A SEARCH FOR SPECTRAL HYSTERESIS AND ENERGY-DEPENDENT TIME LAGS
FROM X-RAY AND TeV GAMMA-RAY OBSERVATIONS OF Mrk 421
A. U. Abeysekara
1
, S. Archambault
2
, A. Archer
3
, W. Benbow
4
, R. Bird
5
, M. Buchovecky
5
, J. H. Buckley
3
, V. Bugaev
3
,
J. V Cardenzana
6
, M. Cerruti
4
, X. Chen
7,8
, L. Ciupik
9
, M. P. Connolly
10
,W.Cui
11,12
, J. D. Eisch
6
, A. Falcone
13
, Q. Feng
2
,
J. P. Finley
11
, H. Fleischhack
8
, A. Flinders
1
, L. Fortson
14
, A. Furniss
15
, S. Griffin
2
, N. Håkansson
7
, D. Hanna
2
,
O. Hervet
16
, J. Holder
17
, T. B. Humensky
18
, M. Hütten
8
, P. Kaaret
19
,P.Kar
1
, M. Kertzman
20
, D. Kieda
1
, M. Krause
8
,
S. Kumar
17
, M. J. Lang
10
, G. Maier
8
, S. McArthur
11
, A. McCann
2
, K. Meagher
21
, P. Moriarty
10
, R. Mukherjee
22
,
D. Nieto
18
,S.O’Brien
24
, R. A. Ong
5
, A. N. Otte
21
, N. Park
23
, V. Pelassa
4
, M. Pohl
7,8
, A. Popkow
5
, E. Pueschel
24
,
K. Ragan
2
, P. T. Reynolds
25
, G. T. Richards
21
, E. Roache
4
, I. Sadeh
8
, M. Santander
22
, G. H. Sembroski
11
, K. Shahinyan
14
,
D. Staszak
23
, I. Telezhinsky
7,8
, J. V. Tucci
11
, J. Tyler
2
, S. P. Wakely
23
, A. Weinstein
6
, A. Wilhelm
7,8
, D. A. Williams
16
(the VERITAS Collaboration),
and
M. L. Ahnen
26
, S. Ansoldi
27,56
, L. A. Antonelli
28
, P. Antoranz
29
, C. Arcaro
30
, A. Babic
31
, B. Banerjee
32
, P. Bangale
33
,
U. Barres de Almeida
33,57
, J. A. Barrio
34
, J. Becerra González
35,36,58
, W. Bednarek
37
, E. Bernardini
38,39
, A. Berti
27,59
,
B. Biasuzzi
27
, A. Biland
26
, O. Blanch
40
, S. Bonnefoy
34
, G. Bonnoli
29
, F. Borracci
33
, T. Bretz
41,60
, R. Carosi
29
,
A. Carosi
28
, A. Chatterjee
32
, P. Colin
33
, E. Colombo
35,36
, J. L. Contreras
34
, J. Cortina
40
, S. Covino
28
, P. Cumani
40
,
P. Da Vela
29
, F. Dazzi
33
, A. De Angelis
30
, B. De Lotto
27
, E. de Oña Wilhelmi
42
, F. Di Pierro
28
, M. Doert
43
,
A. Domínguez
34
, D. Dominis Prester
31
, D. Dorner
41
, M. Doro
30
, S. Einecke
43
, D. Eisenacher Glawion
41
, D. Elsaesser
43
,
M. Engelkemeier
43
, V. Fallah Ramazani
44
, A. Fernández-Barral
40
, D. Fidalgo
34
, M. V. Fonseca
34
, L. Font
45
,
C. Fruck
33
, D. Galindo
46
, R. J. García LÓpez
35,36
, M. Garczarczyk
38
, M. Gaug
45
, P. Giammaria
28
, N. GodinoviĆ
31
,
D. Gora
38
, D. Guberman
40
, D. Hadasch
47
, A. Hahn
33
, T. Hassan
40
, M. Hayashida
47
, J. Herrera
35,36
, J. Hose
33
, D. Hrupec
31
,
G. Hughes
26
, W. Idec
37
, K. Kodani
47
, Y. Konno
47
, H. Kubo
47
, J. Kushida
47
, D. Lelas
31
, E. Lindfors
44
, S. Lombardi
28
,
F. Longo
27,59
,M.LÓpez
34
,R.LÓpez-Coto
40,61
, P. Majumdar
32
, M. Makariev
48
, K. Mallot
38
, G. Maneva
48
,
M. Manganaro
35,36
, K. Mannheim
41
, L. Maraschi
28
, B. Marcote
46
, M. Mariotti
30
, M. Martínez
40
, D. Mazin
33,62
,
U. Menzel
33
, R. Mirzoyan
33
, A. Moralejo
40
, E. Moretti
33
, D. Nakajima
47
, V. Neustroev
44
, A. Niedzwiecki
37
,
M. Nievas Rosillo
34
, K. Nilsson
44,63
, K. Nishijima
47
, K. Noda
33
, L. Nogués
40
, M. Nöthe
43
, S. Paiano
30
, J. Palacio
40
,
M. Palatiello
27
, D. Paneque
33
, R. Paoletti
29
, J. M. Paredes
46
, X. Paredes-Fortuny
46
, G. Pedaletti
38
, M. Peresano
27
,
L. Perri
28
, M. Persic
27,64
, J. Poutanen
44
, P. G. Prada Moroni
49
, E. Prandini
