.
Subject headings: gamma rays: observations — BL Lacertae objects: individual
(S5 0716+714)
1.Introduction
Blazars, a common term used for flat spectrum radio quasars (FSRQ) and BL Lacertae
objects, appear to be the most energetic types of Active Galactic Nuclei (AGN). In these
objects the dominant radiation component originates in a relativistic jet pointed nearly
towards the observer. The double-peaked spectral energy distribution (SED) of blazars is
attributed to a population of relativistic electrons spiraling in the magnetic field of the jet.
The low energy peak is due to synchrotron emission and the second, high energy peak is often
attributed to inverse Compton scattering of low energy photons in leptonic emission models
(Maraschi et al. 1992; Dermer & Schlickeiser 1993; Bloom & Marscher 1996). Models based
on the acceleration of hadrons can also sufficiently describe the observed SEDs and light
curves (Mannheim 1993; M¨ ucke et al. 2003). For most FSRQs and a large fraction of BL
Lacertae objects (namely LBLs1) the low energy peak is located in the energy range between
submillimeter and optical. On the other hand, for most of the sources detected to emit
VHE γ-rays (HBLs2) the low energy peak is located at UV to X-rays energies (Padovani
2007). The high energy peak is typically at MeV–GeV energies. Blazars are highly variable
1LBL = low frequency peaking BL Lacertae
2HBL = high frequency peaking BL Lac

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in all wavebands and the relation between variability in different bands is a key element in
discriminating between different models.
The MAGIC Collaboration is performing Target of Opportunity observations of sources
in a high flux state in the optical and/or X-ray band. Optically triggered observations have
resulted in the discovery of VHE γ-rays from Mrk 180 (Albert et al. 2006) and 1ES 1011+496
(Albert et al. 2007a). In this paper we report the results of observations of S5 0716+714 in
November 2007 and April 2008. The observation at the latter date resulted in the discovery
of VHE γ-rays from the source as announced in Teshima et al. (2008).
The BL Lac object S5 0716+714 has been studied intensively at all frequency bands. It
is highly variable with rapid variations observed from the radio to X-ray bands (Wagner et al.
1996). It has therefore been target to several multiwavelength campaigns, the most recent
one organized by the GLAST-AGILE Support Program in July-November 2007 (Villata et al.
2008; Giommi et al. 2008a). Due to the very bright nucleus, which outshines the host galaxy,
the redshift of S5 0716+714 is still uncertain. The recent photometric detection of the host
galaxy (Nilsson et al. 2008) suggests a redshift of z = 0.31 ± 0.08 which is consistent with
the redshift z = 0.26 determined by spectroscopy for three galaxies close to the location
of S5 0716+714 (Stickel et al. 1993). In the SED of S5 0716+714 the synchrotron peak is
located in the optical band and is, therefore, classified either as LBL (Nieppola et al. 2006)
or as IBL3(Padovani et al. 1995)
S5 0716+714 was detected in the MeV energy range several times at different flux levels
by the EGRET detector on board the Compton Gamma-ray Observatory (Hartman et al.
1999). In 2008 AGILE reported the detection of a variable γ-ray flux with a peak flux
density above the maximum reported from EGRET (Chen et al. 2008). S5 0716+714 is also
on the Fermi-LAT bright source list (Abdo et al. 2009). Observations at VHE γ-ray energies
by HEGRA resulted in an upper limit of F(> 1.6TeV) = 3.13 × 10−12photons/cm2/s
(Aharonian et al. 2004). In this paper we present the first detection of VHE γ-rays from
S5 0716+714. It is the third optically triggered discovery of a VHE γ-ray emitting blazar
by MAGIC.
2.Observations
The MAGIC (Major Atmospheric Gamma-ray Cherenkov) telescope is a standalone
imaging atmospheric Cherenkov telescope located on the Canary Island of La Palma. MAGIC
3IBL = intermediate frequency peaking BL Lacertae

