The Astrophysical Journal, 762:92 (13pp), 2013 January 10 doi:10.1088/0004-637X/762/2/92
C
2013. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
RAPID TeV GAMMA-RAY FLARING OF BL LACERTAE
T. Arlen
1
, T. Aune
2
, M. Beilicke
3
, W. Benbow
4
, A. Bouvier
2
, J. H. Buckley
3
, V. Bugaev
3
, A. Cesarini
5
,L.Ciupik
6
,
M. P. Connolly
5
, W. Cui
7
, R. Dickherber
3
,J.Dumm
8
, M. Errando
9
, A. Falcone
10
, S. Federici
11,12
, Q. Feng
7
,J.P.Finley
7
,
G. Finnegan
13
, L. Fortson
8
, A. Furniss
2
, N. Galante
4
, D. Gall
14
,S.Griffin
15
,J.Grube
6
,G.Gyuk
6
,D.Hanna
15
,
J. Holder
16
, T. B. Humensky
17
, P. Kaaret
14
, N. Karlsson
8
, M. Kertzman
18
, Y. Khassen
19
, D. Kieda
13
, H. Krawczynski
3
,
F. Krennrich
20
, G. Maier
11
, P. Moriarty
21
, R. Mukherjee
9
, T. Nelson
8
, A. O’Faol
´
ain de Bhr
´
oithe
19
,R.A.Ong
1
,
M. Orr
20
, N. Park
22
, J. S. Perkins
23,24
,A.Pichel
25
, M. Pohl
11,12
, H. Prokoph
11
, J. Quinn
19
, K. Ragan
15
, L. C. Reyes
26
,
P. T. Reynolds
27
, E. Roache
4
, D. B. Saxon
16
, M. Schroedter
4
, G. H. Sembroski
7
, D. Staszak
15
, I. Telezhinsky
11,12
,
G. Te
ˇ
si
´
c
15
, M. Theiling
7
, K. Tsurusaki
14
, A. Varlotta
7
,S.Vincent
11
, S. P. Wakely
22
, T. C. Weekes
4
, A. Weinstein
20
,
R. Welsing
11
, D. A. Williams
2
, and B. Zitzer
28
(The VERITAS Collaboration),
and
S. G. Jorstad
29,30
, N. R. MacDonald
29
, A. P. Marscher
29
,P.S.Smith
31
, R. C. Walker
32
, T. Hovatta
33
, J. Richards
7
,
W. Max-Moerbeck
33
, A. Readhead
33
,M.L.Lister
7
, Y. Y. Kovalev
34,35
, A. B. Pushkarev
36,37
,M.A.Gurwell
38
,
A. L
¨
ahteenm
¨
aki
39
, E. Nieppola
39
, M. Tornikoski
39
, and E. J
¨
arvel
¨
a
39
1
Department of Physics and Astronomy, University of California, Los Angeles, C A 90095, USA
2
Santa Cruz Institute for Particle Physics and Department of Physics, University of California, Santa Cruz, CA 95064, USA
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
School of Physics, National University of Ireland Galway, University Road, Galway, Ireland
6
Astronomy Department, Adler Planetarium and Astronomy Museum, Chicago, IL 60605, USA
7
Department of Physics, Purdue University, West Lafayette, IN 47907, USA; qfeng@purdue.edu, cui@purdue.edu
8
School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA
9
Department of Physics and Astronomy, Barnard College, Columbia University, NY 10027, USA
10
Department of Astronomy and Astrophysics, 525 Davey Lab, Pennsylvania State University, University Park, PA 16802, USA
11
DESY, Platanenallee 6, D-15738 Zeuthen, Germany
12
Institute of Physics and Astronomy, University of Potsdam, D-14476 Potsdam-Golm, Germany
13
Department of Physics and Astronomy, University of Utah, Salt Lake City, UT 84112, USA
14
Department of Physics and Astronomy, University of Iowa, Van Allen Hall, Iowa City, IA 52242, USA
15
Physics Department, McGill University, Montreal, QC H3A 2T8, Canada
16
Department of Physics and Astronomy and the Bartol Research Institute, University of Delaware, Newark, DE 19716, USA
17
Physics Department, Columbia University, New York, NY 10027, USA
18
Department of Physics and Astronomy, DePauw University, Greencastle, IN 46135-0037, USA
19
School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
20
Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA
21
Department of Life and Physical Sciences, Galway-Mayo Institute of Technology, Dublin Road, Galway, Ireland