30
, I. Puljak
31
, J. R. Garcia
33
, I. Reichardt
30
,
W. Rhode
43
, M. RibÓ
46
, J. Rico
40
, T. Saito
47
, K. Satalecka
38
, S. Schroeder
43
, T. Schweizer
33
, S. N. Shore
49
,
A. Sillanpää
44
, J. Sitarek
37
, I. Snidaric
31
, D. Sobczynska
37
, A. Stamerra
28
, M. Strzys
33
, T. SuriĆ
31
, L. Takalo
44
,
F. Tavecchio
28
, P. Temnikov
48
, T. TerziĆ
31
, D. Tescaro
30
, M. Teshima
33,62
, D. F. Torres
50
, N. Torres-Albà
46
, T. Toyama
33
,
A. Treves
27
, G. Vanzo
35,36
, M. Vazquez Acosta
35,36
, I. Vovk
33
, J. E. Ward
40
, M. Will
35,36
,M.H.Wu
42
, R. Zanin
46,bd
(the MAGIC Collaboration),
T. Hovatta
51,52
, I. de la Calle Perez
53
, P. S. Smith
54
, E. Racero
53
, and M. BalokoviĆ
55
1
Department of Physics and Astronomy, University of Utah, Salt Lake City, UT 84112, USA
2
Physics Department, McGill University, Montreal, QC H3A 2T8, Canada
3
Department of Physics, Washington University, St. Louis, MO 63130, USA
4
Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for Astrophysics, Amado, AZ 85645, USA
5
Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA
6
Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA
7
Institute of Physics and Astronomy, University of Potsdam, D-14476 Potsdam-Golm, Germany
8
DESY, Platanenallee 6, D-15738 Zeuthen, Germany
9
Astronomy Department, Adler Planetarium and Astronomy Museum, Chicago, IL 60605, USA
10
School of Physics, National University of Ireland Galway, University Road, Galway, Ireland
11
Department of Physics and Astronomy, Purdue University, West Lafayette, IN 47907, USA
12
Department of Physics and Center for Astrophysics, Tsinghua University, Beijing 100084, China
13
Department of Astronomy and Astrophysics, 525 Davey Lab, Pennsylvania State University, University Park, PA 16802, USA
14
School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA
15
Department of Physics, California State University—East Bay, Hayward, CA 94542, USA
16
Santa Cruz Institute for Particle Physics and Department of Physics, University of California, Santa Cruz, CA 95064, USA
17
Department of Physics and Astronomy and the Bartol Research Institute, University of Delaware, Newark, DE 19716, USA
18
Physics Department, Columbia University, New York, NY 10027, USA
19
Department of Physics and Astronomy, University of Iowa, Van Allen Hall, Iowa City, IA 52242, USA
20
Department of Physics and Astronomy, DePauw University, Greencastle, IN 46135-0037, USA
21
School of Physics and Center for Relativistic Astrophysics, Georgia Institute of Technology, 837 State Street NW, Atlanta, GA 30332-0430, USA
22
Department of Physics and Astronomy, Barnard College, Columbia University, NY 10027, USA
23
Enrico Fermi Institute, University of Chicago, Chicago, IL 60637, USA
24
School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
25
Department of Physical Sciences, Cork Institute of Technology, Bishopstown, Cork, Ireland
26
ETH Zurich, CH-8093 Zurich, Switzerland
27
Università di Udine, and INFN Trieste, I-33100 Udine, Italy
28
INAF National Institute for Astrophysics, I-00136 Rome, Italy
The Astrophysical Journal, 834:2 (18pp), 2017 January 1 doi:10.3847/1538-4357/834/1/2
© 2016. The American Astronomical Society. All rights reserved.
