# Multi-wavelength study of the MGRO J2019+37

HOU Chao1;1) CHEN Song-Zhan1 YUAN Qiang1
CAO Zhen1 HE Hui-Hai1 SHENG Xiang-Dong1<EMAIL_ADDRESS>1 Institute of High
Energy Physics, Chinese Academy of Sciences, Beijing 100049, China

###### Abstract

MGRO J2019+37 within the Cygnus region is a bright and extended source
revealed by Milagro at 12-35 TeV. This source is almost as bright as Crab
Nebula in northern sky, while it is not confirmed by ARGO-YBJ around TeV. Up
to now, no obvious counterpart at low energy wavelengths has been found.
Hence, MGRO J2019+37 becomes mysterious and its VHE $\gamma$-ray emission
mechanism is attractive. In this paper, a brief summary of the multi-
wavelength observations from Radio to $\gamma$-ray is presented. All the
available data from the XMM-Newton and INTEGRAL at X-ray, and the _Fermi_ -LAT
at $\gamma$-ray bands were used to get constraint on its emission flux at low
energy wavelengths. Then, its possible counterparts and the VHE emission
mechanism are discussed.

###### keywords:

MGRO J2019+37, multi-wavelength, VHE $\gamma$-ray, radiation mechanism

###### pacs:

9

8.70.Rz, 95.55.Ka, 95.30.Gv

00footnotetext: $\scriptstyle\copyright$2013 Chinese Physical Society and the
Institute of High Energy Physics of the Chinese Academy of Sciences and the
Institute of Modern Physics of the Chinese Academy of Sciences and IOP
Publishing Ltd

##  1 Introduction

Cygnus region is an active massive star formation and destruction portion on
the Galactic plane, with coordinate range of (l$\in[65^{\circ},85^{\circ}]$,
b$\in[-2^{\circ},+2^{\circ}]$) and distance of 1$-$2 kpc from us. It is the
brightest diffuse $\gamma$-ray source in the northern sky as revealed by
_Fermi_ -LAT at GeV, ARGO-YBJ at TeV and Milagro at 15 TeV [2, 3, 4]. As
containing a great deal of molecular clouds and being rich in potential
cosmic-ray acceleration sites, such as Wolf-Rayet stars, OB associations
andsupernova remnants, the Cygnus region becomes a hot land for scientists to
study the origin of cosmic ray [9, 5].

MGRO J2019+37 detected by Milagro within the Cygnus region is towards the Cyg
OB1 association. It is the brightest source of the three new extended sources
discovered by Miagro experiment when it surveyed the northern Galactic
plane[10]. The extension is $\sigma$= $0.32^{\circ}\pm 0.12^{\circ}$ for a
symmetric two-dimensional Gaussian shape. Its measured flux is about 80% Crab
unit at 20 TeV [11]. This source is suspected to be associated with the GeV
pulsar J2021+3651 [12]. About 0.9∘ away from MGRO J2019+37, Tibet AS$\gamma$
collaboration reported a preliminary 5.8$\sigma$ excess in [13]. While only
marginal signal was reported in their later formal result [6, 7].

The ARGO-YBJ experiment is a full coverage extensive air shower (EAS) array
with a large field of view (FOV) at a high altitude of 4300 m. The threshold
is around 300 GeV, which is much lower than any previous EAS arrays. The other
two bright extended sources discovered by Milagro, i.e., MGRO J2031+41 and
MGRO J1908+06, have been confirmed by ARGO-YBJ with significance greater than
5$\sigma$[9, 14, 8]. MGRO J2031+41 is also located in Cygnus region just
nearby MGRO J2019+37. The energy spectra of these two sources measured by
ARGO-YBJ are consistent with that measured by Milagro. Besides these, ARGO-YBJ
detected another extended source HESS J1841$-$055 [15]. Unexpectedly, ARGO-YBJ
detected little signal from the brightest Milagro source MGRO J2019+37, and
the derived flux upper limits at the 90% confidence level (c.l.) are lower
than the Milagro flux at energies below 5 TeV [9]. The ARGO-YBJ upper limits
do not conflict with the Milagro $1\sigma$ error region in a new analysis
applied to the Milagro data from 2005 to 2008 [16], while they constrain the
flux should be lower than the best-fitting value derived by Milagro. In such a
situation, a peak structure is formed with the energy as high as about 10 TeV.