22
Enrico Fermi Institute, University of Chicago, Chicago, IL 60637, USA
23
CRESST and Astroparticle Physics Laboratory NASA/GSFC, Greenbelt, MD 20771, USA
24
Center for Space Science and Technology, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
25
Instituto de Astronomia y Fisica del Espacio, Casilla de Correo 67 - Sucursal 28, (C1428ZAA) Ciudad Aut
´
onoma de Buenos Aires, Argentina
26
Physics Department, California Polytechnic State University, San Luis Obispo, CA 94307, USA
27
Department of Applied Physics and Instrumentation, Cork Institute of Technology, Bishopstown, Cork, Ireland
28
Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA
29
Institute for Astrophysical Research, Boston University, 725 Commonwealth Ave., Boston, MA 02215, USA
30
Astronomical Institute, St. Petersburg State University, Universitetskij Pr. 28, Petrodvorets, 198504 St. Petersburg, Russia
31
Steward Observatory, University of Arizona, Tucson, AZ 85716, USA
32
National Radio Astronomy Observatory, PO Box O, Socorro, NM 87801, USA
33
Cahill Center for Astronomy & Astrophysics, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, USA
34
Astro Space Center of Lebedev Physical Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia
35
Max-Planck-Institute for Radio Astronomy Auf dem Huegel 69, D-53121 Bonn, Germany
36
Pulkovo Astronomical Observatory, Pulkovskoe Chaussee 65/1, 196140 St. Petersburg, Russia
37
Crimean Astrophysical Observatory, 98409 Nauchny, Ukraine
38
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
39
Aalto University Mets
¨
ahovi Radio Observatory, Mets
¨
ahovintie 114, FIN-02540 Kylm
¨
al
¨
a, Finland
Received 2012 September 11; accepted 2012 November 13; published 2012 December 19
ABSTRACT
We report on the detection of a very rapid TeV gamma-ray flare from BL Lacertae on 2011 June 28 with the
Very Energetic Radiation Imaging Telescope Array System (VERITAS). The flaring activity was observed during a
34.6 minute exposure, when the integral flux above 200 GeV reached (3.4 ± 0.6) × 10
−6
photons m
−2
s
−1
, r oughly
125% of the Crab Nebula flux measured by VERITAS. The light curve indicates that the observations missed
the rising phase of the flare but covered a significant portion of the decaying phase. The exponential decay time
was determined to be 13 ± 4 minutes, making it one of the most rapid gamma-ray flares seen from a TeV blazar.
The gamma-ray spectrum of BL Lacertae during the flare was soft, with a photon index of 3.6 ± 0.4, which is
in agreement with the measurement made previously by MAGIC in a lower flaring state. Contemporaneous radio
observations of the source with the Very Long Baseline Array revealed the emergence of a new, superluminal
1
The Astrophysical Journal, 762:92 (13pp), 2013 January 10 Arlen et al.
component from the core around the time of the TeV gamma-ray flare, accompanied by changes in the optical
polarization angle. Changes in flux also appear to have occurred at optical, UV, and GeV gamma-ray wavelengths
at the time of the flare, although they are difficult to quantify precisely due to sparse coverage. A strong flare was
seen at radio wavelengths roughly four months later, which might be related to the gamma-ray flaring activities.
We discuss the implications of these multiwavelength results.
