L25
The Astrophysical Journal, 675: L25–L28, 2008 March 1

 2008. The American Astronomical Society. All rights reserved. Printed in U.S.A.
MAGIC OBSERVATIONS OF THE UNIDENTIFIED g-RAY SOURCE TeV J2032
4130
J. Albert,1 E. Aliu,2 H. Anderhub,3 P. Antoranz,4 C. Baixeras,5 J. A. Barrio,4 H. Bartko,6 D. Bastieri,7
J. K. Becker,8 W. Bednarek,9 K. Berger,1 C. Bigongiari,7 A. Biland,3 R. K. Bock,6,7 G. Bonnoli,10 P. Bordas,11
V. Bosch-Ramon,11 T. Bretz,1 I. Britvitch,3 M. Camara,4 E. Carmona,6 A. Chilingarian,12 S. Commichau,3
J. L. Contreras,4 J. Cortina,2,13 M. T. Costado,14,15 V. Curtef,8 F. Dazzi,7 A. De Angelis,16 C. Delgado,14
R. de los Reyes,4 E. Domingo-Santamarı´a,2 B. De Lotto,16 M. De Maria,16 F. De Sabata,16 D. Dorner,1 M. Doro,7
M. Errando,2 M. Fagiolini,10 D. Ferenc,17 E. Ferna´ndez,2 R. Firpo,2 M. V. Fonseca,4 L. Font,5 N. Galante,6
R. J. Garcı´a-Lo´pez,14,15 M. Garczarczyk,6 M. Gaug,14 F. Goebel,6 M. Hayashida,6 A. Herrero,14,15 D. Ho¨hne,1 J. Hose,6
C. C. Hsu,6 S. Huber,1 T. Jogler,6 R. Kosyra,6 D. Kranich,3 A. Laille,17 E. Leonardo,10 E. Lindfors,18 S. Lombardi,7
F. Longo,16 M. Lo´pez,4 E. Lorenz,3,6 P. Majumdar,6 G. Maneva,19 N. Mankuzhiyil,16 K. Mannheim,1 M. Mariotti,7
M. Martı´nez,2 D. Mazin,2 C. Merck,6 M. Meucci,10 M. Meyer,1 J. M. Miranda,4 R. Mirzoyan,6 S. Mizobuchi,6
A. Moralejo,2 D. Nieto,4 K. Nilsson,18 J. Ninkovic,6 E. On˜a-Wilhelmi,2,13,20 N. Otte,6,21 I. Oya,4 M. Panniello,14,22
R. Paoletti,10 J. M. Paredes,11 M. Pasanen,18 D. Pascoli,7 F. Pauss,3 R. Pegna,10 M. Persic,16,23 L. Peruzzo,7
A. Piccioli,10 E. Prandini,7 N. Puchades,2 A. Raymers,12 W. Rhode,8 M. Ribo´,11 J. Rico,2 M. Rissi,3 A. Robert,5
S. Ru¨gamer,1 A. Saggion,7 T. Y. Saito,6 A. Sa´nchez,5 P. Sartori,7 V. Scalzotto,7 V. Scapin,16 R. Schmitt,1
T. Schweizer,6 M. Shayduk,6,21 K. Shinozaki,6 S. N. Shore,24 N. Sidro,2 A. Sillanpa¨a¨,18 D. Sobczynska,9 F. Spanier,1
A. Stamerra,10 L. S. Stark,3 L. Takalo,18 P. Temnikov,19 D. Tescaro,2 M. Teshima,6 D. F. Torres,13,25 N. Turini,10
H. Vankov,19 A. Venturini,7 V. Vitale,16 R. M. Wagner,6 W. Wittek,6 F. Zandanel,7 R. Zanin,2 and J. Zapatero5
Received 2007 December 14; accepted 2008 January 15; published 2008 February 5
ABSTRACT
We observed the first known very high energy (VHE) g-ray-emitting unidentified source, TeV J2032
4130,
for 94 hr with the MAGIC telescope. The source was detected with a significance of 5.6 j. The flux, position,
and angular extension are compatible with the previous ones measured by the HEGRA telescope system 5 years
ago. The integral flux amounts to (4.5  0.3stat  0.35sys) # 1013 photons cm s above 1 TeV. The source2 1
energy spectrum, obtained with the lowest energy threshold to date, is compatible with a single power law with
a hard photon index of G p 2.0  0.3stat  0.2sys.