1
29
Università di Siena, and INFN Pisa, I-53100 Siena, Italy
30
Università di Padova and INFN, I-35131 Padova, Italy
31
Croatian MAGIC Consortium, Rudjer Boskovic Institute, University of Rijeka, University of Split and University of Zagreb, Croatia
32
Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Salt Lake, Sector-1, Kolkata 700064, India
33
Max-Planck-Institut für Physik, D-80805 München, Germany
34
Universidad Complutense, E-28040 Madrid, Spain
35
Inst. de Astrofísica de Canarias, E-38200 La Laguna, Tenerife, Spain
36
Universidad de La Laguna, Dpto. Astrofísica, E-38206 La Laguna, Tenerife, Spain
37
University of Łódź, PL-90236 Lodz, Poland
38
Deutsches Elektronen-Synchrotron (DESY), D-15738 Zeuthen, Germany
39
Humboldt University of Berlin, Institut für Physik Newtonstr. 15, D-12489 Berlin Germany
40
Institut de Fisica d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology, Campus UAB, E-08193 Bellaterra (Barcelona), Spain
41
Universität Würzburg, D-97074 Würzburg, Germany
42
Institute for Space Sciences (CSIC/IEEC), E-08193 Barcelona, Spain
43
Technische Universität Dortmund, D-44221 Dortmund, Germany
44
Finnish MAGIC Consortium, Tuorla Observatory, University of Turku and Astronomy Division, University of Oulu, Finland
45
Unitat de Física de les Radiacions, Departament de Física, and CERES-IEEC, Universitat Autònoma de Barcelona, E-08193 Bellaterra, Spain
46
Universitat de Barcelona, ICC, IEEC-UB, E-08028 Barcelona, Spain
47
Japanese MAGIC Consortium, ICRR, The University of Tokyo, Department of Physics and Hakubi Center, Kyoto University, Tokai University,
The University of Tokushima, Japan
48
Inst. for Nucl. Research and Nucl. Energy, BG-1784 Sofia, Bulgaria
49
Università di Pisa, and INFN Pisa, I-56126 Pisa, Italy
50
ICREA and Institute for Space Sciences (CSIC/IEEC), E-08193 Barcelona, Spain
51
Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
52
Aalto University Department of Radio Science and Engineering, P.O. BOX 13000, FI-00076 AALTO, Finland
53
European Space Astronomy Centre (INSA-ESAC), European Space Agency (ESA), Satellite Tracking Station, P.O. Box—Apdo 50727,
E-28080 Villafranca del Castillo, Madrid, Spain
54
Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
55
Cahill Center for Astronomy & Astrophysics, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, USA
Received 2016 September 26; accepted 2016 November 10; published 2016 December 22
ABSTRACT
Blazars are variable emitters across all wavelengths over a wide range of timescales, from months down to minutes.
It is therefore essential to observe blazars simultaneously at different wavelengths, especially in the X-ray and
gamma-ray bands, where the broadband spectral energy distributions usually peak. In this work, we report on three
“target-of-opportunity” observations of Mrk 421, one of the brightest TeV blazars, triggered by a strong flaring
event at TeV energies in 2014. These observations feature long, continuous, and simultaneous exposures with
XMM-Newton (covering the X-ray and optical/ ultraviolet bands) and VERITAS (covering the TeV gamma-ray
band), along with contemporaneous observations from other gamma-ray facilities (MAGIC and Fermi-Large Area
Telescope) and a number of radio and optical facilities. Although neither rapid flares nor significant X-ray/TeV
correlation are detected, these observations reveal subtle changes in the X-ray spectrum of the source over the
course of a few days. We search the simultaneous X-ray and TeV data for spectral hysteresis patterns and time
delays, which could provide insight into the emission mechanisms and the source properties (e.g., the radius of the
emitting region, the strength of the magnetic field, and related timescales). The observed broadband spectra are
consistent with a one-zone synchrotron self-Compton model. We find that the power spectral density distribution at
4×10
−4
Hz from the X-ray data can be described by a power-law model with an index value between 1.2 and
1.8, and do not find evidence for a steepening of the power spectral index (often associated with a characteristic
length scale) compared to the previously reported values at lower frequencies.