Key words: galaxies: active – galaxies: individual (BL Lacertae, VER J2202+422) – gamma rays: galaxies
Online-only material: color figures
1. INTRODUCTION
Blazars form a subclass of active galactic nuclei (AGNs) that
feature a relativistic jet pointing roughly at the observer. They
are known for being highly variable at all wavelengths. In the
most extreme cases, the timescales of gamma-ray variability can
be as short as a few minutes at very high energies (100 GeV;
VHE). Such variability has been detected in several BL Lacertae
objects (BL Lac objects), including Mrk 421 (Gaidos et al.
1996), Mrk 501 (Albert et al. 2007), and PKS 2155-304
(Aharonian et al. 2007), and more recently in the flat-spectrum
radio quasar (FSRQ) PKS 1222+21 (Aleksi
´
cetal.2011). The
rapid variability poses serious challenges to the theoretical
understanding of gamma-ray production in blazars. On the
one hand, rapid gamma-ray variability implies very compact
emitting regions that can be most naturally associated with
the immediate vicinity of the central supermassive black hole.
On the other hand, the regions must be sufficiently outside
the broad-line regions (BLRs) that gamma rays can escape
attenuation due to external radiation fields (which, for FSRQs,
are particularly strong). Many models have been proposed to
resolve these issues (Ghisellini & Tavecchio 2008; Giannios
et al. 2009; Tavecchio et al. 2011;Barkovetal.2012; Nalewajko
et al. 2012; Narayan & Piran 2012).
The spectral energy distributions (SEDs) of blazars show two
characteristic peaks, with one in the infrared (IR)–X-ray fre-
quency range and the other in the MeV–TeV gamma-ray range,
respectively. The lower-energy peak is believed to be associ-
ated with synchrotron radiation from relativistic electrons in the
jet, and the higher-energy peak with inverse-Compton radia-
tion from the same electrons in leptonic models; the situation
is more complex in hadronic models. Going from high-power
quasars to low-power BL Lac objects, the peaks shift systemati-
cally to higher frequencies. Most of the known TeV gamma-ray
blazars are BL Lac objects. They have been historically di-
vided into high-frequency-peaked BL Lac objects (HBLs) and
low-frequency-peaked BL Lac objects (LBLs; Padovani &
Giommi 1995; Fossati et al. 1998). BL Lacertae, the archetypi-
cal source of the class, is an LBL in this classification scheme.
BL Lacertae (also known as 1ES 2200+420) is an AGN
located at a redshift of z = 0.069 (Miller et al. 1978). In 1998,
the Crimean Observatory reported a detection of the source
at >100% of the Crab Nebula flux above 1 TeV (Neshpor
et al. 2001). Subsequently, the MAGIC Collaboration reported
another detection during an active state in 2005, but at a much
lower flux level (only about 3% of the Crab Nebula flux) (Albert
et al. 2007). Triggered by activities seen with the Fermi-LAT
(Cutini 2011) and AGILE (Piano et al. 2011) at GeV gamma-ray
energies, as well as in the optical (Larionov et al. 2011), near-IR
(Carrasco et al. 2011), and radio (Angelakis et al. 2011) in 2011
May, we began to monitor BL Lacertae more regularly at TeV
gamma-ray energies with Very Energetic Radiation Imaging
Telescope Array System (VERITAS). In this work, we report
the detection of a rapid, intense VHE gamma-ray flare from the
direction of the source on MJD 55740 (2011 June 28), as well
as the results from the multiwavelength observations that were
conducted around the time of this flare.
2. OBSERVATIONS AND DATA ANALYSIS
2.1. Very High Energy Gamma Ray
VERITAS is an array of four 12 m imaging atmospheric
Cherenkov telescopes located in southern Arizona. Each tele-
scope is equipped with a f ocal-plane camera with 499 photomul-
tiplier tubes, covering a 3.
◦
5 field of view (Holder et al. 2008).
VERITAS is sensitive to VHE radiation in the energy range from
∼100 GeV to ∼30 TeV, being capable of making a detection at
a statistical significance of 5 standard deviations (5σ ) of a point
source of 1% of the Crab Nebula flux in ∼25 hr.
