Issue |
A&A
Volume 507, Number 1, November III 2009
|
|
---|---|---|
Page(s) | 241 - 250 | |
Section | Galactic structure, stellar clusters, and populations | |
DOI | https://doi.org/10.1051/0004-6361/200912448 | |
Published online | 03 September 2009 |
A&A 507, 241-250 (2009)
Radio continuum and near-infrared study
of the MGRO J2019+37
region![[*]](/icons/foot_motif.png)
J. M. Paredes1
- J. Martí2,3 - C. H. Ishwara-Chandra4
- J. R. Sánchez-Sutil3 - A. J. Muñoz-Arjonilla2,3
- J. Moldón1 - M. Peracaula5
- P. L. Luque-Escamilla3 -
V. Zabalza1 - V. Bosch-Ramon6
- P. Bordas1 - G. E. Romero7,8, - M. Ribó1
1 - Departament d'Astronomia i Meteorologia and Institut de Ciències
del Cosmos (ICC), Universitat de Barcelona (UB/IEEC),
Martí i Franquès 1, 08028 Barcelona, Spain
2 - Departamento de Física, EPS, Universidad de Jaén, Campus Las
Lagunillas s/n, Edif. A3, 23071 Jaén, Spain
3 - Grupo de Investigación FQM-322, Universidad de Jaén, Campus Las
Lagunillas s/n, Edif. A3, 23071 Jaén, Spain
4 - NCRA, TIFR, Post Bag 3, Ganeshkhind, Pune-411 007, India
5 - Institut d'Informàtica i Aplicacions, Universitat de Girona,
Girona, Spain
6 - Max Planck Institut für Kernphysik, Saupfercheckweg 1, Heidelberg
69117, Germany
7 - Instituto Argentino de Radioastronomía (CCT La Plata, CONICET),
C.C.5,
(1894) Villa Elisa, Buenos Aires, Argentina
8 - Facultad de Ciencias Astronómicas y Geofísicas, UNLP, Paseo del
Bosque, 1900 La Plata, Argentina
Received 7 May 2009 / Accepted 31 July 2009
Abstract
Context. MGRO J2019+37 is an
unidentified
extended source of very high energy gamma-rays originally reported by
the Milagro Collaboration as the brightest TeV source in the Cygnus
region. Its extended emission could be powered by either a single or
several sources. The GeV pulsar AGL J2020.5+3653,
discovered by AGILE and associated with
PSR J2021+3651,
could contribute to the emission from MGRO J2019+37.
Aims. Our aim is to identify radio and near-infrared
sources in
the field of the extended TeV source MGRO J2019+37,
and
study potential counterparts to explain its emission.
Methods. We surveyed a region of about 6 square
degrees with the
Giant Metrewave Radio Telescope (GMRT) at the frequency
610 MHz.
We also observed the central square degree of this survey in the
near-infrared -band
using the 3.5 m telescope in Calar Alto. Archival X-ray
observations of some specific fields are included. VLBI observations of
an interesting radio source were performed. We explored possible
scenarios to produce the multi-TeV emission from
MGRO J2019+37 and studied which of the sources could
be
the main particle accelerator.
Results. We present a catalogue of 362 radio sources
detected
with the GMRT in the field of MGRO J2019+37, and the
results of a cross-correlation of this catalog with one obtained at
near-infrared wavelengths, which contains
sources, as well as
with available X-ray observations of the region. Some peculiar sources
inside the
1
uncertainty region of the TeV emission from
MGRO J2019+37 are discussed in detail, including the
pulsar PSR J2021+3651 and its pulsar wind nebula
PWN G75.2+0.1, two new radio-jet sources, the H II
region Sh 2-104 containing two star clusters, and
the
radio source NVSS J202032+363158. We also find that
the
hadronic scenario is the most likely in case of a single accelerator,
and discuss the possible contribution from the sources mentioned above.
Conclusions. Although the radio and GeV pulsar
PSR J2021+3651 / AGL J2020.5+3653
and its
associated pulsar wind nebula PWN G75.2+0.1 can
contribute to the emission from MGRO J2019+37,
extrapolation of the GeV spectrum does not explain the detected
multi-TeV flux. Other sources discussed here could contribute to the
emission of the Milagro source.
Key words: gamma rays: observations - H II regions - infrared: stars - radio continuum: stars - X-rays: binaries
1 Introduction
The Galactic very-high-energy (VHE) -ray sources discovered by the
latest generation of Cherenkov observatories (HESS, MAGIC,
Milagro) are currently an actively studied topic in modern high-energy
astrophysics. Among
the
75 detected
sources, nearly one third remain yet as unidentified. A
significant number of them have extended morphologies on 0.1-1
scales in
the TeV energy band, ensuring that the identification of counterparts
at lower
energies is a very difficult task. The most representative of this new
population
of Galactic sources is TeV J2032+4130, inside whose
error box both
compact and extended radio sources on arcsecond scales were found
(Paredes
et al. 2007). XMM-Newton
observations of this source also detected faint extended X-ray emission
(Horns et al.
2007a).
An addition to the population of extended, unidentified TeV sources was
reported by the Milagro collaboration, following the discovery in the
Cygnus region
of the most extended TeV source known so far
(Abdo
et al. 2007a,b).