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has a standard trigger threshold of 60GeV for observations at low zenith angles, an angular
resolution of ∼ 0.1◦on the event by event basis and an energy resolution above 150GeV of
∼ 25% (see Albert et al. 2008a, for details).
The Tuorla blazar monitoring program4(Takalo et al. 2007) monitors S5 0716+714
on a nightly basis using the KVA 35cm telescope at La Palma5and the Tuorla 1meter
telescope. At the end of October 2007 (22th, MJD=54395) the optical flux had more than
doubled (from 19mJy to 42mJy) in less than a month and, according to a predefined criteria
MAGIC was alerted. Due to moon and weather constraints, the MAGIC observations started
11 days later, when the optical flux had already decreased significantly (see Fig. 1). The
observations were performed in the wobble mode (Daum et al. 1997) pointing 0.4 deg offset
from the source to enable simultaneous estimation of the background from the same field of
view. MAGIC observed the source during 14 nights for a total of 16.8 hours. During some
nights the observing conditions were rather poor and the affected data were rejected from
the analysis. The exposure time for good quality data amounts to 10.3 hours. The zenith
angle range of these observations was from 42 to 46 degrees.
In April 2008 a new bright and fast optical flare occurred. The optical flux almost
doubled within three nights (14th of April, MJD=54570: 29 mJy, 17th April: 52 mJy), and
at 17th of April reached its historical maximum value. MAGIC started the observations 5
nights later, when the moon conditions allowed. The source was observed during 9 nights
with zenith angles from 47 to 55 degrees for a total of 7.1 hours. Unfortunately, during
the last 6 nights of the observations there was strong calima wind carrying fine sand from
Sahara desert and these data were, therefore, of bad quality and rejected from the analysis.
The total exposure time of good quality data for this observation period amounts to only
2.8 hours. The total live time of S5 0716+714 MAGIC observations in 2007 and 2008 after
data quality cuts was 13.1 hours.
3.Data Analysis and Results
The MAGIC data were analyzed using the standard analysis chain as described in
Albert et al. (2008a,b); Aliu et al. (2009). In order to suppress the background showers
produced by charged cosmic rays, a multivariate classification method known as Random
Forest is used (Albert et al. 2008c). For every event, the algorithm takes as input a set of
4http://users.utu.fi/kani/1m
5http://tur3.tur.iac.es/

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5441054420
]
-1
s
-2
F (>400GeV) [cm
-10
0
10
-12
10
×
]
-1
s
-2
[cm
-12
10
×
0.7)
±
= (0.8
2007
F
545785458054582
]
-1
s
-2
[cm
-12
10
×
2.2)
±
= (7.5
2008
F
2008
Time [mjd]
5440054450545005455054600
F [mJy]
0
20
40
60
Time [mjd]
MAGIC
KVA, R-band
2007
Fig. 1.— The light curve of S5 0716+714 as measured from November 2007 until April 2008.
The day-by-day γ-ray light curve from MAGIC is shown in the upper panel for the 2007
data (left panel) and 2008 data (right panel), whereas the optical KVA data are shown in
the lower panel. The simultaneous optical with the MAGIC data are marked black. The
error bars (1σ) of the optical fluxes are smaller than the points and thus not visible.
image parameters, and produces one single parameter as output, called Hadronness. The
background rejection is then achieved by a cut in Hadronness, which was optimized using
Crab Nebula data taken under comparable conditions.
The cut in |Alpha| that defines the signal region was also optimized in the same way. An
additional cut removed the events with a total charge of less than 200 photoelectrons (phe)
in order to assure a better background rejection. We used standard cuts in Hadronness
and |Alpha|, which are determined to give the best significance for a point-like source with
a flux on the 10% level of the Crab Nebula flux. For the given cuts and the relatively
large zenith angles of the observations the analysis threshold corresponds to 400GeV. The
resulting |Alpha| distribution after all cuts for the overall S5 0716+714 data sample in