Key words: BL Lacertae objects: individual (Markarian 421) – galaxies: active – gamma rays: general – radiation
mechanisms: non-thermal
1. INTRODUCTION
Relativistic outflows in the form of bipolar jets are an
important means of carrying energy away from many accreting
compact objects in astrophysics. Such objects range from X-ray
binaries of a few solar masses, to the bright central regions with
black holes of millions of solar masses in some galaxies,
known as active galactic nuclei (AGNs). Blazars, an extreme
sub-class of the AGN family, are oriented such that one of the
relativistic jets is pointed almost directly at the observer,
resulting in a bright, point-like source (e.g., Padovani &
Giommi 1995).
The spectral energy distribution (SED) of blazars typically
exhibits two peaks in the νF
ν
representation (e.g., Fossati et al.
1998). The lower-energy peak in the SED of blazars is
commonly associated with synchrotron radiation from
56
Also at the Department of Physics of Kyoto University, Japan.
57
Now at Centro Brasileiro de Pesquisas Físicas (CBPF/MCTI), R. Dr.
Xavier Sigaud, 150— Urca, Rio de Janeiro—RJ, 22290-180, Brazil.
58
Now at NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
and Department of Physics and Department of Astronomy, University of
Maryland, College Park, MD 20742, USA.
59
Also at University of Trieste.
60
Now at Ecole polytechnique fédérale de Lausanne (EPFL), Lausanne,
Switzerland.
61
Now at Max-Planck-Institut für Kernphysik, P.O. Box 103980, D-69029
Heidelberg, Germany.
62
Also at Japanese MAGIC Consortium.
63
Now at Finnish Centre for Astronomy with ESO (FINCA), Turku, Finland.
64
Also at INAF-Trieste and Dept. of Physics & Astronomy, University of
Bologna.
2
The Astrophysical Journal, 834:2 (18pp), 2017 January 1 Abeysekara et al.
relativistic electrons/positrons (electrons hereafter) in the jet.
The higher-energy peak could be the result of inverse-Compton
scattering from the same electrons (in leptonic models, e.g.,
Marscher & Gear 1985; Maraschi et al. 1992; Böttcher &
Dermer 1998), or of radiation from hadronic processes, e.g., π
0
decay (e.g., Sahu et al. 2013), photopion processes (e.g.,
Mannheim et al. 1991; Dimitrakoudis et al. 2014), or proton
synchrotron emission (e.g., Aharonian 2000). Current instru-
ments are usually unable to measure the broadband SED with
the necessary energy coverage and time resolution (e.g.,
Böttcher et al. 2013), therefore variability plays a crucial role
in distinguishing between these models (e.g., Mastichiadis
et al. 2013).
Blazars are variable emitters across all wavelengths over a
wide range of timescales. On long timescales ( days to months),
radio observations with high angular resolution have suggested
a connection between knots with distinct polarization angles
and outbursts of radio flux, sometimes with an optical and/or
gamma-ray counterpart (e.g., Rani et al. 2015). Correlated
multiwavelength (MWL) variability studies are important for
investigating the particles and magnetic field in the jets, as well
as their spatial structure (e.g., Błażejowski et al. 2005;
Katarzyński et al. 2005; Arlen et al. 2013). For example, in
synchrotron self-Compton (SSC) models for high-frequency-
peaked BL Lac objects (HBLs), X-ray and very-high-energy
(VHE; 100 GeV–100 TeV) fluxes are highly correlated and
most strongly variable when the electron injection rate changes.
A general correlation between the X-ray and TeV fluxes on
longer timescales has been observed with no systematic lags.
However, Fossati et al. (2008
) found “an intriguing hint” that
the correlation between X-ray and TeV fluxes may be different
for variability with different timescales. Specifically, the data
suggest a roughly quadratic dependence of the VHE flux on the
X-ray flux for timescales of hours, but a less steep, close to
linear relationship, for timescales of days (e.g., Fossati et al.
2008; Aleksić et al. 2015b; Baloković et al. 2016).
On shorter timescales, blazar variability has been observed in
both X-ray and gamma-ray bands (e.g., Gaidos et al. 1996; Cui
2004; Pryal et al. 2015). Especially interesting are the fast TeV
flares with doubling times as short as a few minutes, the
production mechanisms of which are even less well understood
than the variability on longer timescales. One major obstacle to
understanding such flares lies in the practical challenge in
organizing simultaneous MWL observations on short time-
scales. First, it is difficult, if not impossible, to predict when a
blazar will flare, due to the stochastic nature of its emission.