Prior to the intensified monitoring campaign with VERITAS,
BL Lacertae had also been observed on a number of occasions,
mostly with the full array. The data from those observations are
also used in this work to establish a longer baseline. The total live
exposure time (after quality selection) amounts to 20.3 hr from
2010 September to 2011 November, with zenith angles ranging
from 10
◦
to 40
◦
. The source was not detected throughout the
time period, except for one night on MJD 55740 (2011 June 28),
when the automated real-time analysis revealed the presence
of a rapidly flaring gamma-ray source in the direction of BL
Lacertae. On that night, BL Lacertae was observed only with
three telescopes in the “wobble” mode (Aharonian et al. 2001)
with 0.
◦
5 offset, because one telescope was temporarily out of
commission. Starting at 10:22:24 UTC, two 20 minute runs
were taken on the source under good weather conditions, with
the zenith angle varying between 10
◦
and 13
◦
. No additional runs
were possible due to imminent sunrise. The total live exposure
time was 34.6 minutes.
The data were analyzed using the data analysis package
described in Cogan (2008). The analysis procedure includes
raw data calibration, image parameterization (Hillas 1985),
event reconstruction, background rejection and signal extraction
(Daniel 2008). The standard data quality cuts (identical for the
four- and three-telescope configuration), which were previously
optimized for a simulated s oft point source of ∼6.6% of the
Crab Nebula flux at 200 GeV and a photon index of 4, were
applied to the shower images. The cuts used were: an integrated
charge lower cut of 45 photoelectrons, a distance (between the
image centroid and the center of the camera) upper cut of 1.
◦
43,
a minimum number of pixels cut of 5 for each image, inclusive,
mean scaled width and length cuts 0.05 < MSW < 1.15, and
0.05 < MSL < 1.3, respectively. A cut of θ
2
< 0.03 deg
2
on the size of the point-source search window was made,
where θ is the angle between the reconstructed gamma-ray
direction and the direction to the source. A specific effective area
corresponding to these cuts and the relevant array configuration
was generated from simulations and was used to calculate the
flux. The reflected-region background model (Berge et al. 2007)
was applied for background estimation, a generalized method
from Li & Ma (1983) was used for the calculation of statistical
significance, and upper limits were calculated using the method
2
The Astrophysical Journal, 762:92 (13pp), 2013 January 10 Arlen et al.
described by Rolke et al. (2005). The results were confirmed
by an independent secondary analysis with a different analysis
package, as described in Daniel (2008).
2.2. High Energy Gamma Ray
The Fermi Large Area Telescope (LAT) is a pair-conversion
high-energy 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 2.4 sr, and an effective area
of ∼8000 cm
2
for >1 GeV. In its nominal (survey) mode, the
Fermi-LAT covers the full sky every 3 hr.
During the time window when VERITAS detected a rapid
flare on MJD 55740 (2011 June 28), BL Lacertae was in
the field of view of the LAT for about 16 minutes (MJD
55740.431–55740.442). In analyzing the simultaneous LAT
data, we selected Diffuse class photons with energy between
0.2 and 10 GeV in a 16
◦
× 16
◦
region of interest (ROI)
centered at the location of BL Lacertae. Only events with
rocking angle <52
◦
and zenith angle <100
◦
were selected. The
data were processed using the publicly available Fermi-LAT
tools (v9r23p1) with standard instrument response functions
(P7SOURCE_V6). For such a short exposure, a very simple model
containing the source of interest and the contribution of the
galactic (using file gal_2yearp7v6_v0.fits) and isotropic
(using file iso_p7v6source.txt) diffuse emission was used.
The contribution of the other known gamma-ray sources in the
ROI is assumed to be negligible compared to that of BL Lacertae
and the diffuse emission.
The model is fitted to the data using a binned likelihood
analysis (gtlike), where the only free parameters are the
spectral normalization and the power-law index of BL Lacertae.