VERITAS is a narrow FOV imaging atmospheric Cherenkov telescope with excellent
energy and angular resolution ranging from hundreds of GeV to multi TeV. In
2007, VERITAS had surveyed the Cygnus region with a sensitivity of 6.3% Crab
unit, but no emission from MGRO J2019+37 was detected [17]. In 2010, with
further deep observations better than 1% Crab unit, VERITAS revealed a complex
TeV emitting structure at the position of MGRO J2019+37 which is likely
powered by multiple sources [18].

The MGRO J2019+37 region was surveyed by the Giant Metrewave Radio Telescope
(GMRT) at the frequency of 610 MHz and the 3.5 m telescope in Calar Alto at
the near-infrared $K_{s}$-band. A catalogue of 362 radio sources and $\sim
3\times 10^{5}$ near-infrared sources were detected [19]. Some peculiar
sources are noticeable, such as the pulsar PSR J2021+3651, two new radio-jet
sources, the radio source NVSS J202032+363158 and the HII region Sh 2-104
containing two star clusters.

The MGRO J2019+37 region was also observed by XMM-Newton at 1$\sim$8 keV with
a high sensitivity. A $\sim$20’ extended emission around PSR J2021+3651 and an
UCHII region in Sh 2-104 were detected. In the GeV band, EGRET, _Fermi_ -LAT
and AGILE all detected some point sources with this extended region. Among
them, PSR J2021+3651 detected by $Fermi$-LAT is a spin-powered radio pulsar
whose spectrum has a cutoff at 10 GeV.

To sum up, no similar morphology as that of TeV emission is found at radio,
optical, X-ray and GeV $\gamma$-ray bands. Therefore, no definite counterpart
of MGRO J2019+37 is found at low energy bands.

The existing observation by the low energy bands telescopes doesn’t give a
flux constraint especially for the MGRO J2019+37 extended region. In order to
better understanding the TeV emission mechanism, a multi-wavelength
observation, especially at X-ray and $\gamma$-ray bands, is quite necessary.
For such a large extended region, observations for MGRO J2019+37 had better to
be implemented by detectors with wide FOV. In this paper, all the available
data from the INTEGRAL at hard X-ray and the _Fermi_ -LAT at $\gamma$-ray
bands, which are wide FOV, are used to constrain the emission flux from MGRO
J2019+37. In addition, the soft X-ray data from the narrow FOV detector XMM-
Newton are also analyzed. Then, its possible origins and the corresponding
emission mechanism are discussed.

##  2 Multi-wavelength analysis

Since the angular resolution of ground based particle detector is relatively
poor, it is difficult to identify the low energy counterpart of a TeV
$\gamma$-ray source given there are usually more than one low energy sources
in the error box of the TeV source. Variability is a good identifier to find
the counterpart for variable sources. However, it becomes more difficult for a
steady source. In this case, we may need a more detailed study of the
properties of all the observed sources, such as spectrum, flux, pulsation and
even polarization to explore their possible connection with the target source.
In the following, we try to find the low energy counterpart of MGRO J2019+37
and discuss its multi-wavelength emission mechanism.

###  2.1 _Fermi_ -LAT GeV gamma-ray

_Fermi_ -LAT is an imaging high-energy $\gamma$-ray telescope covering the
energy band from 20 MeV to 300 GeV. The angular resolution is about
$3.5^{\circ}$ at 100 MeV, improved to about $0.1^{\circ}$ at 10 GeV [20]. The
FOV of _Fermi_ -LAT covers about 20% of the sky at a time. It scans
continuously, and covers the whole sky every three hours. The LAT data from a
region of interest (ROI) centered on MGRO J2019+37
($304.63^{\circ},36.88^{\circ}$) with a radius of $20^{\circ}$ were downloaded
from Fermi Science Support Center111http://fermi.gsfc.nasa.gov/ssc/. The
observational time is from 4 August 2008 to 4 August 2012.

The data analysis was performed following the standard procedure with a binned
maximum-likelihood method. The model adopted in the likelihood fitting
included the diffuse backgrounds with both the Galactic and isotropic
components, as well as the point sources in 2FGL catalog [2]. Within
$1^{\circ}$ region of MGRO J2019+37, there are three point sources in 2FGL
catalog, which are 2FGL J2021+3651, 2GFL J2018+3626 and 2FGL J2015.6+3709.
2FGL J2021+3651 is identified as a pulsar, and 2FGL J2015.6+3709 is identified
as an active galactic nucleus (AGN). 2GFL J2018+3626 is unidentified, and it
may be the counterpart of MGRO J2019+37.