The TeV emission from this area covers
several square degrees and includes diffuse emission and at least one
new source,
MGRO J2019+37, located to within an accuracy of 0.4
.
After the
Crab Nebula, MGRO J2019+37 is the strongest source
detected by Milagro. The Tibet AS-
experiment confirmed the detection of
this source by measuring a 5.8
signal compatible with the position of
MGRO J2019+37 (Amenomori
et al. 2008).
On the other hand, VERITAS inferred an upper limit that is compatible
with the Milagro detection for a hard-spectrum extended source (Kieda et al.
2008).
The origin of all these types of emission and their association with
astrophysical sources
is unclear. Although a possible connection with the anisotropy of
Galactic
cosmic rays was proposed (Amenomori
et al. 2006), the TeV -ray flux measured at
12 TeV from the diffuse emission of the Cygnus region (after
excluding MGRO J2019+37), exceeds that predicted by
a conventional
model of cosmic ray production and propagation (Abdo
et al. 2007a). This
strongly infers the existence of hard-spectrum cosmic-ray sources
and/or
other types of TeV
-ray
sources in the region. It is unclear whether
the emission originates in either a single extended source or a
combination
of several point sources. MGRO J2019+37 is
positionally coincident
with the EGRET sources 3EG J2021+3716 and
3EG J2016+3657 (see
Fig. 1).
These sources could represent the GeV counterparts to the TeV
source MGRO J2019+37, which may be a multiple
source. Only one of
them, 3EG J2021+3716, appears in the bright
gamma-ray source list published by the Fermi Gamma-ray Space
Telescope
(Abdo
et al. 2009). Previous observations with AGILE
illustrated its
pulsar nature and inferred an association of this source with
PSR J2021+3651 (Halpern
et al. 2008).
![]() |
Figure 1:
Radio map obtained with the GMRT at 610 MHz (greyscale)
convolved with
a circular restoring beam of 30
|
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To explain steady VHE -ray
emission, hadronic models have been
developed by several authors (e.g., Butt
et al. 2003; Aharonian
& Atoyan 1996; Bordas
et al. 2009; Torres
et al. 2004). The electromagnetic radiation produced
by both hadronic jets from microquasars and Galactic cosmic rays, and
their interaction with the ISM were explored by Bosch-Ramon
et al. (2005). The interaction
between the high energy protons, accelerated at the jet termination
shock, and
the interstellar hydrogen nuclei produces charged and neutral pions (
,
and
); the first
set will decay to electrons and positrons and
the second set to photons. The primary radiation,
-decay
photons, is
in the
-ray
band, but the secondary particles can produce significant
fluxes of synchrotron (from radio frequencies to X-rays) and
bremsstrahlung
emission (from soft
-rays
to the TeV range), and in general lower efficiency, inverse Compton
(IC) emission by interaction with ambient
infrared photons. Detectable fluxes of extended and steady emission
should be
produced by this mechanism. Other scenarios involve a jet-driven
termination
shock at which relativistic electrons produce synchrotron and TeV
IC emission
(Aharonian &
Atoyan 1998). In this context, X-ray observations provide a
crucial constraint of the IC emission.
To understand the nature of the Milagro source in the Cygnus region, we
performed a multiwavelength campaign comprising a deep radio survey at
610 MHz using the Giant Metrewave Radio Telescope (GMRT)
interferometer
covering the
MGRO J2019+37 field,
near-infrared observations in the
band using the
3.5 m telescope at
Calar Alto of the central square degree, and archival X-ray data.
This paper is organized as follows. In Sect. 2, we report on previous radio surveys of the Cygnus region, while in Sect. 3 we present the GMRT survey and the results obtained. In Sect. 4, we provide an overview of the near-infrared survey, and in Sect. 5 we report on the cross-correlations both between our GMRT survey and the near-infrared survey, and between the GMRT survey and previous X-ray observations. We comment on particularly interesting sources in Sect. 6 and we discuss their possible contribution to the TeV emission of MGRO J2019+37 in Sect. 7. We finish with our conclusions in Sect. 8.
2 Previous radio surveys of the Cygnus region
At radio frequencies, the Cygnus region has been imaged many times,
sometimes as
part of Galactic surveys. However, these studies were carried out at
poor
angular resolution and/or a relatively high limiting flux density. Some
of the
most representative of previous surveys are: the Canadian Galactic
Plane Survey
(CGPS) performed with the Synthesis Telescope at the Dominion Radio
Astrophysical Observatory (DRAO) at 408 and 1420 MHz, with
angular resolutions
of 5
3 and 1
6, and limiting flux densities
of 9 and 1 mJy,
respectively, at declination of +40
(Taylor et al.
2003); the Westerbork
Synthesis Radio Telescope (WSRT) 327 MHz survey with an
angular resolution of
1
and a limiting flux density of 10 mJy (Taylor et al.
1996); and the DRAO 408
and 1430 MHz survey with angular resolution of
and
,
respectively, and limiting flux densities of
150 and 45 mJy, respectively (Wendker
et al. 1991). The most recent survey of this
region is the WSRT 350 and 1400 MHz continuum survey of the
Cygnus OB2
association, with angular resolutions of
and
,
and limiting flux densities of 10-15 and 2 mJy,
respectively (Setia Gunawan
et al. 2003). The WSRT survey does not cover the
MGRO J2019+37 field.