Second, it takes time to coordinate target-of-opportunity (ToO)
observations with X-ray satellites and ground-based telescopes
in response to a spontaneous flaring event. Third, most of the
current X-ray satellites have relatively short orbital periods, and
are frequently interrupted by Earth occultation and the South
Atlantic Anomaly passage, while observations from ground-
based Cherenkov telescopes may be affected by the weather, or
precluded by daylight. These gaps in the observations increase
the chances of missing a fast flare and introduce bias into
timing analyses (e.g., cross-correlation and power spectrum).
The XMM-Newton satellite has a long orbital period (48 hr),
capable of providing observations of >10 hr with no exposure
gaps. It is therefore uniquely well-suited for monitoring and
studying sub-hour variability, and is chosen as the primary
X-ray instrument in this work. It is also worth noting that fast
automated analyses of multi-wavelength data from TeV
gamma-ray blazars are done regularly, providing the potential
to deploy ToO observations at short notice if a strong flare is
detected from a blazar.
Spectral hysteresis and energy-dependent time lags observed
in blazars have also provided unique insights into the different
timescales associated with particle acceleration and energy loss
(e.g., Kirk et al. 1998; Böttcher & Chiang
2002), which can
then be used to test different blazar models. However, such
studies have been limited to X-ray observations, as a large
number of photons are needed to provide a constraining result
(e.g., Takahashi et al. 1996; Kataoka et al. 2000; Cui 2004;
Falcone et al. 2004). The increased sensitivity of the current
generation of Cherenkov telescopes, such us VERITAS and
MAGIC, has motivated the search of fast TeV gamma-ray
variability and hysteresis of blazars in this work.
Within the framework of a 6 month long multi-instrument
campaign, the MAGIC telescopes observed on 2014 April 25 a
VHE gamma-ray flux reaching eight times the flux above
300 GeV of the Crab Nebula (Crab units, C. U.) from the TeV
blazar Mrk 421 (e.g., Punch et al. 1992), which is about 16
times brighter than usual. This triggered a joint ToO program
by XMM-Newton, VERITAS, and MAGIC. Three, approxi-
mately 4 hr long, continuous and simultaneous observations in
both X-ray and TeV gamma-ray bands were carried out on
2014 April 29, May 1, and May 3. This was the third time in
eight years that Mrk 421 triggered the joint ToO program.
Compared to the last two triggers in 2006 and 2008 (Acciari
et al. 2011), the source flux observed by VERITAS was
significantly higher at 1–2.5 C. U. in 2014. In this work, we
focus on the simultaneous VERITAS–XMM-Newton data
obtained from the ToO observations in 2014 (listed in Table 1),
and complement this study with other contemporaneous MWL
observations (including those of MAGIC) of Mrk 421. The
details of the large flare observed with MAGIC on 2014 April
25 will be reported elsewhere.
2. OBSERVATIONS AND DATA ANALYSIS
2.1. VERITAS and MAGIC
VERITAS is an array of four 12 m ground-based imaging
atmospheric Cherenkov telescopes in southern Arizona, each
equipped with a camera consisting of 499 photomultiplier tubes
(PMTs)(Holder 2011). It is sensitive to gamma-rays in the
energy range from ∼100 GeV to ∼30 TeV with an energy
resolution of ∼15%, and covers a 3
°.5 field-of-view with an
angular resolution (68% containment) of ∼0°.1. It is capable of
making a detection at a statistical significance of five standard
deviations (5σ) of a point source of 1% C. U. in ∼25 hr. The
systematic uncertainty on the energy calibration is estimated at
Table 1
Summary of the Simultaneous ToO Observations of Mrk 421 in 2014
UTC Date MJD VERITAS XMM-EPN
2014 Apr 29 56776 03:19-08:02 04:24-08:00
2014 May 01 56778 03:24-06:10 03:46-07:53
2014 May 03 56780 03:31-06:05 03:35-07:42
Note. Columns 1 and 2 are the UTC and MJD dates of the observations,
respectively. Columns 3 and 4 are the start and end time of the VERITAS and
XMM-Newton observations, respectively.
3
The Astrophysical Journal, 834:2 (18pp), 2017 January 1 Abeysekara et al.