The contribution of the galactic and isotropic diffuse emission
was fixed to a normalization of 1.0, which is compatible with the
values obtained when analyzing the same field of view during
longer timespans. The results are used to construct an energy
spectrum of BL Lacertae. We also performed an unbinned
likelihood analysis and obtained similar spectral results.
For comparison, we repeated the analyses for a longer period
(of 24 hr) centered at the time of the VERITAS observations, as
well as for times prior to the VERITAS-detected flare (between
2011 May 26 and 2011 June 26, or MJD 55707–55738). For the
latter, we adopted a source model that incorporates all s ources
in the 2FGL catalog within the ROI and within 5
◦
of the ROI
edges. The spectral results were extracted by adopting a custom
spectral code (SED_scripts) available on the Fermi-LAT Web
site. In all cases, the LAT spectrum of BL Lacertae can be well
described by a power law, which justifies the assumption made
in the likelihood analyses.
A daily-binned light curve integrated above 0.1 GeV was
derived covering the period MJD 55652–55949 (2011 April
01–2012 January 23) using the likelihood method described
above. In each one-day bin, the flux and the corresponding 1σ
error are calculated if the test statistic (TS) value is greater than
1, otherwise an upper limit is calculated.
2.3. X-Ray and Ultraviolet
BL Lacertae was also observed with the XRT and UVOT
instruments on board the Swift satellite (Gehrels et al. 2004)
contemporaneously with the gamma-ray flare in 18 exposures
between MJD 55704 (2011 May 23) and MJD 55768 (2011
July 26), including six ∼2 ks Target of Opportunity (ToO)
observations on six nights following the VHE flare on MJD
55740. The combination of the X-ray telescope (XRT) and
UV/optical telescope (UVOT) provided useful coverage in
soft X-rays and UV, although none of the observations were
simultaneous with the VERITAS observations during the flare.
We analyzed the XRT data using the HEASOFT package
(version 6.11). The event files are calibrated and cleaned using
the calibration files from 2011 September 5. The data were
taken in the photon-counting (PC) mode, and were selected from
grades 0 to 12 over the energy range 0.3–10 keV. Since the rates
did not exceed 0.5 counts s
−1
, pile-up effects were negligible.
Source counts were extracted with a 20 pixel radius circle
centered on the source, while background counts were extracted
from a 40 pixel radius circle in a source-free region. Ancillary
response files were generated using the xrtmkarf task, with
corrections applied for the point-spread function (PSF) losses
and CCD defects. The corresponding response matrix from the
XRT calibration files was applied. The spectrum was fitted with
an absorbed power law model, allowing the neutral hydrogen
(H i) column density (N
H
) to vary. The best-fitted value of N
H
is (0.24 ± 0.01) × 10
22
cm
−2
, which is in agreement with the
result of N
H
= 0.25 × 10
22
cm
−2
presented by Ravasio et al.
(2003), but is larger than the value of N
H
= 0.18 × 10
22
cm
−2
from the Leiden/Argentine/Bonn (LAB) survey of galactic H i
(Kalberla et al. 2005).
The UVOT cycled through each of the optical and the UV pass
bands V, B, U, UVW1, UVM2, and UVW2. Data were taken in
the image mode discarding the photon timing information. Only
data from UVW2 band are shown in this work; the other bands
roughly track UVW2. The photometry was computed using an
aperture of 5
following the general prescription of Poole et al.
(2008) and Breeveld et al. (2010). Contamination by background
light arising from nearby sources was removed by introducing
ad hoc exclusion regions. Adopting the N
H
value provided
by the XRT analysis and assuming E(B − V ) = 0.34 mag
(Maesano et al. 1997), we estimated R
V
= 3.2(G
¨
uver &
¨
Ozel
2009). Then, the optical/UV galactic extinction coefficients
were applied (Fitzpatrick 1999). The host galaxy contribution
has been estimated using the PEGASE-HR code (Le Borgne
et al. 2004) extended for the ultraviolet UVOT filters. Moreover,
there is no pixel saturation in the source region and no significant
photon loss. Therefore, it is possible to constrain the systematics
to below 10%.