3 GMRT 610 MHz Radio Survey
3.1 Observations
The MGRO J2019+37 region was observed with
wide-field deep radio
imaging at 610 MHz (49 cm) using the GMRT, located in
Pune (India). We designed
an hexagonal pattern of 19 pointings to cover the region of about
centred
on the MGRO J2019+37 peak of
emission. The observations were carried out in July 2007, but were
affected by a series of power failures in the array and compensatory
time was scheduled in August 2007.
The flux density scale was set using the primary amplitude calibrators 3C 286 and 3C 48, which were observed at the beginning and end of each observing session. On the other hand, phase calibration was performed by repeated observations of the nearby phase calibrator J2052+365. Each pointing was observed for a series of scans to achieve a good coverage in the uv plane, the total time spent on each field being 45 min. The total effective time amounts to 20 h.
Observations were made in two 16-MHz upper and lower sidebands (USB and LSB) centered on 610 MHz, each divided into 128 spectral channels. The data of each side-band were separately edited with standard tasks of the Astronomical Image Processing System (AIPS) package. There were no major radio frequency interference (RFI) problems. However, we did find that narrow band RFI affected a few channels across the band, which were completely flagged. Once poor antennas, baselines, or channels were removed, the bandpass correction was used to extend the calibration to all channels. After the bandpass calibration, the central channels of each sideband were averaged, leading to a data file of 5 compressed channels, of a bandwidth small enough to avoid bandwidth smearing problems in our images. Standard calibration for continuum data was performed beyond this point. At the end of the self-calibration deconvolution iteration scheme, we combined both USB and LSB images of each pointing and mosaicked the entire region using the AIPS task FLATN.
We produced different maps of between high and low angular resolution
of the GMRT
mosaic. Our highest quality image has an rms of
0.2 mJy beam-1 with a 5
resolution because of the long baselines of the GMRT. A low angular
resolution
version was also produced using a restoring beam of 30
to enhance the extended radio sources in the field. This map has an
rms of 0.5 mJy beam-1.
3.2 Results
Figure 1 shows a low angular resolution radio image of the MGRO J2019+37 field, together with the position of sources at other wavelengths. The location of MGRO J2019+37 is consistent with those of the EGRET sources 3EG J2016+3657 and 3EG J2021+3716. The first of them is positionally coincident with the blazar-like source B2013+370 (G74.87+1.22) (Halpern et al. 2001; Mukherjee et al. 2000), although this blazar is well outside the inner box of MGRO J2019+37. The second is marginally coincident with the pulsar wind nebula PWN G75.2+0.1 (Hessels et al. 2004). High-energy gamma-ray pulsations originating in the pulsar were detected by AGILE and Fermi (Abdo et al. 2009; Halpern et al. 2008). There are other known strong and/or extended radio sources in the field, such as the brightest one inside the MGRO J2019+37 center of gravity box, NVSS J202032+363158, and the H II region Sh 2-104 (also known as Sh 104). Other interesting sources not obvious at first glance become evident when considering the whole field in detail. Some of them display a resolved morphology, and in Sect. 6 we discuss these objects in more detail.
3.3 Radio catalogue
We applied the SEXtractor automatic procedure (Bertin &
Arnouts 1996) to our
5
resolution mosaic (with pixel size of 1
)
to produce a list of
sources with peak flux density higher than about ten times the local
noise
after primary beam correction. Objects with less
than 5 connected pixels above
the threshold were not included. The output was visually inspected
and all candidate detections inferred to be false (i.e., deconvolution
artifacts near bright sources) were simply deleted by hand. We used the
local background analysis in SEXtractor to take into account the
uneven background because of beam response effects. Considering the 5
beam
size of the mosaic that we used, and the signal-to-noise ratio that we
required for
detection, we estimate that the positions obtained have a typical
uncertainty
of 0
5
or smaller.
The resulting list, considered to be very reliable although not
complete at the lowest flux density levels, contains 362 radio
sources. Among them, 203 are
fainter than 10 mJy and the majority were previously
undetected at radio
wavelengths. We present the catalogue in Table 2 of the online
material accompanying this paper. The first and second columns provide
the
catalogue number and the source name. The third and fourth columns give
the
J2000.0 position in right ascension order. The fifth and sixth columns
provide
the peak flux density and the local noise, respectively. The seventh
and eighth
columns list the integrated flux density and its error. Uncertainties
quoted
for the peak and integrated flux densities are based on the formal
errors of
the fit and allow the reliability of the detection to be judged.
However,
they do not include the error due to primary beam correction as a
function of angular distance to the phase centre because of unknown
antenna
offsets, which is estimated to be around 10% of the flux density values
(see for instance Paredes
et al. 2008). In Fig. 2, we show
the
source distribution histogram as a function of .