The left panel of Fig. 2.1 shows the residual $\gamma$-ray counts map of the
sky around MGRO J2019+37, after subtracting the diffuse backgrounds. To reduce
the influence of pulsar PSR J2021+3651, which has a spectral cutoff at about
10 GeV [21], we adopt an energy threshold of 10 GeV to show the counts map and
for the spectral analysis below. The counts map is smoothed with a
$0.3^{\circ}$ width Gaussian kernel. From the counts map we see that 2GFL
J2018+3626 has few photons above $10$ GeV. The spectrum given in 2FGL catalog
of this source is also very soft at high energies. Since other two sources
located in the vicinity of MGRO J2019+37 have been identified as a pulsar and
an AGN, we think that none of these three sources will serve as the
counterpart of MGRO J2019+37. The right panel of Fig. 2.1 gives the residual
map after further subtracting the known 2FGL sources. It shows that there is
not any excess at the location of MGRO J2019+37.

Residual counts map of $\gamma$-ray photons above 10 GeV using the data
observed by _Fermi_ -LAT, after subtracting the diffuse backgrounds (left) and
the diffuse backgrounds together with the known 2FGL sources (right). The
circle shows the target region of MGRO J2019+37 with diameter of $1^{\circ}$,
and the crosses label the known sources in LAT 2FGL catalog. (Color online)

Count rate map of X-ray photons using the data observed by INTEGRAL IBIS/ISGRI
from 2002 to 2012, smoothed with a $0.25^{\circ}$ Gaussian kernel. The left
panel is for 20$-$60 keV, and the right panel is for 60$-$200 keV. The circle
shows the target region of MGRO J2019+37 with diameter of $1^{\circ}$, and the
squares represent the postions of known sources in the INTEGRAL catalog. The
point source located in MGRO J2019+37 circle, IGR J20188+3647, is identified
as a transient point source. (Color online)

Then we add MGRO J2019+37 in the model and re-do the likelihood analysis. MGRO
J2019+37 is modeled with a two-dimensional Gaussian template with
$\sigma=0.35^{\circ}$ as revealed in TeV observation [10]. No strong signal
with significance $>2\sigma$ is found. Therefore we derive the upper limits of
MGRO J2019+37 with _Fermi_ -LAT data. Assuming a power-law index of $2$, we
find the 95% c.l. upper limits of the fluxes are $2.1\times 10^{-10}$ cm-2s-1
for $10-31$ GeV, $8.0\times 10^{-11}$ cm-2s-1 for $31.1-97$ GeV, and
$5.0\times 10^{-11}$ cm-2s-1 for $97-300$ GeV, respectively.

###  2.2 INTEGRAL IBIS/ISGRI hard X-ray

IBIS (Imager on Board the INTEGRAL Satellite) is one of the instruments of the
INTEGRAL X-ray telescope. It works in the energy range from $\sim$15 keV to
several MeV. The FOV of IBIS is $8.33^{\circ}\times 8.00^{\circ}$
($19^{\circ}\times 19^{\circ}$) for fully (50%) coded mode, and the angular
resolution (full width half maximum, FWHM) is $0.2^{\circ}$. There are two
detectors: the Integral Soft $\gamma$-Ray Imager (ISGRI), which is a semi-
conductor array optimized for lower energies (18 keV-1 MeV), and the PIxelated
Ceasium Iodide (CsI) Telescope (PICsIT), which is a crystal scintillator
sensitive for higher energies (175 keV-10 MeV) [22].

The data and standard analysis software OSA10.0 are downloaded from the
INTEGRAL official website222http://www.isdc.unige.ch/integral/archive. In this
analysis we adopt the ISGRI data recorded from 2002 to 2012 for all observing
numbers. Due to the quality of achieved data, we restrict the analysis in the
energy range $20-200$ keV, and divide the data into two bands, 20$-$60 keV and
60$-$200 keV respectively. Fig. 2.1 shows the count rate maps in these two
energy bands. These maps are smoothed with a 0.25∘ width Gaussian kernel. The
circle shows MGRO J2019+37 region with diameter of $1^{\circ}$, and the white
squares shows the positions of the sources discovered by INTEGRAL. IGR
J20188+3647, which is identified as a supergiant fast X-ray transient (SFXT)
source, shows a fast rise (10 minutes) followed by a slow decay (50 minutes)
[23], and thus may be not the counterpart of the extended TeV source MGRO
J2019+37. According to these count rate maps, there is no significant excess
coincident with MGRO J2019+37. We will estimate the upper limits of MGRO
J2019+37 in the hard X-ray band.