Subject heading: gamma rays: observations
1. INTRODUCTION
The TeV source J2032
4130 (Aharonian et al. 2002) was
the first unidentified very high energy (VHE) g-ray source, and
also the first discovered extended TeV source, likely to be
Galactic.
Intensive observational campaigns at different wavelengths
1 Universita¨t Wu¨rzburg, D-97074 Wu¨rzburg, Germany.
2 IFAE, Edifici Cn., E-08193 Bellaterra (Barcelona), Spain.
3 ETH Zurich, CH-8093, Switzerland.
4 Universidad Complutense, E-28040 Madrid, Spain.
5 Universitat Auto`noma de Barcelona, E-08193 Bellaterra, Spain.
6 Max-Planck-Institut fu¨r Physik, D-80805 Munich, Germany.
7 Universita` di Padova and INFN, I-35131 Padova, Italy.
8 Universita¨t Dortmund, D-44227 Dortmund, Germany.
9 University of Ło´dz´, PL-90236 Lodz, Poland.
10 Universita` di Siena, and INFN Pisa, I-53100 Siena, Italy.
11 Universitat de Barcelona, E-08028 Barcelona, Spain.
12 Yerevan Physics Institute, AM-375036 Yerevan, Armenia.
13 Corresponding author.
14 Inst. de Astrofisica de Canarias, E-38200 La Laguna, Tenerife, Spain.
15 Depto. de Astrofisica, Universidad, E-38206 La Laguna, Tenerife, Spain.
16 Universita` di Udine, and INFN Trieste, I-33100 Udine, Italy.
17 University of California, Davis, CA 95616.
18 Tuorla Observatory, Turku University, FI-21500 Piikkio¨, Finland.
19 Inst. for Nucl. Research and Nucl. Energy, BG-1784 Sofia, Bulgaria.
20 Current address: APC (CNRS) Paris, France.
21 Humboldt-Universita¨t zu Berlin, D-12489 Berlin, Germany.
22 Deceased.
23 INAF–Osservatorio Astronomico and INFN, I-34131 Trieste, Italy.
24 Universita` di Pisa, and INFN Pisa, I-56126 Pisa, Italy.
25 ICREA and Institut de Cie`ncies de l’Espai (IEEC-CSIC), 08193 Barce-
lona, Spain.
have been carried out on TeV J2032
4130. Butt et al. (2003)
presented an analysis of the CO, H i, and infrared emissions,
together with first observations by Chandra (5 ks) and a rean-
alysis of VLA data. These observations showed that the TeV
source region is positionally coincident with an outlying group
of stars (from the Cygnus OB2 core), although they failed to
identify a counterpart. Mukherjee et al. (2003) analyzed the
same Chandra data and provided optical follow-up observa-
tions of several of the brightest X-ray sources, confirming that
most were either O stars or foreground late-type stars. A deeper
Chandra observation (50 ks; Butt et al. 2006) found hundreds
of starlike sources and yet no diffuse X-ray counterpart
emission.
A deep (∼50 ks) XMM-Newton exposure has also been ob-
tained (Horns et al. 2007). After the subtraction of the contri-
bution of known sources from the data, an extended X-ray
emission region with a FWHM size of ∼12
 was reported. The
centroid of the emission is colocated with the position of TeV
J2032
4130 and was proposed as the counterpart of the TeV
source. The question of whether the result reported by Horns
et al. can be interpreted as a truly diffuse background, or
whether it could be a result of unresolved X-ray sources, re-
mains disputable.
Paredes et al. (2007) and Martı´ et al. (2007) have provided
deep radio observations covering the TeV J2032
4130 vicinity
using the Giant Metrewave Radio Telescope and discovered a
population of radio sources, some in coincidence with X-ray
detections by Butt et al. (2006) and with optical/IR counter-
parts. At least three of these sources are nonthermal, and one
L26 ALBERT ET AL. Vol. 675
TABLE 1
Observing Periods, Zenith Angle Ranges,
and Observation Modes
Year
Time
(hr)
Zenith Angle
(deg) Mode
2005 . . . . . . 18.1 13–30 ON/OFF
2006 . . . . . . 60.1 11–44 Wobble
2007 . . . . . . 15.5 11–30 Wobble
has a hard X-ray energy spectrum. They found extended non-
thermal diffuse emission in the radio band apparently con-
necting with one or two radio sources. It is yet to be determined
whether one or more of these sources is similar to some of the
known g-ray binaries (e.g., Aharonian et al. 2006; Albert et
al. 2006a).