20%, and that on the spectral index is estimated at 0.2
(Madhavan 2013).
VERITAS has been monitoring Mrk421 regularly for
approximately 20 hr every year, as part of several long-term
MWL monitoring campaigns (e.g., Acciari et al. 2011; Aleksić
et al. 2015b). The general strategy is to take a 30 minute
exposure on every third night when the source is visible, with
coordinated, simultaneous X-ray observations (usually with the
X-Ray Telescope (XRT) on board the Swift satellite).In
contrast, the three long and simultaneous observations with
XMM-Newton and VERITAS on 2014 April 29, May 1, and
May 3 are specific attempts to catch rapid flares on top of
elevated flux states simultaneously in the X-ray and TeV bands.
Due to high atmospheric dust conditions at the VERITAS
site on May 1, only data from the ToO observations on April 29
and May 3 have been used. The VERITAS observations on
these two nights were taken in “wobble ” mode (Fomin et al.
1994) with the source offset 0°. 5 from the center of the field-of-
view. The zenith angles of the observations were between 10°
and 40°. After deadtime correction, the total exposure time
from these observations is 6.14 hr. The data were analyzed
using the data analysis procedures described in Cogan (2008).
Standard gamma-ray selection cuts, previously optimized for
sources with a power-law spectrum of a photon index 2.5, were
applied to reject cosmic-ray (CR) background events. The
reflected-region background model (Berge et al. 2007) was
used to estimate the number of CR background events that
passed the cuts, and a generalized method from Li & Ma
(1983) was used for the calculation of statistical significance.
The VERITAS results are shown in Table 2.
To parameterize the curvature in the VERITAS-measured
TeV spectra, a power-law model with an exponential cutoff has
been used to fit the daily spectra:
dN
dE
K
E
E
e .1
0
E
E
cutoff
=
a-
-
⎛
⎝
⎜
⎞
⎠
⎟
()
However, we used a power-law model in the hysteresis study in
Section 3.3, as it adequately describes each 10 minute
integrated spectrum without the cutoff energy as an extra
degree of freedom.
The Major Atmospheric Gamma-ray Imaging Cherenkov
(MAGIC) telescope system consists of two 17 m telescopes,
located at the Observatory Roque de los Muchachos, on the
Canary Island of La Palma (28.8 N, 17.8 W, 2200 m a.s.l.).
Stereoscopic observations provide a sensitivity of detecting a
point source at ∼0.7% C. U. above 220 GeV in 50 hr of
observation, and allow measurement of photons in the energy
range from 50 GeV to above 50 TeV. The night-to-night
systematic uncertainty in the VHE flux measurement by
MAGIC is estimated to be of the order of 11% (Aleksić
et al. 2016).
Mrk421 was observed by MAGIC for six nights from 2014
April 28 to May 4, as part of a longer MWL observational
campaign. The source was observed in “wobble” mode, with
0°. 4 offset with respect to the nominal source position (Fomin
et al. 1994). After discarding data observed in poor weather
conditions, the total analyzed data amounted to 3.3 hr of
observations, with exposures per observation ranging from 14
to 38 minutes, and zenith angles spanning from 9° to 42°.
The MAGIC data were analyzed using the standard MAGIC
analysis and reconstruction software(Zanin et al. 2013). The
integral flux was computed above 560 GeV, the same as the
energy threshold found in the VERITAS long-term light curve,
in order to use all the observations including those at large
zenith angles. The source gamma-ray flux varied between 1.3
and 2.2 C. U. above 560 GeV for different days in this period,
with no significant intra-night variability. This flux value is 3–5
times larger than the typical VHE flux of Mrk421 (Acciari
et al. 2014; Aleksić et al. 2015a). These observations are not
simultaneous with the XMM-Newton observations. The source
is known to change spectral index with flux level
(Krennrich
et al. 2002), and hence we computed the photon flux above
560 GeV using the measured spectral shape above 400 GeV,
which ranged from 2.8 to 3.3.
2.2. Fermi-Large Area Telescope (LAT)
Fermi-LAT is a pair-conversion high-energy (HE) gamma-
ray telescope covering an energy range from about 20 MeV to
more than 300 GeV (Atwood et al. 2009). It has a large field-
of-view of 2.4sr that covers the full sky every 3hr in the
nominal survey mode. Thus, Fermi-LAT provides long-term
sampling of the entire sky. However, it has a small effective
area of ∼8000 cm
2
for >1 GeV, which is usually not sufficient
to resolve variability on timescales of hours or less.