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[deg]
α
0 1020 30405060708090
# entries / 2 [deg]
0
100
200
300
400
500
S5 0716+714, MAGIC 2007 - 2008
Observation time: T = 13.1 hrs
events: 1548
on
N
events (scaled with 0.33): 1295.7
off
N
Excess events: 252
39
±
20.8
±
43.5
±
σ
Significance: S = 5.8
ON source
OFF source
Fig. 2.— |Alpha| distribution after all cuts for the total MAGIC data sample in 2007 –
2008. A γ-ray excess with a significance of 5.8σ is found.
2007 – 2008 is shown in Fig. 2. An overall excess of 252 γ-like events corresponding to a
significance of S = 5.8σ was found (following Eq. 17 in Li & Ma (1983), Non= 1548, Noff
= 3887, alpha = 0.33). Most of the signal comes from the 2008 data sample: the analysis of
the 2008 data alone results in 176 excess events over 422 background events corresponding
to S = 6.9σ. From the 2007 data alone an excess corresponding to S = 2.2σ was found.
The day-by-day light curve as measured by MAGIC data is shown in Fig. 1 (upper panel)
together with the optical KVA light curve (lower panel). In November 2007 the MAGIC flux
above 400GeV is at F2007(> 0.4TeV) = (0.8 ± 0.7stat± 0.2syst) × 10−11[cm−2s−1], whereas
the flux is about 9 times higher in 2008: F2008(> 0.4TeV) = (7.5 ± 2.2stat± 2.3syst) ×
10−11[cm−2s−1]. No significant variability is seen on time scales shorter than 6 months.
Given the limited effective exposure times these observations are not sensitive to variability
on shorter time scales. The individual MAGIC points would have low significance, and an
intra-night variability by a factor of at least ten would have been required to detect it. In
the optical band, instead, a clear variability on time scales from days to months is visible
with two distinct flares: the first in October 2007, and the second in April 2008 (Fig. 1,

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lower panel).
The differential energy spectrum is calculated only for the April 2008 data set. The
measured and unfolded for detector effects (Albert et al. 2007c) spectrum is shown in Fig. 3.
The data point at E = 1.3TeV has a significance below 1σ and was, therefore, converted
into an upper limit corresponding to a 95% confidence level. The measured spectrum can
be well fitted by a simple power law (with the differential flux given in units of TeV−1cm−2
s−1):
dN
dE dAdt= (2.4 ± 0.8) × 10−11(E/500GeV)−3.5±0.5
The errors are statistical only. The systematic uncertainties are estimated to be 0.2 on the
photon index and 30% on the absolute flux level. Due to the energy-dependent attenua-
tion of VHE γ-rays with low-energy photons of the extragalactic background light (EBL,
Gould & Schr´ eder 1967), the VHE γ-ray flux of distant sources is significantly suppressed.
We calculated the deabsorbed, i.e. intrinsic, spectrum of S5 0716+714 using an EBL model
of Franceschini et al. (2008) and assuming a redshift of z = 0.26. The resulting intrinsic
spectrum (shown in Fig. 3, red points) has a fitted photon index of Γ = 1.8 ± 0.6, which is
well within the range of other extragalactic sources measured so far.
(1)
As the source redshift is still uncertain, we used the MAGIC spectra to calculate upper
limits to the redshift. We assumed two different maximum values for a possible hardness of
the intrinsic spectrum: 1.5, being a canonical value for a γ-ray spectrum emitted by electrons
with a spectral index of 2.0; and 0.666, being the limiting case for a γ-ray spectrum emitted
by a monoenergetic electron distribution. Using the method described in Mazin & Goebel
(2007) we obtain the following upper limits for the redshift: z < 0.46 (for an assumed
intrinsic spectrum with a power law photon index of 1.5) and z < 0.59 (for an assumed
intrinsic spectrum with a power law photon index of 2/3). Both limits agree with the redshift
determined from the host galaxy detection (z = 0.31±0.08) and from the spectroscopy of 3
nearby galaxies (z = 0.26).
4.Discussion
MAGIC observed the blazar S5 0716+714 in November 2007 and April 2008, the obser-
vations resulting in the discovery of a very high energy γ-ray excess with a significance of 5.8σ.
During the November 2007 MAGIC observations the average optical flux was ∼ 20mJy, while
in April 2008 the optical flux was ∼ 45mJy. The same trend is also visible in the MAGIC
data: the flux in April 2008 is significantly higher than in November 2007. This seems
to support the indication seen in previous MAGIC observations for other BL Lac objects