A circle with radius of 0.54∘ centered on MGRO J2019+37, which encloses 68% of
the events from MGRO J2019+37 extended area taking into account of the point
spread function (PSF) of ISGRI, is used to calculate the count rate. The
expected background count rate is estimated from six other circular regions
with the same radius but 1.28∘ away from the center of MGRO J2019+37. We use
Helene method [24] to calculate the 95% c.l. upper limit of the count rate.
Since there is no spectral analysis script for extended source analysis in
OSA10.0 software, we adopt a method proposed by Swift-BAT collaboration [25]
to derive the flux of an extended source through comparing the count rate with
that of the standard candle Crab Nebula. The source flux can be calculated by

${\rm source~{}flux=\frac{source~{}count~{}rate}{Crab~{}count~{}rate}\times
Crab~{}flux}$ (1)

in each energy band. To test this method, we apply it to the point sources Cyg
X-1 and Cyg X-3, which are close to MGRO J2019+37 as shown in Fig. 2.1. We
compare the fluxes derived by this method with that derived by the standard
method for point source analysis conducted by the OSA10.0. Although the
spectral shapes of Cyg X-1 and Cyg X-3 are very different from that of Crab
Nebula, the fluxes obtained by the two methods are consistent with each other
within 10%. To estimate the flux upper limit from MGRO J2019+37, a power-law
spectrum with index of 2.0 is assumed in this work, which is very closed to
the spectrum of Crab Nebula. Therefore, the systematic error is expected to be
smaller than 10%. The final 95% c.l. upper limits are $1.4\times 10^{-4}$
cm-2s-1 for $20-60$ keV and $6.0\times 10^{-5}$ cm-2s-1 for $60-200$ keV.

###  2.3 XMM-Newton soft X-ray

XMM-Newton has observed MGRO J2019+37 extended region with the European Photon
Imaging Camera (EPIC). EPIC has one pn and two MOS cameras, which covers the
energies from 0.2 to 12 keV with an energy resolution of 0.15 keV at 1 keV.
Their FOV is 30’, and the on-axis resolution angle is about 6” (FWHM) and 15”
(half-power diameter).

There are four archival observations of the MGRO J2019+37 extended region. Two
of the observations focus on the pulsar wind nebula (PWN) G75.2+0.1. The third
one points to IGR J20188+3647 and the fourth points to MGRO J2019+37. The
pulsar PSR J2021+3651 and its PWN G75.2+0.1, HII regions sh2-104 and WR141
have been detected by EPIC in this extended region. The mosaic image of this
region can be found in Fig. 2.3, which is presented in [26]. There are no
other candidate sources in $1^{\circ}$ region of MGRO J2019+37. The PWN and
HII region might be VHE emitters. Therefore we choose PWN G75.2+0.1 and HII
region sh2-104 for spectral analysis.

XMM-Newton background subtracted and exposure corrected X-ray (1keV-8keV)
mosaic image of the MGRO J2019+37 region, which is presented in [26]. The
central cross and box indicate the Gravity Center and its positional
uncertainty including statistic and systematic errors of the TeV emission from
MGRO J2019+37 [10]. The black cross indicates the position of PWN G75.2+0.1.
This picture also label the HII region Sh 2-104 and the Wolf Rayet star WR
141. North is up and East is left. (Color online)

The spectral fitting results obtained using XMM-Newton data.  source name
model $n_{\rm H}$/($4\times 10^{22}$ cm-2) index flux/($2-10$ keV, erg
cm-2s-1) PWN G75.2+0.1 absorbed power-law 0.31 1.44${}_{-0.17}^{+0.18}$
$2.62\times 10^{-12}$ sh2-104 absorbed power-law 2.70 2.09${}_{-0.73}^{+0.88}$
$4.54\times 10^{-13}$

XMM-Newton MOS2 spectrum for PWN G75.2+0.1.

XMM-Newton pn spectrum for HII region sh2-104.