![]() |
Figure 2:
Number of sources versus
|
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4 Near-infrared survey
We also carried out a near-infrared (NIR) survey of the central square
degree of the region using the OMEGA2000 wide field camera
(
)
on the 3.5 m telescope at Centro Astronómico
Hispano Alemán (CAHA) in Calar Alto (Spain) on
25 September 2007. This
instrument consists of a Rockwell HAWAII2 HgCdTe detector with
pixels
sensitive from 0.8 to 2.5
m. The observations were performed in the
-band
(2.15
m)
to minimize the interstellar
absorption. Individual frames were sky-subtracted, flat-field
corrected, and
then combined into a final mosaic using the AIPS
task FLATN. The
ensamble of
pointings covers almost completely the center of
gravity and uncertainty region of the TeV emission from the source
MGRO J2019+37. The average limiting magnitude across
the mosaic is
mag, and the total
field of view is
.
Astrometric solutions for the final frames were
determined within
0
1
by identifying about twenty reference stars in
each pointing, for which positions were retrieved from the 2MASS
catalogue
(Skrutskie
et al. 2006). A catalogue of
315 000 near-infrared
sources was
produced using the SEXtractor package.
5 Radio, near-infrared, and X-ray cross-correlation catalogue
We performed a cross-correlation between the radio and near-infrared
source
catalogues. Considering the 0
5
uncertainty in the radio positions and the
0
1
uncertainty in the NIR ones, we used a conservative maximum
offset of 0
6
for associations (neglecting systematics between both
catalogues). There are 42 of the 362 detected radio sources inside the
area
imaged in the near infrared. A total of 6 of these 42 sources have a
near-infrared counterpart candidate within 0
6
of their radio position.
Their magnitudes are listed in the ninth column of Table 2 of
the online material. The chance coincidence probability of finding a
NIR source
closer than 0
6
to a given radio source is estimated to be the number of NIR
sources multiplied by the area of the uncertainty in positions occupied
by the
42 radio sources, divided by the total area of the region:
.
Therefore, of the six radio sources with NIR counterpart, we expect
that one of
them is a random coincidence.
We also obtained source lists of all X-ray observations of the region
performed by Chandra and XMM-Newton,
computed by the
celldetect
and edetect_chain
tasks from CIAO 4.0 and SAS 8.0, respectively. A
total of 41 of the 362 radio sources are located in fields
observed in X-rays, which cover an area of 314 arcmin2(1 130 973 arcsec2)
and contain 519 X-ray sources. We found that 5
of the 41 radio sources have an X-ray counterpart candidate
within 5
(the typical uncertainty for XMM-Newton). Their
X-ray fluxes are listed
in the tenth column of Table 2 of the online material. The
chance coincidence probability of finding a radio source closer than 5
to a
given X-ray source is estimated to be the number of radio sources
multiplied by
the area of the uncertainty in positions occupied by the 5 X-ray
sources,
divided by the total area of the region:
arcsec2)=1.4.
Therefore, of the five X-ray sources with radio
counterpart we also expect that one of them is a random coincidence.
A single triple radio/near-infrared/X-ray coincidence has been found (source number 115 in Table 2 of the online material).
6 Individual sources in the MGRO J2019+37 field
The most interesting radio sources that appear in the uncertainty region of the TeV emission (red box in Fig. 1) are described below.
6.1 PSR J2021+3651 / PWN G75.2+0.1
The radio pulsar PSR J2021+3651 has a rotation
period P=0.104 s, a
characteristic age of kyr,
and a spin-down luminosity
erg s-1.
It is coincident with the unidentified
source GeV 2020+3658 (Roberts
et al. 2002), which overlaps with the EGRET
source 3EG J2021+3716 (see Fig. 1). Chandra
observations of this pulsar detected a
pulsar
wind nebula named PWN G75.2+0.1 (Hessels
et al. 2004). Chandra
observations of the pulsar and its PWN detected rings and jets around
PSR J2021+3651, and inferred a distance to the
pulsar of 3-4 kpc
(Van Etten
et al. 2008), in contrast to the 12 kpc
implied by the pulsar
dispersion measure (Roberts
et al. 2002). XMM-Newton
observations show
emission extending to a distance of
10-15 arcmin, whereas
radio observations with
the VLA at 1.4 GHz show a radio nebula coincident with the
X-ray extension
(Roberts
et al. 2008).
AGILE detected the source
AGL J2020.5+3653 at energies above
100 MeV range, which shows pulsations and was associated with
the pulsar
PSR J2021+3651 (Halpern
et al. 2008). The photon spectrum of the source
can be fitted by a power-law of photon index
in the
range 100-1000 MeV, while a turndown is seen above
1.5 GeV. This source
appears as 1AGL J2021+3652 in the first AGILE
catalog of high
confidence gamma-ray sources (Pittori
et al. 2009). Fermi also
detected
the source 0FGL 2020.8+3649 in positional
coincidence with the pulsar
(Abdo
et al. 2009).
We found neither a (low-frequency) radio nor a near-infrared source at
the
position of PSR J2021+3651. The nearest
near-infrared source is at a
distance of 3
9
and has a
magnitude of 17.3. In the
radio, from our 610 MHz GMRT data we can establish an upper
limit to
any possible point-like counterpart of 1.0 mJy by multiplying
the background
emission level by a factor of 5. The radio flux density of the extended
emission found with the
VLA at 1.4 GHz amounts to
700 mJy in an area of
about 100 arcmin2,
which for a uniform distribution yields 7 mJy arcmin-2.