Several theoretical explanations for the TeV emission from
J2032
4130 have been given. Among them, those related to
extragalactic counterparts, e.g., a radio galaxy (Butt et al. 2006)
or a proton blazar (Mukherjee et al. 2003), face the difficulty
of explaining the extended appearance of the source. Gamma-
ray production in hypothetical jet termination lobes of Cyg X-
3 was explored (Aharonian et al. 2002), but the putative north-
ern lobe of Cyg X-3 (now considered a mere thermal H ii
region; Martı´ et al. 2006) is far from the location of the TeV
source. A yet unknown pulsar wind nebula (PWN) was pro-
posed by Bednarek (2003), although no clear PWN signal was
observed. A distant microquasar was proposed by Paredes et
al. (2007), perhaps related to one of the X-ray/radio sources
they discovered. If such an association is accepted, the exten-
sion of the source could be explained by the diffusion of ac-
celerated particles into a hypothetical nearby molecular en-
hancement (see Bosch-Ramon et al. 2005). Torres et al. (2004)
and Domingo-Santamarı´a & Torres (2006) studied the rela-
tionship between the TeV emission and the known massive
stars in the area, through the interaction of relativistic protons
with wind ions. The distribution of stars in the neighborhood
favors this interpretation (Butt et al. 2006). An explanation
involving the excitation of giant dipole resonances of relativ-
istic heavy nuclei in radiation-dominated environments has also
been suggested (Anchordoqui et al. 2007).
2. PREVIOUS VERY HIGH ENERGY g-RAY OBSERVATIONS
We start by making a brief summary of what has been
claimed by other experiments observing at the highest energies.
The HEGRA IACT, using 4 years of data (from 1999 to
2002), found a source to the north of Cygnus X-3, steady in
flux over the years, extended, with radius 6.2
  ′1.2stat
, and exhibiting a hard energy spectrum with a photon′0.9sys
index of Gp 1.9  0.1stat  0.3sys (Aharonian et al. 2005).
Its integral flux above 1 TeV amounts to ∼5% of the Crab
Nebula, assuming a Gaussian profile for the intrinsic source
morphology. The center of the source position was determined
quite accurately at a p 20h31m57.0s   ands s6.2 13.7stat sys
d p 4129
56.8   (J2000.0).′ ′1.1 1.0stat sys
The Whipple collaboration reported an excess at the position
of the HEGRA unidentified source (3.3 j) in their archival data
of 1989 and 1990 (Lang et al. 2004), with a flux level of ∼12%
of the Crab Nebula for GeV. The detected flux is inE 1 600
conflict with the HEGRA flux level and steady nature of the
source, assuming they all have the same origin. This large
difference between the detected flux levels, if physical, might
suggest episodic emission (with low duty cycle) or variability
over timescale measured in years. Nevertheless, the existence
of g-ray variability is difficult to reconcile with the extended
appearance of the source. The large difference might also be
due in part to unspecified systematic errors on the flux deter-
mination. Recently, the Whipple collaboration reported new
observations of this field done with their 10 m telescope for
65.5 hr during 2003 and 2005 (Konopelko et al. 2007). Their
data are consistent with either a pointlike or an extended source
with less than 6 angular size. Regarding the position, the HE-′
GRA and the latest Whipple data are barely in agreement: their
centers of gravity are ∼9 apart, and only agree when adding′
up the spatial uncertainties in both data sets in opposite direc-
tions. Konopelko et al. do not provide a energy spectrum for
this source, but give a 8% Crab-level flux (although with no
energy threshold specified) under the assumption of a steep
(Crab-like) energy spectrum.
The Cygnus region shows an excess in the MILAGRO data
(Abdo et al. 2007). The flux at 20 TeV in a 3 # 3 region
centered at the HEGRA position is (9.8  2.9stat  2.7sys) #
10 TeV cm s assuming a differential energy spectrum15 1 2 1
E . This flux is 3 times the HEGRA flux extrapolated at 202.6
TeV. The Tibet air shower detector recently reported evidence
for an excess also in their VHE g-ray candidate set from this
region (Amenomori et al. 2006).
In this rich observational and theoretical context we report
here on MAGIC telescope observations of TeV J2032
4130.