2.4. Optical
As part of the Steward Observatory spectropolarimetric mon-
itoring project (Smith et al. 2009), BL Lacertae was observed
regularly with the 2.3m Bok Telescope and the 1.54m Kuiper
Telescope in Arizona. Measurements of the V-band flux density
and optical linear polarization are from the Steward Observatory
public data archive (http://james.as.arizona.edu/psmith/Fermi/).
The data were reduced and calibrated following the procedures
described by Smith et al. (2009). We note that there is a 180
◦
de-
generacy in polarization angle, so we shifted some polarization
angles by 180
◦
to minimize the change between two consecu-
tive measurements. No corrections to the data have been made
for the contribution from the host galaxy, or interstellar polar-
ization, extinction, and reddening. However, these issues have
little effect on variability studies.
2.5. Radio
BL Lacertae was observed with the Very Long Baseline Array
(VLBA) at 43 GHz, roughly once a month, as part of the monitor-
ing program of gamma-ray bright blazars at Boston University
3
The Astrophysical Journal, 762:92 (13pp), 2013 January 10 Arlen et al.
(http://www.bu.edu/blazars/VLBAproject.html). Two extra ep-
ochs of imaging were added via Director’s Discretionary Time
on 2011 July 6 and 29. The data were correlated at the National
Radio Astronomy Observatory in Socorro, NM, and then ana-
lyzed at Boston University following the procedures outlined by
Jorstad et al. (2005). The calibrated total and polarized intensity
images were used to investigate the jet kinematics and to cal-
culate the polarization parameters (degree of polarization p and
position angle of polarization χ) for the whole source imaged at
43 GHz with the VLBA and for individual jet components. The
uncertainties of polarization parameters were computed based
on the noise level of total and polarized intensity images and do
not exceed 0.6% and 3.
◦
5 for degree of polarization and position
angle of polarization, respectively.
BL Lacertae is also in the sample of the Monitoring Of Jets
in Active galactic nuclei with VLBA Experiments (MOJAVE)
program. For this work, we only used results from polarization
measurements at 15.4 GHz. The data reduction procedures are
described by Lister et al. (2009). Briefly, the flux density of
the core component is derived from a Gaussian model fit to the
interferometric visibility data. Polarization properties of the core
are then derived by taking the mean Stokes Q and U flux densities
of the nine contiguous pixels that are centered at the Gaussian
peak pixel position of the core fit. The results include fractional
linear polarization, electric vector position angle (note the 180
◦
degeneracy), and polarized flux densities. The flux density has
an uncertainty of ∼5%, and the position angle of polarization
has an uncertainty of ∼3
◦
.
For better sampling, we used data from blazar monitoring
programs with the Owens Valley Radio Observatory (OVRO)
at 15.4 GHz, with the Mets
¨
ahovi Radio Observatory (MRO) at
37 GHz, and with the Submillimeter Array (SMA) at 230 and
350 GHz, respectively. The OVRO 40 m uses off-axis dual-
beam optics and a cryogenic high electron mobility transistor
(HEMT) low-noise amplifier with a 15.0 GHz center frequency
and 3 GHz bandwidth. The two sky beams are Dicke-switched
using the off-source beam as a reference, and the source is
alternated between the two beams in an ON–ON fashion to
remove atmospheric and ground contamination. Calibration
is achieved using a temperature-stable diode noise source to
remove receiver gain drifts and the flux density scale is derived
from observations of 3C 286 assuming the Baars et al. (1977)
value of 3.44 J y at 15.0 GHz. The systematic uncertainty of
about 5% in the flux density scale is not included in the error
bars. Complete details of the reduction and calibration procedure
are f ound in Richards et al. (2011).