On the other hand,
the rms of our low-resolution radio map at 610 MHz shown in
Fig. 1,
with a beam size of 30
,
is 0.5 mJy beam-1.
This provides a conservative 5-
upper limit of either 2.5 mJy beam-1
or
9 mJy arcmin-2. This upper
limit implies that if the radio emission is
uniformly distributed, its spectral index must be above -0.3. This
value is
compatible with the radio emission being produced by the synchrotron
mechanism,
as expected in this nebula.
6.2 Jet-like radio sources
We discovered two jet-like radio sources located well inside the
uncertainty region of MGRO J2019+37. Their J2000.0
positions are
,
(source
A) and
,
(source B).
In Fig. 3,
we show a
GMRT high resolution image of each of them superimposed on the
near-infrared
image. Both sources appear to be unresolved in the NVSS
1.4 GHz catalogue
(Condon et al.
1998). Based on our GMRT survey, the NVSS survey, and the
Westerbork Northern
Sky Survey (WENSS; Rengelink
et al. 1997) data at 327 MHz, we estimate a
spectral index of
for source A, clearly indicating a
non-thermal nature. It is interesting to note that the source is not
detected
in the VLA Low-Frequency Sky Survey (VLSS; Cohen et al.
2007) at 74 MHz, with a
3-
upper limit of 1.2 Jy. With the spectral index above, we would
expect
a flux density of 1.9 Jy, clearly indicative of a turnover at
lower radio frequencies, which could be produced by intrinsic
self-absorption
or by Galactic foreground free-free absorption. The GMRT and NVSS data
for
source B provide a spectral index of -
,
compatible with the
non-detection in WENSS, and suggesting a non-thermal nature for this
radio
source.
![]() |
Figure 3:
Radio and near-infrared image composition of the jet-like radio
sources A ( top) and B ( bottom).
The GMRT radio contours are superimposed on the |
Open with DEXTER |
Source A (Fig. 3-top) shows a double-sided morphology, sources #141 and #142 in Table 2 of the online material, with a slight bending towards the south-east. This structure resembles ones typically seen in radio galaxies with a non-negligible pressure from the intergalactic medium. Unfortunately, there is no clear extended NIR counterpart in the axis joining the radio lobes that could be identified with the parent galaxy, and no firm conclusion can be obtained from the present data.
Source B (Fig. 3-bottom)
shows a morphological and spectral
similarity to the radio lobes of the `great annihilator'
1E 1740.7-2942, a microquasar at the Galactic center
(Mirabel
et al. 1992). The two lobes correspond to sources
#193 and #194 in
Table 2 of the online material. We did not detect a radio
core in this source but, as for the one present in
1E 1740.7-2942, it
could have a flat spectrum and the flux density at such a low frequency
is
expected to be very low compared to that of the radio lobes. As can be
seen in the
figure, there are two near-infrared objects close to the central
position of
the source. Their J2000.0 coordinates and magnitudes are: ,
,
,
and
,
,
.
Their proximity
significantly biases the photometry. The bright source is point-like
and offset
from the axis traced by the radio lobes. The faint source is aligned
with the axis and fuzzy, implying that the origin of the double radio
source is most likely a radio galaxy.
Previous ROSAT pointed observations (Obs. Id.
500248P
conducted on 24 October 1993) did not detect any of
these two
radio-jet sources, placing a
3-
upper limit of
erg cm2 s-1
on their
persistent flux in the energy range 0.1-2.4 keV. With the
present data, we
cannot elucidate whether the sources are Galactic or extragalactic,
although
there are hints of their extragalactic nature.
![]() |
Figure 4:
Top: composite radio and near-infrared image
centred on the
Sh 2-104 region. The contours correspond to 10, 20,
35, 55, 80,
100, 125, 155 times the rms noise of 0.3 mJy beam-1
of our GMRT 610 MHz
(49 cm wavelength) image. We overlay our |
Open with DEXTER |
6.3 H II region Sh 2-104
Sh 2-104, also known as Sh 104,
is an optically visible
H II region of
diameter at a distance of
kpc
(Deharveng
et al. 2003). There is a central O6 V star
suspected of being responsible for ionizing the region (Lahulla
et al. 1985). The appearance of
Sh 2-104 in the optical and in the radio bands is
very similar,
although the radio images show the presence of an ultra compact H II (UCHII)
region at the eastern border, which is not visible in the optical image
(Deharveng
et al. 2003). The interaction between the expanding
H II region
Sh 2-104 and the UCHII region may be responsible for
triggered star
formation in the latter, resulting in a deeply embedded young cluster.
This
region has also been detected as a high luminosity (
)
IRAS source.
Our GMRT observations (see Fig. 4) detect a structure similar to that found at 1.46 GHz with the VLA (Fich 1993) and at 1.4 GHz within the NVSS radio continuum survey (Condon et al. 1998).
We also observed Sh 2-104 in the near-infrared -band.