The data from both the MOS and pn instruments were analyzed using the XMM
Science Analysis Software (SAS333http://xmm.esa.int/sas/) version 13.0.1 with
the most recent calibration files. The event files were created from
observation data files (ODFs) using the SAS tasks epchain and emchain. The
events were then filtered to retain only patterns 0 to 4 for the pn data
(0.2$-$15 keV ) and patterns 0 to 12 for the MOS data (0.2$-$12 keV). The data
were further filtered to remove the time intervals of high background rates.
The observational IDs of the data we finally used for spectral analysis are
0404540101 (MOS2 for G75.2+0.1) and 0510011401 (pn for sh2-104). Ancillary
response files and redistribution matrix files are calculated for the
corresponding detector regions. Then we extract the MOS and pn spectra of the
interested source regions.

For PWN G75.2+0.1, we select the extended region with an ellipse (major radius
6.0’ and minor radius 3.7’), to subtract the central pulsar. The background is
extracted from the off-source region near the source. We employ the X-ray
spectral fitting package XSPEC version
12.7.1444http://heasarc.nasa.gov/xanadu/xspec/ to extract the source spectrum.
An absorbed power-law model is used to fit the data. The best-fit result and
residual of the MOS2 observation of PWN G75.2+0.1 are shown in Fig. 2.3. For
sh2-104, a 0.7’ circular area is selected as the source region. Procedure
similar to that for PWN G75.2+0.1 analysis is adopted to extract the spectrum.
The results are shown in Fig. 2.3. The fitting results are listed in Table
2.3.

##  3 Discussion

The multi-wavelength observational results of MGRO J2019+37 are shown in Fig.
3.1. Here the XMM-Newton results are the de-absorbed power-law spectrum from
PWN G75.2+0.1. According to Milagro and ARGO-YBJ data, we find that the VHE
$\gamma$-ray spectrum peak at $\sim 10$ TeV, which is higher than most of the
known TeV emitters 555http://tevcat.uchicago.edu/. Up to now (February 2014),
only one of the 147 TeV sources shows similar peak energy as MGRO J2019+37,
i.e. the peak energy of the PWN Vela X is around 13 TeV [27].

VERITAS has resolved the emission of MGRO J2019+37 into a complex $\gamma$-ray
emission region, which is likely composed of multiple sources. If these
sources are independent with each other, we will expect that the probability
to have such a special spectrum with peak energy around 10 TeV is very low.
Therefore, it is natural to expect that this complex $\gamma$-ray emission
might have the same origin. MGRO J2019+37 is bright and extended in TeV band,
without counterpart in other wavelengths. All these properties make it be a
mysterious “dark” accelerator to radiate VHE $\gamma$-ray. Understanding its
VHE $\gamma$-ray emission mechanism will be interesting and important.

It is worth noting that the observation periods do not fully overlap for these
detectors. Assuming a distance of 1$-$2 kpc for this source, the angular
extension $\sigma=0.32^{\circ}$ will correspond to a length scale of 5$-$10
pc, which implies a shortest variation time scale of $15-30$ years. Therefore,
we do not expect a significant flux variation over the whole extended region
during the observation time, 2002-2012, of this work.

PWN is a prominent class of VHE $\gamma$-ray sources. G75.2+0.1, PWN of the
pulsar PSR J2021+3651, is a possible candidate to power MGRO J2019+37. The
spin down power is $\dot{E}=3.4\times 10^{36}{\rm erg\ s^{-1}}$. The
$\gamma$-ray flux integrated from 1 to 100 TeV based on Milagro spectrum is
$F_{\gamma}=3.2\times 10^{-11}{\rm erg\ cm^{-2}s^{-1}}$. The estimated
distance of PSR J2021+3651 ranges from 2 to 12 kpc [28, 29]. The $\gamma$-ray
luminosity of this source is:

$\L_{\gamma}=F_{\gamma}\times(4\pi{d}^{2})=1.5\times 10^{34}(d/2\,{\rm
kpc})^{2}{\rm erg\ s^{-1}}.$ (2)

The efficiency of VHE $\gamma$-ray emission to the spin down power is
$L_{\gamma}/\dot{E}=0.44\%(d/2\,{\rm kpc})^{2}$. This is consistent with the
range of the $\gamma$-ray efficiency $\eta_{\gamma}=10^{-4}-0.1$ found for
other PWNs [30].