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Albert et al. (2006, 2007a,b), that there is a connection between optical high states and VHE
γ-ray high states.
In April 2008 S5 0716+714 was also in a historical high state in X-rays (Giommi et al.
2008b) and the optical polarization angle started to rotate immediately after the optical
maximum had been reached (Larionov et al. 2008). However, the radio flux at 37 GHz did
remain at a quiescent level (A. L¨ ahteenm¨ aki, priv.comm). This energy dependent behavior
is very similar to the one seen in BL Lac in 2005, which was attributed to an emission feature
moving in a helical path upstream of the VLBA core (Marscher et al. 2008). The high VHE
γ-ray flux of S5 0716+714 observed by MAGIC might originate from such a moving emission
feature, but a more detailed study of the multiwavelength light curves from spring 2008 are
needed to confirm this.
We tried modeling the SED with a one-zone synchrotron self Compton (SSC) model
with relativistic electrons following a smoothed broken power law energy distribution (see
Tavecchio et al. (2001) for a full description). The fit parameters are: the minimum electron
energy γmin= 104, the break energy in the electron spectrum γb= 1.9 · 104, the maximum
electrom energy γmax = 7 · 105, the electron spectral index below and above the break
energy n1 = 2, n2 = 4.5, respectively, the magnetic field B = 0.13G, the normalization
factor K = 1.4 · 105particles/cm3, the radius of the emitting region R = 7 · 1015cm and
the Doppler factor δ = 30. To account for the observed TeV flux corrected for intergalactic
absorption the model (continuous line in Fig. 4) predicts a high intensity peak in the 10GeV
range. Although the source was flaring in optical at the time of the MAGIC detection,
such high γ-ray flux (more than 10 times larger than observed by EGRET and Fermi, open
blue bow-tie in Fig. 4) appears somewhat implausible. Therefore, we also considered the
emission from a structured jet, modeled as a fast ”spine” surrounded by a slower moving
”layer” (Ghisellini et al. 2005). Photons emitted by the layer are subject to IC scattering
by the relativistic electrons of the spine thus introducing an additional contribution to the
IC spectrum at energies higher than SSC can reach. The computed model is shown as a
dashed line in Fig. 4. The GeV excess is substantially reduced and the spectral shape in
the γ-ray domain is consistent with the spectral indices measured at both GeV and TeV
energies, though with presumably different intensity states.
The discovery of high flux of VHE γ-rays by MAGIC from the blazar S5 0716+714
in April 2008 should be compared to detailed light curves from other wavelengths (radio,
optical and X-rays) as well as VLBA maps and optical polarization light curve in order to
further investigate the origin of this interesting event.

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Acknowledgement
We would like to thank the Instituto de Astrofisica de Canarias for the excellent work-
ing conditions at the Observatorio del Roque de los Muchachos in La Palma. The support
of the German BMBF and MPG, the Italian INFN and Spanish MICINN is gratefully ac-
knowledged. This work was also supported by ETH Research Grant TH 34/043, by the
Polish MNiSzW Grant N N203 390834, and by the YIP of the Helmholtz Gemeinschaft.
The authors would also like to thank the anonymous referee for the valuable comments and
suggestions, which help to improve the paper.
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This preprint was prepared with the AAS LATEX macros v5.2.

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Energy, E [GeV]
100
200300400
1000
2000
]
-1
s
-2
cm
-1
differential flux, dN/dE [TeV
-13
10
-12
10
-11
10
-10
10
-9
10
-8
10
-7
10
MAGIC, S5 0716+714
power law fit:
Γ
-
500GeV
10
×
0.8)
0.54
E

0
dN/dE = N
]
-1
s
-2
cm
-1
[TeV
-11
±
±
= (2.4
= 3.45
0
N
Γ
S5 0716+714, measured
Power law fit to data
S5 0716+714, deabsorbed
Crab Nebula spectrum
Fig. 3.— The differential energy spectrum of S5 0716+714. Only data from April 2008
are used. The measured (black points) as well as deabsorbed (using the EBL model of
Franceschini et al. (2008) and assuming z=0.26, red points) spectra are shown. The results
from a power law fit to the measured spectrum are shown in the plot. The Crab Nebula
spectrum is shown for comparison (green dashed line).

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Fig. 4.— The broad band SED of S5 0716+714. In red is the KVA (from 23 april 2008),
Swift/UVOT and Swift/XRT (from 29 April 2008) and the deabsorped MAGIC (from 22 to
24 April 2008) data. The MAGIC measured flux for April 2008 is shown in black and the
MAGIC upperlimit for November 2007 is shown with grey arrow. The grey symbols show
historical data (see Tavecchio & Ghisellini 2009). The bow-tie shows the flux from the Fermi
bright list. The solid line shows the overall emission calculated with a one-zone SSC model
using the April 2008 data (red), see text for model parameters. The dashed line shows the
emission calculated with the spine-layer model described in Tavecchio & Ghisellini (2009).
We assume the same parameters reported there, but a volume three times larger.