The images obtained are deeper than those from 2MASS. Figure 4
shows our near-infrared images of the field of
Sh 2-104 with the radio
emission contours superimposed. In the eastern region of the ring (to
the left
side), the near-infrared image shows the well known cluster associated
with the
UCHII region, which must contain at least one massive OB star
(Deharveng
et al. 2003). In the central part of the image, the
single O6 V star of
Lahulla
et al. (1985), which corresponds to
2MASS J20174184+3645264
(Skrutskie
et al. 2006),
now appears to be resolved as several point-like objects,
indicative of the presence of a cluster. Therefore, apart from this
ionizing early-type star, the new cluster candidate could also
contribute to the formation of
the H II region (e.g., additional early
type stars, wind shocks).
Furthermore, an elongated arc along the south of
Sh 2-104 as well as
to the east of the UCHII region can be discerned in the NIR images.
These
features may be related to the interaction between the expanding H II
region and the interstellar medium.
Despite its deep coverage at other wavelengths,
Sh 2-104 was poorly explored in the X-ray domain.
Previous X-ray observations of this region
by ROSAT detected a source
(2RXP J201742.3+364513;
Rosat consortium 2000)
located at ,
with
a positional error of
12
,
overlapping with the central star
2MASS J20174184+3645264 and the cluster candidate
(see bottom-right of
Fig. 4).
The count rate of
count s-1
in the energy range 0.1-2.0 keV,
provides a flux of
erg cm-2 s-1
based on the
assumption of a thermal spectrum with a temperature of 1.5 keV
(a typical value
for colliding wind regions). On the other hand, OB stars are
known to be X-ray
sources, presumably because of shocks in their stellar winds (see
Güdel 2004
for a review). According to the complete study by
Berghöfer
et al. (1997) of more than 200 isolated
OB stars detected in ROSAT data,
for an O6 V star, with bolometric luminosity of
erg s-1(Martins
et al. 2005), the corresponding X-ray luminosity is
erg s-1.
Considering a distance of 4.0 kpc to both
Sh 2-104 and its ionizing central star
2MASS J20174184+3645264, the expected X-ray flux is
erg cm-2 s-1,
fully compatible with the detected
X-ray flux from 2RXP J201742.3+364513.
6.4 NVSS J202032+363158
The source NVSS J202032+363158 is the brightest
compact radio source
within the error box of the TeV peak emission of
MGRO J2019+37 in our
GMRT observations. Its flux density at 610 MHz is
833 mJy and has not been
resolved. This source appears in the VLSS at 74 MHz, in the
WENSS at 327 MHz,
in the CGPS at 408 MHz (Taylor et al.
1996), in the Effelsberg survey of the
Cygnus X region at 1420 MHz (Wendker
et al. 1991), and in the Green
Bank 4.85 GHz
northern sky surveys 87GB (Gregory &
Condon 1991) and GB6 (Gregory
et al. 1996). In Table 1, we
summarize the detected flux densities within these
surveys, and we plot the corresponding spectrum in Fig. 5.
Assuming a stable flux density, the radio spectrum of this source can
be
described by ,
and it is, therefore, clearly a non-thermal emitter. Despite very low
frequencies being sampled, no evidence of the turnover frequency below
1 GHz is
obvious from this simple power-law fit.
By inspecting of the NRAO archives, we found a previous VLA snapshot
(6 min on
source) of this radio source at the 20 cm wavelength in
B configuration (providing a nominal synthesized beam of 4
)
observed on 25 March 1989.
This observation was calibrated using standard AIPS
tasks, including phase self-calibration. A uniformly weighted image is
shown in
Fig. 6-left.
As can be seen, this radio source is resolved,
displaying a one-sided radio jet extending a few arcsec towards the
north,
with a core component of
250 mJy and a
secondary component of about
70 mJy. To enhance the compact structure of the source, we
obtained an
image for the longest baselines of the same VLA run, using a uv-range
of
30-50 k
.
The image, shown in Fig. 6-right,
clearly shows
a compact core and a secondary component, with peak flux densities
of 170 and
20 mJy, respectively, resembling the large-scale jet of a
microquasar.
To explore the source at higher angular resolutions, we observed the
core of
NVSS J202032+363158 at 1.6 GHz
(18 cm wavelength) with the European
VLBI Network in eVLBI mode (eEVN). This is a technique in which the
signals from
distant radio telescopes are directly streamed into the central data
processor
for real-time correlation, instead of being recorded on disk or tape.
The
observation took place on 3 March 2007
from 5:00 to 13:00 UT (centered on
MJD 54163.375), and was performed using 6 antennas: Cm, Mc,
Jb-2, On-85, Tr,
and Wb. Scans on NVSS J202032+363158 were
interleaved with scans on
the compact phase calibrator J2015+3710, with a 6-min cycle time
(66 s on
the calibrator and 246 s on the source). The data were
recorded using dual
polarization and 2-bit sampling, at 256 Mbps. A total
bandwidth of 32 MHz per
polarization was provided by 4 sub-bands. The e-VLBI data were
processed at the
Joint Institute for VLBI in Europe (JIVE) correlator in real time,
using an
integration time of 2 s. The target source was correlated with
the position
obtained from the VLA-B (30-50 k)
data:
and
,
for a total maximum uncertainty of 100 mas. During
observations, we experienced synchronization problems and the
correlation had to be
restarted several times. A few antennas were dropped out of the
correlation
jobs during the gaps used for measuring the system temperatures. Due to
these
disconnections, part of the data, which is not recorded onto disks for
these
experiments, was lost during the correlation, and the true on-source
time is
estimated to be around 3 hours.