In the PWN scenario, we could expect a leptonic origin of the multi-wavelength
emission of MGRO J2019+37. The $\gamma$-ray morphology of a PWN might not be
necessarily the same as the X-ray morphology, and the $\gamma$-ray luminosity
can be also much higher than the X-ray luminosity [31]. If the supernova
explosion occurs in an inhomogeneous medium, the resulting asymmetric reverse
shock will push the pulsar to the direction away from the higher density
medium. In such a scenario, the particles responsible for the $\gamma$-ray
could be the “relic” of the PWN, while those responsible for X-ray could be
newly accelerated ones. Therefore, the X-ray image shows displacement compared
with the $\gamma$-ray image. The particle spectrum to produce X-ray may be
also different from that to produce $\gamma$-ray.

It is also possible that the $\gamma$-ray emission is produced by hadronic
cosmic ray interactions, and the X-ray emission is produced by high energy
electrons. In this scenario, there should be more degrees of freedom of the
modeling because there is no direct connection with the multi-wavelength data.
In the following, we will discuss both the leptonic and hadronic models and
explain the multi-wavelength data of MGRO J2019+37.

###  3.1 Leptonic model

A simple leptonic model is constructed to interpret both the X-ray emission
from PWN G75.2+0.1 and $\gamma$-ray emission from MGRO J2019+37. We assume a
uniform distribution of electrons in the vicinity of MGRO J2019+37. VHE
$\gamma$-ray emission is produced by the inverse Compton scattering of
electrons with the interstellar radiation field (ISRF666We adopt the local
results of the ISRF as an approximation.), including the cosmic microwave
background, infrared and starlight [32]. The X-ray emission is produced by the
synchrotron radiation of the electrons, within a confined region surrounding
the PWN where the magnetic field strength is expected to be higher than the
average value in the interstellar medium. A filling factor $f$ is introduced
to describe the fraction of the X-ray emitting volume to the $\gamma$-ray
emitting volume.

Multi-wavelength spectrum of MGRO J2019+37. The shaded region indicates the
best-fit spectrum and the $1\sigma$ uncertainty region as measured by Milagro
[16]. The arrows above 1 TeV show the upper limits derived by the ARGO-YBJ
[9]. The arrows at keV and GeV show the upper limits derived using data
observed by INTEGRAL-IBIS and _Fermi_ -LAT, respectively. The triangles show
the flux of PWN G75.2+0.1 using XMM-Newton data. The solid lines show the two
leptonic model expectations as described in the text. (Color online)

The electron spectrum is assumed to be a power-law function with an
exponential cutoff $dN/dE\propto E^{-\alpha}\exp(-E/E_{c})$, where $N$ is the
electron number, $E$ is the electron energy, $\alpha$ is spectral index and
$E_{c}$ is cutoff energy. Since most of the observations give upper limits, we
cannot put good constraints on the model parameters. We choose two values of
the electron spectral indices $\alpha=1.5$ and $2.0$ for illustration. It is
possible for a PWN to give an electron spectrum harder than $2$, e.g., the
Crab nebula [33]. For $\alpha=1.5$, we have $E_{c}\approx 50$ TeV, the total
energy of electrons above 1 GeV $W_{e}(>1\,{\rm GeV})\approx 3.2\times
10^{46}(d/2\,{\rm kpc})^{2}$ erg, the magnetic field to produce X-ray emission
of G75.2+0.1 $B\approx 30\mu$G, and the filling factor $f\approx 0.3\%$. For
$\alpha=2.0$, we have $E_{c}\approx 90$ TeV, $W_{e}(>1\,{\rm GeV})\approx
8.4\times 10^{46}(d/2\,{\rm kpc})^{2}$ erg, $B\approx 30\mu$G, and $f\approx
0.24\%$. The model expectations are shown in Fig. 3.1.