We analyzed the Fermi-LAT Pass 8 data in the week of the
VERITAS observations and produced daily averaged spectra
and a daily binned light curve. We selected events of class
source and type front+back with an energy between 0.1 and
300 GeV in a 10° region of interest (RoI) centered at the
location of Mrk421, and removed events with a zenith angle
>90°. The data were processed using the publicly available
Fermi-LAT science tools (v10r0p5) with instrument response
functions (P8R2_SOURCE_V6). A model with the contribu-
tions of all sources within the RoI with a test statistic value
greater than 3, a list of 3FGL sources within a source region of
a radius of 20° from Mrk421, and the contribution of the
Galactic ( using file gll_iem_v06.fit) and isotropic (using
file iso_P8R2_SOURCE_V6_v06.txt) diffuse emission
was used. This model was fitted to LAT Pass 8 data between
2014 April 1 and June 1 using an unbinned likelihood analysis
(gtlike). The test-statistic maps were examined to ensure no
unmodeled transient sources were present in the RoI during the
period analyzed. All other best-fit parameters in the model were
then fixed, except the spectral normalization and the power-law
index of Mrk421, in order to perform a spectral and temporal
maximum likelihood analysis.
Table 2
Summary of VERITAS Observations of Mrk 421 (the Analysis Details are given in Section 3)
Date Exposure Significance Non Noff α Gamma-ray Rate Background Rate
(minutes) σ photons min
−1
CRs min
−1
2014 Apr 29 237.4 97.4 2481 538 0.1 10.2±0.2 0.21
2014 May 01 146.4 KKKK K K
2014 May 03 131.0 74.3 1443 315 0.1 10.8±0.3 0.22
4
The Astrophysical Journal, 834:2 (18pp), 2017 January 1 Abeysekara et al.
2.3. XMM-Newton and Swift-XRT
The XMM-Newton satellite carries the European Photon
Imaging Camera (EPIC) pn X-ray CCD camera (Strüder et al.
2001), including two metal-oxide-silicon (MOS) cameras and a
pn camera. The reflection grating spectrometers (RGS) with
high-energy resolution are installed in front of the MOS
detector. The incoming X-ray flux is divided into two portions
for the MOS and RGS detectors. The EPIC-pn (EPN) detector
receives the unobstructed beam and is capable of observing
with very high time resolution. The Optical/ultraviolet (UV)
Monitor (OM) onboard the XMM-Newton satellite provides the
capability to cover a 17′ ×17′ square region between 170 and
650 nm (Mason et al. 2001). The OM is equipped with six
broad-band filters (U, B, V, UVW1, UVM2, and UVW2).
Three ToO observations were taken simultaneously with the
VERITAS observations on 2014 April 29, May1, and May3.
To fully utilize the high time resolution capability of XMM-
Newton in both the X-ray and optical/UV bands, all three ToO
observations of Mrk 421 were taken in EPN timing mode and
OM fast mode. MOS and RGS were also operated during the
observations, but the data were not used due to the relatively
low timing resolution and the lack of X-ray spectral lines from
the source. The EPN camera covers a spectral range of
approximately 0.5–10 keV and, with the UVM2 filter, the OM
covers the range of about 200–
270 nm.
XMM-Newton EPN and OM data were analyzed using
Statistical Analysis System (SAS) software version 13.5
(Gabriel et al. 2004). An X-ray loading correction and a rate-
dependent pulse height amplitude (RDPHA) correction were
performed using the SAS tool epchain. We ran the SAS task
epproc to produce the RDPHA results, which applies
calibrations using known spectral lines and is likely more
accurate than the alternative charge transfer inefficiency
corrections.