We performed the post-correlation data reduction using the AIPS
software package and Difmap. We applied
ionospheric corrections to the
visibility data, and the system temperatures were used to obtain the a
priori
visibility amplitude calibration. All stations produced fringes with
the 1-Jy
phase calibrator, situated at 1
2 from the target, and we
therefore transferred
the solutions for the phases to the target source. We improved the
amplitude
calibration using correction factors for each antenna obtained from the
self-calibration of J2015+3710. Self-calibration of the
NVSS J202032+363158 data was impossible because of
the lack of bright
sources in the primary beam of the antennas. The phased-referenced
natural-weighted image that we obtained had a synthesized beam of
mas
at a position angle of 30
3, and an rms noise of
0.20 mJy beam-1. No
significant detections were found within a distance of 5
from the
correlated phase center.
There is no near-infrared counterpart candidate to
NVSS J202032+363158. The nearest sources are both at
4
1,
with
magnitudes of 14.2 and 17.3 in the
-band.
Table 1: Non-simultaneous flux density measurements of the source NVSS J202032+363158 obtained from different surveys.
![]() |
Figure 5: Radio spectrum of NVSS J202032+363158 based on the flux densities compiled in Table 1. The straight line is a simple power-law fit. |
Open with DEXTER |
![]() |
Figure 6:
(Left): image of the source
NVSS J202032+363158 at
21 cm obtained using uniform weights on B-configuration VLA
data. The source is
resolved, displaying a one-sided radio jet extending a few arc-sec
towards the
north, with a core component of |
Open with DEXTER |
7 Could any of the selected individual sources power the TeV emission from MGRO J2019+37?
MGRO J2019+37 covers a sky region of approximately .
The extended emission could be produced by either a
single powerful accelerator, or by the superposition of several
point-like
sources. Although we focus on the individual sources presented in
Sect. 6,
we cannot exclude some of the additional radio sources listed in
Table 2 of the online
material being responsible for, or contributing to, the Milagro source.
If MGRO J2019+37 is a single extended source, and
not a
combination of different sources, the origin of the
>12 TeV emission is
likely to be hadronic. The time required to fill a region of a size of 1
(or (1-5
cm at
2-10 kpc distance) with electrons of
100 TeV
by means of diffusion is
![]() |
(1) |
where R20=R/1020 cm is the source size, and B-6=B/10-6 G is the ISM turbulent magnetic field. For realistic ISM densities of



![]() |
(2) |
For



Once the most probable emission scenario is decided, we will be able to see whether the different objects proposed in Sect. 6 could act as the accelerator.
The spin-down luminosity of PSR J2021+3651 is
marginally in agreement
with the energetic requirements stated above. Nevertheless, for this
object to
act as the accelerator, most of this luminosity should be in the form
of protons
(as in, e.g., Horns et al.
2007b). In addition, the accelerated protons should
escape the 10'
nebula in a time shorter than or equal to the age of the
pulsar,
17 kyr,
which may not be possible if the turbulent magnetic
field in the nebula reaches value of several 10
G or higher.
On the other
hand, the turndown in the AGILE
GeV spectrum questions the association of
AGL J2020.5+3653 as the only counterpart to
MGRO J2019+37. Extrapolation of the last two data
points
in the spectrum shown
in Halpern
et al. (2008) provide a flux at 20 TeV a
factor of 3500 below the
reported MGRO J2019+37 flux (Abdo
et al. 2007b). Even ignoring the
turndown and fitting the entire spectrum with a single power-law, there
is still
a one order of magnitude difference. Therefore, if no additional
components are
present in the GeV-TeV spectrum of
PSR J2021+3651/PWN G75.2+0.1,
this source alone can hardly
explain the multi-TeV emission from MGRO J2019+37.
The massive star and the star-forming region (MSR; SFR) associated with
the H II region
Sh 2-104 could be responsible for the extended
Milagro source if they
were capable of injecting 1037 erg s-1
in the form of relativistic
protons into their surroundings. Assuming an efficiency of a 10% for
the kinetic
energy converted to non-thermal proton energy in the shocks present
inside the
H II/SFR region, about 100 massive
(proto)stars
producing jets or winds with velocities of
108 cm s-1
and mass-loss rates of
would be required to reach the needed proton luminosities. It seems
unlikely that Sh 2-104 can harbor such a high number
of massive (proto)stars.
However, Sh 2-104 may be part of a larger MSR or SFR
that have not yet been detected, and in that case, the larger whole MSR
or SFR could represent the emitter
of the whole Milagro source through wind collisions or jet/medium
interactions (see, e.g., Romero
2008; Torres et al.
2004), respectively. In
this scenario, thermal free-free radio emission from the whole SFR
would be
expected. The non-detection of this SFR in our GMRT observations could
be
explained by free-free absorption in the ionized regions and the
surrounding material of the MSR/SFR, although the development of the
particular details of this scenario are beyond the scope of this work.