3. MAGIC OBSERVATIONS AND RESULTS
The MAGIC single-dish Imaging Air Cerenkov Telescope
(see, e.g., Cortina et al. 2005 for a detailed description) is
located on the Canary Island of La Palma. Its angular (energy)
resolution is approximately 0.09 (20%), and the trigger (anal-
ysis) threshold is 55 (60) GeV at zenith in dark conditions (see
Albert et al. 2008). One of the unique characteristics of MAGIC
is its capability of observing under moderate moonlight illu-
mination (Albert et al. 2007a), albeit with a slightly elevated
threshold.
The field of view of TeV J2032
4130 was observed with
MAGIC for more than 100 hr distributed in 2005, 2006, and
2007 (see Table 1). During the first period in summer 2005,
the observation was carried out in ON/OFF mode, that is, the
source was observed on-axis while observations from an empty,
nearby field of view were used to estimate the background. In
summer 2006 and 2007, the data were taken in Wobble mode,
using five positions around the HEGRA position instead of the
usual two symmetrical position in order to monitor a wider
field of view. Quality cuts based on the trigger and after-clean-
ing rates were applied in order to remove bad weather runs
and data spoiled by car or satellite light flashes. After these
quality cuts the total observation time is 93.7 hr. The energy
range for which we report these results is significantly above
the aforementioned trigger and analysis threshold energies due
to the fact that the observations were scheduled during moon-
light and at relatively high zenith angles (up to 44 ).
The data analysis was carried out using the standard MAGIC
analysis and reconstruction software (Bretz & Wagner 2003).
It follows the general stream explained in Albert et al. (2006b,
2006c, 2006d). After calibration and two levels of image clean-
ing tail cuts (for image core and boundary pixels, see Fegan
1997), the camera images are parameterized by the so-called
image parameters (Hillas 1985). The random forest method
was applied for the g/hadron separation (Albert et al. 2007b).
Using this method a parameter, dubbed hadronness (H), can
be calculated for every event and is a measure of the probability
No. 1, 2008 MAGIC OBSERVATIONS OF TeV J2032
4130 L27
Fig. 1.—Distribution of the -parameter for events coming from the di-2v
rection of TeV J2032
4130 (SIZE 1 800 photoelectrons), the background
distribution subtracted (black points). A convolved radial Gaussian fit Fp
is indicated by the solid black line with jsrcp 5.0
2 2 2A exp [0.5v /(j 
 j )]psf src
 1.7
. The jpsf was measured from MC simulation and validated with Crab
Nebula observations to be jpsf p 5.2
  0.1
 (dashed black line).
Fig. 2.—Gaussian-smoothed (j p 4
) map of g-ray excess events (back-
ground subtracted) for energies above 500 GeV. The MAGIC position is shown
with a black cross. The surrounding black circle corresponds to the measured
1 j width. The last position reported by Whipple is marked with a white cross
while the HEGRA position is shown with a blue cross in the center of the
field of view. The error bars, in all cases, correspond to the linear sum of the
statistical and systematic errors. The green crosses correspond to the positions
of Cyg X-3, WR 146, and the EGRET source 3EG J2033
4118. The ellipse
around the EGRET source marks the 95 confidence contour.%
TABLE 2
Events Recorded above SIZE 1 800
Photoelectrons
Year Non Noff Nex fnorm Nj
2005 . . . . . . . . 641 576 65 0.47 2.2
2006 . . . . . . . . 688 559 129 0.20 4.8
2007 . . . . . . . . 175 136 39 0.20 2.9
Overall . . . . . . 1504 1271 233 0.27 5.6
Notes.— , , and refer to the number ofN N Non off ex
events recorded in the direction of the source, the nor-
malized background, and the g-ray excess, respec-
tively. The normalization ratio and significancefnorm
are also shown.Nj
TABLE 3
Upper Limit for Sources in the FOV,
above 500 GeV
Target Name
Flux
(Crab Nebula)
Cyg X-3 . . . . . . . . . . . . . . . . 0.011
WR 146 . . . . . . . . . . . . . . . . 0.010
3EG J2033
4118 . . . . . . 0.009
that the event is not g-like. The g-like sample is selected for
images with H below a specified value, which is optimized
using a sample of Crab Nebula data processed with the same
analysis stream. An independent sample of Monte Carlo g-
showers was used to determine the cut efficiency. Since part
of our observations was recorded during partial moonshine, we
have corrected the efficiency loss due to the increase of ambient
light following the procedure outlined in Albert et al. (2007a).