The 37 GHz observations were made with the 13.7 m
diameter Mets
¨
ahovi radio telescope, which is a radome-enclosed
paraboloid antenna situated in Finland (24 23
38
E, +60 13
05
). The measurements were made with a 1 GHz-band dual
beam receiver centered at 36.8 GHz. The observations are
ON–ON observations, alternating the source and the sky in
each feed horn. A typical integration time to obtain one flux
density data point is between 1200 s and 1400 s. The detection
limit of the telescope at 37 GHz is on the order of 0.2 Jy under
optimal conditions. Data points with a signal-to-noise ratio <4
are treated as non-detections. The flux density scale is set by
observations of DR 21. Sources NGC 7027, 3C 274 and 3C 84
are used as secondary calibrators. A detailed description of the
data reduction and analysis is given in Ter
¨
asranta et al. (1998).
The error estimate in the flux density includes the contribution
from the measurement rms and the uncertainty of the absolute
calibration.
Observations of BL Lacertae at frequencies near 230 and
350 GHz are from the SMA, a radio interferometer consisting
of eight 6 m diameter radio telescopes located just below the
summit of Mauna Kea, Hawaii. These data were obtained and
calibrated as part of the normal monitoring program initiated
by the SMA (see Gurwell et al. 2007). Generally, the signal-
to-noise ratio of these observations exceeds 50 and is often
well over 100, and the true error on the measured flux density
is limited by systematic rather than signal-to-noise effects.
Visibility amplitudes are calibrated by referencing to standard
sources of well-understood brightness, typically solar system
objects such as Uranus, Neptune, Titan, Ganymede, or Callisto.
Models of the brightness of these objects are accurate to within
around 5% at these frequencies. Moreover, the SMA usually
processes only a single polarization at one time, and there is
evidence that BL Lacertae in 2011 exhibited a fairly strong
(∼15%) linear polarization. For a long observation covering a
significant range of parallactic angle, the effect of the linear
polarization would be largely washed out, providing a good
measure of the flux density. However, not all observations of
BL Lacertae covered a significant range of parallactic angle,
and thus in some cases we would expect a potential absolute
systematic error up to 10%. In most cases, we expect that the
total systematic error i s around 7.5%.
3. RESULTS
3.1. Gamma-ray Properties
The VERITAS analysis showed an excess of 212 γ -like
events, corresponding to 11.0 ± 0.8 γ/min and a 21.1σ de-
tection of BL Lacertae in the first observation run on MJD
55740 (2011 June 28), with an effective exposure of 19.3 min-
utes starting at 10:22:24 UTC. The second run, with an ef-
fective exposure of 15.3 minutes, yielded an excess of only 33
γ -like events, corresponding to a 4.1σ detection. The VERITAS
analysis of 19.7 hr data from 2010 September to 2011 Novem-
ber, excluding the two flaring runs, showed an excess of 21
γ -like events, and a statistical significance of 0.28σ .
Focusing on the two flaring runs, we produced a light curve
with four-minute bins as shown in the inset of Figure 1.
The fluxes were computed with a lower energy threshold of
200 GeV. The observations missed the rising phase of the
flare. In four-minute bins, the highest flux that was measured is
(3.4±0.6)×10
−6
photons m
−2
s
−1
, which corresponds to about
125% of the Crab Nebula flux above 200 GeV, as measured with
VERITAS. To quantify the decay time, the light curve was fitted
with an exponential function I (t) = I
0
× exp (−t/τ
d
), and the
best-fit decay time was τ
d
= 13 ± 4 minutes.
To determine the position of the flaring gamma-ray source,
we fitted a two-dimensional Gaussian function to the uncorre-
lated map (binned to 0.
◦
05) of excess events (after acceptance
correction) from both runs. The best-fit right ascension and dec-
lination (J2000) are 22
h
02
m
37
s
and +42
◦
15
25
, respectively,
with a statistical uncertainty of ∼0.