Observations searching for maser emission with instruments such as Apex
or
Nanten could help us to detect this hypothetical star-forming region.
We note that the accelerator itself might be outside MGRO J2019+37, as in the case of the stellar cluster Berkeley 87 mentioned in Abdo et al. (2007a). This cluster could accelerate the protons that would then escape from it diffusing towards, and ultimately interacting with, a denser region located near the Milagro source best-fit model position.
We found three non-thermal radio sources with jet-like
structures in
the field of MGRO J2019+37: sources A and B, and
source
NVSS J202032+363158. Although some arguments support
the
extragalactic nature of sources A and B, we cannot exclude the
possibility that they are Galactic in nature. VLBI observations of
NVSS J202032+363158 provide an upper limit
of 1 mJy beam-1 to the flux
for a beam size of 20 mas.
Therefore, this source
did not exhibit a compact core during our observations. It was either
completely
resolved or is a variable radio source, since no radio emission is
expected in the
high/soft state of microquasars. In any case, these three sources could
be
hadronic microquasars whose jets would interact with the ISM
accelerating
protons (e.g., Bordas et al.
2009; Heinz
& Sunyaev 2002). The accelerated protons may escape
from the accelerating region colliding with the surrounding regions of
the ISM,
rendering very high-energy emission (e.g., Bosch-Ramon
et al. 2005). From the
energetic point of view, although these sources could explain the
Milagro source, the
lack of clear X-ray counterparts needs to be explained, if accretion is
taking
place in these objects. It might be the case that accretion is
inefficient in
producing X-rays (as could be the case in LS 5039;
e.g., Bosch-Ramon
et al. 2007). Finally, a microquasar located outside
the Milagro region
could be powering the multi-TeV radiation.
The constraint on a hadronic origin for the Milagro emission does not apply if the source consists of different accelerators/emitters. In this case, several leptonic emitters, which may or may not coincide with (some of) the sources discussed here, could be behind MGRO J2019+37.
From this analysis, we conclude that several objects should be
considered when trying to understand the origin of
MGRO J2019+37, although the nature
of the accelerator/emitter remains uncertain. Insights into this
question could be provided by further multiwavelength studies, by
future
imaging atmospheric Cherenkov telescopes (MAGIC-II, HESS-II), and by ,
which should be able to constrain the source position and
morphology more tightly, and explore in detail its physics by obtaining
spectral
information across a broad wavelength range. Finally, neutrino
detections with future neutrino instruments
could provide additional evidence to support the hadronic scenario.
8 Conclusions
We have carried out a deep radio survey of about 6 square degrees region in the direction of MGRO J2019+37, and a near-infrared survey of the central square degree. This has provided a catalogue of 362 radio sources and a catalogue of 315 000 NIR sources. The radio and NIR data presented here detect a large number of previously unknown sources and shed additional light on known objects. We have found that if a single accelerator is powering MGRO J2019+37, the most likely origin of the multi-TeV emission is hadronic in nature. We have shown that the extrapolation of the spectrum of the pulsar AGL J2020.5+3653 does not explain the detected flux from MGRO J2019+37. This indicates either that there is an additional component in the GeV-TeV spectrum of the pulsar and/or that other sources, such as those discussed here, could contribute to the emission of the Milagro source. The results presented in this paper may be useful in interpreting future data provided by the Fermi satellite of the gamma-ray sources in this remarkable region of the Galactic plane. The physical understanding of the most relevant sources in the field is currently a work in progress, in addition to the analysis of new XMM-Newton and AGILE observations.
AcknowledgementsWe thank the staff of the GMRT who have made these observations possible. GMRT is run by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research. Based on observations collected at the Centro Astronómico Hispano Alemán (CAHA) at Calar Alto, operated jointly by the Max-Planck Institut für Astronomie and the Instituto de Astrofísica de Andalucía (CSIC) This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the NASA and NSF in the USA. e-VLBI developments in Europe are supported by the EC DG-INFSO funded Communication Network Developments project 'EXPReS' Contract No. 02662. The European VLBI Network is a joint facility of European, Chinese, South African and other radio astronomy institutes funded by their national research councils. We acknowledge support by DGI of the Spanish Ministerio de Educación y Ciencia (MEC) under grants AYA2007-68034-C03-01, AYA2007-68034-C03-02 and AYA2007-68034-C03-03, FEDER funds and Junta de Andalucía under PAIDI research group FQM-322. J.M. was supported by the Spanish Ministerio de Ciencia e Innovacion (MICINN) under fellowship BES-2008-004564. M.P. and M.R. acknowledge financial support from MEC and European Social Funds through a Ramón y Cajal research contract. V.B.-R. gratefully acknowledges support from the Alexander von Humboldt Foundation. P.B. was supported by the DGI of MEC (Spain) under fellowship BES-2005-7234. G.E.R. is supported by the Argentine Agencies CONICET (PIP 5375)and ANPCyT (PICT 03-13291). We thank the anonymous referee for his useful comments.
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Footnotes
- ...
region
- Table 2 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/507/241
- ...
- Member of CONICET.
All Tables
Table 1: Non-simultaneous flux density measurements of the source NVSS J202032+363158 obtained from different surveys.
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