The -distribution was calculated, v being the angular dis-2v
tance between the source direction and the reconstructed arrival
direction of the showers. The reconstruction of individual g-
ray arrival directions makes use of the DISP method (Domingo-
Santamaria et al. 2005). The expected number of background
events is calculated using five regions symmetrically placed
for each wobble position with respect to the center of the cam-
era and referred to as antisources. Figure 1 shows the distri-
bution of the parameter for the excess observed from the2v
direction of the source, for a SIZE cut of 800 photoelectrons.
This relatively high SIZE cut was selected in order to optimize
the sensitivity for a source with such a hard energy spectrum
observed during moonlight. Therefore, the total number of g-
like excesses after Hillas cuts and applying a cut in v2 ! 0.05
is , for which a total significance of 5.6 j is obtained.N p 233ex
Table 2 shows the number of excesses above background for
the different observing periods.
The excess is fitted to a Gaussian function folded with the
telescope PSF, as obtained from Monte Carlo simulations and
validated with Crab Nebula observations. The source is ex-
tended with respect to the MAGIC PSF. Its intrinsic size as-
suming a Gaussian profile is jsrcp 5.0
  . The′ ′1.7 0.6sta sys
exact shape of the source, even if similar to the keV diffuse
emission reported by Horns et al. (2007), cannot be completely
trusted due to limited statistics and telescope pointing
systematics.
Figure 2 shows the Gaussian-smoothed (jp 4
) map (0.65
# 0.65) of g-ray emission (background subtracted) around
TeV J2032
4130 for energies GeV. The position ofE 1 500
a few previously observed g-ray source candidates are also
shown, namely, Cyg X-3, the EGRET source 3EG J2033
4118
(with its confidence contour at 95 ), the Wolf-Rayet star WR%
146, and the Whipple and HEGRA experimental positions. The
regions around Cyg X-3, WR 146, and 3EG J2033
4118 have
been further investigated by us and no detection is obtained
for a steady emission. The upper limit fluxes (Rolke et al. 2005),
for a 95 confidence level, above 500 GeV for a pointlike%
source at these positions are given in Table 3.
To determine the best position of the MAGIC detection the
excess map was fitted to a 2D bell-shaped function. The result
is shown in the sky map with a black cross and a circle in-
dicating its size. The best-fit coordinates are R.A.p 20h32m20s
 11  11 and decl. p 4130
36.0  s s ′ ′1.2 1.8stat sys stat sys
(J2000.0; for more details on the systematic uncertainties in
the source position determination, see Bretz & Wagner 2003).
L28 ALBERT ET AL. Vol. 675
Fig. 3.—Differential energy spectrum from TeV J2032
4130 as measured
by the MAGIC telescope in black solid line. The gray shadow shows the 1 j
error in the fitted energy spectrum. The flux observed by Whipple in 2005
and in the MILAGRO scan are marked with colored squares (blue and pink,
respectively). The gray dotted line represents the Crab Nebula energy spectrum
measured by MAGIC. The blue line shows the HEGRA energy spectrum.
Theoretical one-zone model predictions are depicted with dashed lines.
The position found is compatible within errors with the one
determined by HEGRA, and barely compatible with the claims
by Whipple mentioned above (in Konopelko et al. 2007).
The TeV J2032
4130 energy spectrum was obtained using
the Tikhonov unfolding technique (Tikhonov & Arsenin 1979).
It can be fitted (x2/dof p 0.3) by a power-law function. The
differential flux (TeV cm s ) is1 2 1
2.00.3dN E
13p (4.5 0.3)# 10 . (1)( )dE dA dt 1 TeV
The errors quoted are only statistical. The systematic error is
estimated to be 35 in the flux level and 0.2 in the photon%
index (see Albert et al. 2008). The differential energy spectrum
is shown in Figure 3. The HEGRA TeV J2032
4130 and
MAGIC Crab Nebula measured spectra (in Albert et al. 2008)
are shown with the blue solid line and black dotted line, re-
spectively. The MAGIC energy spectrum is compatible both
in flux level and photon index with the one measured by
HEGRA.
Crab Nebula data from the same periods and zenith angle
distributions were studied with the same analysis chain to check
for any systematic deviation due to the long observation period.
No indication of time variability was observed: the source in-
tegral flux is constant within errors, at 3 of the Crab Nebula%
flux. The relative systematic uncertainty in the ratio of both
fluxes was estimated to be less than 10 . This uncertainty%
comes mainly from the slightly different atmospheric trans-
mission conditions and differences in the detector parameters
during data taking of the source and the Crab Nebula.