65
Note that even after the RDPHA corrections,
residual absorption features (not associated with the source) can
still be present in the spectrum (see e.g., Pintore et al. 2014).To
account for the source and the residual spectral features, the
X-ray spectra were fitted using XSPEC version 12.8.1 with a
model including a power law, a photoelectric absorption
component representing the Galactic neutral hydrogen absorp-
tion, an absorption edge component, and two Gaussian
components. The last three components are only associated
with the instrument. They account for the oxygen K line at
∼0.54 keV, the silicon K line at ∼1.84 keV, and the gold M
line at ∼2.2 keV, respectively. The model can be expressed as
follows:
dN
dE
eKE e
EE
eKEe
EE
,
;
,
;
2
nE
i
K
c
DE E n E
i
K
c
PL
2
PL
2
Gi
i
EE
i
i
c
Gi
i
EE
i
i
H
,
0,
2
2
2
3
H
,
0,
2
2
2
å
å
=
+
+
sa
ps
sa
ps
--
-
-- -
-
s
s
-
-
-
⎧
⎨
⎪
⎪
⎪
⎪
⎩
⎪
⎪
⎪
⎪
⎡
⎣
⎢
⎤
⎦
⎥
⎡
⎣
⎢
⎤
⎦
⎥
()
()
() ()
()
()
where n
H
is the column density of neutral hydrogen; K
PL
and
K
Gi,
are the normalization factor for the power-law component
and the ith Gaussian component, respectively; E
c
,
E
i0,
, and σ
i
are the threshold energy of the absorption edge, the center and
the standard deviation of the ith Gaussian component,
respectively; D is the absorption depth at the threshold energy
E
c
; and α is the photon index of the power-law component in
the model. The edge component at ∼0.5 keV is replaced by a
Gaussian component (the 0th component in Table 4) for data
taken on May 3 since the latter provides a better fit. We fix the
column density of Galactic neutral hydrogen to
N
H
≈1.9×10
20
cm
−2
, which was measured by the Leiden/
Argentine/Bonn (LAB) survey toward the direction of
Mrk421 (Kalberla et al. 2005). It is worth noting that the
best-fit power-law index hardly changes when we set N
H
free.
The count rate measured by the EPN camera with a thin filter
can be converted to flux using energy conversion factors
(ECFs, in units of 10
11
cts cm
2
erg
−1
), which depend on the
filter, the photon index α, the Galactic n
H
absorption, and the
energy range (Mateos et al. 2009). The flux f, in units
of erg cm
−2
s
−1
, can be obtained by f=rate/ECF, where rate
has the units of cts s
−1
. A similar flux conversion factor is used
for the OM UVM2 filter to convert each count at 2310 Å to flux
density 2.20×10
15
erg cm
−2
s
−1
Å
−1
. A 2% systematic
uncertainty error was added to the OM light curve.
The long-term Swift-XRT light curve is produced using an
online analysis tool The Swift-XRT data products generator
66
(Evans et al. 2009). This tool is publicly available and can be
used to produce Swift-XRT spectra, light curves, and images
for a point source. A light curve of Mrk421 was made from all
Swift-XRT observations available from 2005 March 1 to 2014
April 30, integrated between 0.3 and 10 keV, with a fixed bin
width of 50 s. We cut the first 150s of each WT observation,
during which it is possible that the satellite could still be
settling, thus causing non-source-related deflections in the light
curve.
2.4. Steward Observatory
Regular optical observations of a sample of gamma-ray-
bright blazars, including Mrk421, have been carried out at
Steward Observatory since the launch of the Fermi satellite
(Smith et al. 2009), and these data are publicly accessible.
67
For
the 2014 April–May MWL observing campaign, the SPOL
optical, dual-beam spectropolarimeter (Schmidt et al. 1992b)
was used at the Steward Observatory 1.54 m Kuiper Telescope
on Mt. Bigelow, Arizona from April 25 to May 4 UTC. When
the weather permitted, the usual observing frequency of one
observation per night for Mrk421 was increased to four per
night after April 26 so that any rapid changes in linear
polarization and optical flux could be better tracked. The
spectropolarimeter was configured with a 600 l/mm diffraction
grating that yields a dispersion of 4 Å pixel
−1
, spectral
coverage from 4000 to 7550 Å, and resolution of ∼16 Å. The
CCD detector is a thinned, anti-reflection coated 1200×800
STA device with a quantum efficiency of about 0.9 from 5000
to 7000 Å. All polarization observations of Mrk421 were
made with a 3″×50″ slit oriented so that its long (spatial)
dimension is east–west on the sky and the CCD was binned by
two pixels (∼0
9) in the spatial direction. An observation of
Mrk421 typically consists of a 30 s exposure at all 16 positions
of the λ/2-wave plate, properly sorted into four images with
65
See http://xmm2.esac.esa.int/docs/documents/CAL-SRN-0312-1-4.pdf.
66
http://www.swift.ac.uk/user_objects/
67
http://james.as.arizona.edu/~psmith/Fermi/
5
The Astrophysical Journal, 834:2 (18pp), 2017 January 1 Abeysekara et al.