◦
01 along both directions.
The source is thus named VER J2202+422. According to the
Simbad database, BL Lacertae is the only object within a radius
of 2
.
Using data from the first flaring run, we extracted a gamma-
ray s pectrum (Figure 2). It can be fitted with a power law:
dN/dE = (0.58 ± 0.07) × 10
−9
× (E/0.3TeV)
(−3.6±0.4)
cm
−2
s
−1
TeV
−1
. (1)
4
The Astrophysical Journal, 762:92 (13pp), 2013 January 10 Arlen et al.
0 10 20 30 40
Minutes
0
1
2
3
4
10
-6
m
-2
s
-1
55500 55600 55700 55800 55900
Modified Julian Date
0.0
0.5
1.0
1.5
Flux (10
-6
m
-2
s
-1
)
Figure 1. TeV gamma-ray light curve of BL Lacertae (>200 GeV). When the source was not significantly detected, 99% confidence upper limits are shown. The
upper limits were derived by combining data from all observation runs for each night, but for the night of the flare, the fluxes derived from the two individual runs are
shown separately. The inset shows the flare in detail, in four-minute b ins for the first run, and one 16 minute bin for the second run, with minute 0 indicating the start
of the first run. The dashed line shows the best fit to the profile with an exponential function (see the text).
0.1 1.0
Energy (TeV)
10
-13
10
-12
10
-11
10
-10
10
-9
10
-8
Flux density (cm
-2
s
-1
TeV
-1
)
VERITAS flaring run
MAGIC 2005
Figure 2. TeV gamma-ray spectrum of BL Lacertae. The VERITAS data points
are shown as red open diamonds, along with the best-fit power law (solid line).
For comparison, we also show the published MAGIC spectrum of the source
as blue open squares, along with the best-fit power law (dashed line). The two
power laws have comparable slopes.
(A color version of this figure is available in the online journal.)
Also shown in the figure is the gamma-ray spectrum of BL
Lacertae obtained with MAGIC during a lower-flux state (Albert
et al. 2007). The two gamma-ray spectra have nearly the same
slope. This is in contrast with the typical spectral hardening trend
of a flaring blazar (e.g., Giommi et al. 1990). It may reflect the
fact that in both cases the TeV gamma rays fall on the steeply
falling part of the high-energy SED peak, which might not be
sensitive to flux changes. A spectrum was also constructed from
both runs and fitted with a power law:
dN/dE = (0.30 ± 0.03) × 10
−9
× (E/0.3TeV)
(−3.8±0.3)
cm
−2
s
−1
TeV
−1
.
To better constrain the gamma-ray SED, we plotted the
Fermi-LAT spectra of the source averaged over several time
E [GeV]
-1
10 1 10
2
10
3
10
]
-1
s
-2
[erg cm
ν
Fν
-13
10
-12
10
-11
10
-10
10
-9
10
-8
10
VERITAS flare
VERITAS pre-flare
LAT simultaneous
LAT 1-day
LAT pre-flare
Figure 3. Broadband gamma-ray spectrum of BL Lacertae. For VERITAS, the
flare spectrum with EBL correction is shown as blue points with uncertainties,
and the spectrum without EBL correction is shown as lighter blue points. Also
plotted are the 95% confidence upper limits that were derived from 14 VERITAS
observation runs in the month prior to the flare. For the Fermi-LAT data, the
simultaneous, one-day, and pre-flare spectra are shown in blue, light blue, and
gray. The pre-flare spectrum was derived from LAT observations taken over a
period of one month before the TeV gamma-ray flare. The shaded areas show
the 1σ confidence interval of the overall Fermi-derived spectra, independent
from the data points.
(A color version of this figure is available in the online journal.)
periods (16 minutes, one day, and one month) along with the
VERITAS spectra, in Figure 3. The VERITAS spectra dur-
ing the flare both with and without extragalactic background
light (EBL) corrections (Dom
´
ınguez et al. 2011) are shown,
5