For illustrative purposes, the dotted lines in Figure 3 rep-
resent one-zone hadronic and leptonic models of the high-en-
ergy emission, both consistent with observations at lower en-
ergies in the region. Under the hadronic scenario, the are0p
obtained from a proton parent population described by a power
law (index ) with exponential cutoff at 100 TeV. TheGp 2
cutoff value was adopted to be consistent with the upper limit
at the highest energies coming from the HEGRA spectrum.
The inverse Compton spectrum is obtained from an electron
population with equal index and a 40 TeV exponential cutoff
scattering off the CMB photons. As in Aharonian et al. (2005),
we do not consider here the conditions under which particles
are accelerated or how they lose energy. Our leptonic fits (see
also the quoted paper for an SED representation) can only cope
with the data if we are actually looking at a Compton peak
around the energy range of detection, which is not fully dis-
carded within errors. Both models are compatible with the high-
energy emission. Confirming the reality of the diffuse emission
detected at lower energies is crucial to distinguish between
these and more complex models.
4. CONCLUDING REMARKS
MAGIC observations confirm the location of TeV
J2032
4130 found by HEGRA. The MAGIC observation
shows an extended source with a significance of 5.6 j. We find
a steady flux with no significant variability within the 3 year
span of the observations (with the flux being at a similar level
to the HEGRA data of the period 2002–2005). We also present
the source energy spectrum obtained with the lowest energy
threshold to date, which, within errors, is compatible with a
single power law.
We thank the IAC for the excellent working conditions at
the ORM. The support of the German BMBF and MPG, the
Italian INFN, the Spanish CICYT, the ETH Research Grant
TH 34/04 3, and the Polish MNiI Grant 1P03D01028 is grate-
fully acknowledged.
REFERENCES
Abdo, A. A., et al. 2007, ApJ, 658, L33
Aharonian, F. A., et al. 2002, A&A, 393, L37
———. 2005, A&A, 431, 197
———. 2006, A&A, 460, 743
Albert, J., et al. 2006a, Science, 312, 1771
———. 2006b, ApJ, 637, L41
———. 2006c, ApJ, 638, L101
———. 2006d, ApJ, 643, L53
———. 2007a, Astropart. Phys., submitted (astro-ph/0702475)
———. 2007b, Astropart. Phys., submitted (arXiv:0709.3719)
———. 2008, ApJ, in press (arXiv:0705.3244)
Amenomori, M., et al. 2006, Science, 314, 439
Anchordoqui, L. A., et al. 2007, Phys. Rev. D, 75, 063001
Bednarek, W. 2003, MNRAS, 345, 847
Bosch-Ramon, V., Aharonian, F., & Paredes, J. M. 2005, A&A, 432, 609
Bretz, T., & Wagner, W. 2003, in Proc. 28th Int. Cosmic Ray Conf. (Tsukuba),
2947
Butt, Y., et al. 2003, ApJ, 597, 494
———. 2006, ApJ, 643, 238
Cortina, J., et al. 2005, in Proc. 29th Int. Cosmic Ray Conf. (Pune), 5, 359
Domingo-Santamarı´a, E., & Torres, D. F. 2006, A&A, 448, 613
Domingo-Santamarı´a, E., et al. 2005, in Proc. 29th Int. Cosmic Ray Conf.
(Pune), 5, 363
Fegan, D. J. 1997, J. Phys. G, 23, 1013
Hillas, A. M. 1985, in Proc. 19th Int. Cosmic Ray Conf. (La Jolla), 3, 445
Horns, D., et al. 2007, A&A, 469, L17
Konopelko, A., et al. 2007, ApJ, 658, 1062
Lang, M. J., et al. 2004, A&A, 423, 415
Martı´, J., et al. 2006, A&A, 451, 1037
———. 2007, A&A, 472, 557
Mukherjee, R., et al. 2003, ApJ, 589, 487
Paredes, J. M., et al. 2007, ApJ, 654, L135
Rolke, W. A., Lo´pez, A. M., & Conrad, J. 2005, Nucl. Instrum. Methods Phys.
Res., A551, 493
Tikhonov, A. N., & Arsenin, V. Ja. 1979, in Numerical Methods for the Solution
of Ill-posed Problems (Moscow: Nauka)
Torres, D. F., Domingo-Santamarı´a E., & Romero, G. E. 2004, ApJ, 601, L75