The model gives a marginal fit to the data. However, even for $\alpha=1.5$,
the synchrotron spectrum seems softer than the XMM-Newton data of PWN
G75.2+0.1. As shown in Table 2.3, the X-ray spectral index is about $1.44$,
which corresponds to an electron spectral index $\alpha\approx 1.9$. The
softening of the expected synchrotron spectrum should mainly due to the cutoff
energy $E_{c}$. It is possible that, if PWN G75.2+0.1 is the acceleration
source of the high energy electrons, the electron spectrum will be harder and
cutoff at higher energies when close to the acceleration source. Then the
electrons diffuse to the entire region of MGRO J2019+37, become softer and
cutoff earlier. The cooling time scale for electrons in the ISRF can be
estimated as $\tau_{\rm cool}\approx 3\times 10^{5}(U_{\rm ISRF}/{\rm
eV\,cm^{-3}})^{-1}(E/{\rm TeV})^{-1}$ yr, where $U_{\rm ISRF}$ is the energy
density of the ISRF, and $E$ is the energy of the electrons. Given the
lifetime of PSR J2021+3651 is about $17000$ yr, the cooling energy of
electrons can be as high as tens of TeV, which is consistent with the required
value to fit the data. We can also estimate the diffusion scale of the
electrons. Assuming a diffusion coefficient of $10^{30}$ cm2s-1 (approximate
value for TeV particles in the Galactic disk [34]), the diffusion length is
estimated to be $\sim 300(D/10^{30}{\rm cm^{2}s^{-1}})^{0.5}(t/17000\,{\rm
yr})^{0.5}$ pc. Such a value seems too large compared with the spatial
extension of MGRO J2019+37. It seems that the VHE $\gamma$-ray emission should
come from newly accelerated electrons if this scenario works. Detailed
modeling will depend on the time-dependent injection and diffusion of the
electrons which beyond the scope of this work. In this scenario we could
expect an energy dependent $\gamma$-ray morphology to be same as that for HESS
J1825$-$137 [35]. Future $\gamma$-ray facilities with higher sensitivity and
angular resolution may test this scenario [36].

The current spin down power of PSR J2021+3651 is $\dot{E}=3.4\times 10^{36}$
erg s-1. For an estimated age of about $17000$ years, the time integrated spin
down energy is higher than $1.8\times 10^{48}$ erg. Thus the energy fraction
transferred to high energy electrons is then $\eta_{e}\approx(2~{}\rm
to~{}5)\%(d/2\,{\rm kpc})^{2}$. It shows that PSR J2021+3651 should be enough
to power the $\gamma$-ray emission of MGRO J2019+37.

An alternative scenario is that the electrons are accelerated in a diffuse
region, by e.g. the ensemble of massive OB association in the Cygnus region
[37]. In this scenario the non-coincidence of the X-ray image and $\gamma$-ray
image can be easily understood. The existence of acceleration in the extended
region of MGRO J2019+37 may also explain the required hard electron spectrum
to reproduce the Milagro measurement and ARGO-YBJ upper limits. However, the
fit to the XMM-Newton spectrum is not good enough in this scenario.
Furthermore, special treatment of the particle diffusion will be also needed
to avoid too large extension of the source region.

###  3.2 Hadronic model

The $\gamma$-ray emission can be also produced through the decay of neutral
pions which is produced in the inelastic collisions between accelerated cosmic
ray nuclei and the ambient interstellar medium. Still we assume exponential
cutoff power law for the spectrum of the accelerated nuclei (protons for
simplicity). The expected $\gamma$-ray spectra for two illustration values of
proton spectral indices, $\alpha_{p}=1.5$ and $2.0$, are shown in Fig. 3.2.
For $\alpha=1.5$ ($2.0$), the adopted cutoff energy is $200$ ($500$) TeV, and
the total energy of protons above 1 GeV is $6.4(16)\times 10^{49}$ $(d/2\,{\rm
kpc})^{2}(n/{\rm cm}^{-3})^{-1}$ erg.

Two hadronic model expectations of the $\gamma$-ray spectrum of MGRO J2019+37,
compared with the data. (Color online)

Fig. 3.2 shows that the hadronic model can barely fit the $\gamma$-ray data.
The exponential cutoff power-law spectrum of protons seems to be too broad
compared with the peak behavior of the VHE $\gamma$-ray spectrum, unless the
proton spectral index is much harder. The required total energy of protons
seems to be higher than the abovely estimated energy released from spin down
of PSR J2021+3651. It is possible that the total energy release of PSR
J2021+3651 is higher than $1.8\times 10^{48}$ erg because the rotation of the
pulsar should be faster in the past. Another possibility is that these protons
were accelerated by the remnant of the supernova which produced PSR
J2021+3651. The total energy of protons seems to be consistent with the
canonical value of $\sim 10\%$ of the total kinetic energy released from a
typical supernova explosion, say $10^{51}$ erg. However, the constraints from
particle diffusion as discussed in Sec. 3.1 is also applied here. A high
density concentration or a low diffusion coefficient around MGRO J2019+37 may
be helpful to avoid the too large extension problem due to diffusion. Similar
to the leptonic scenario, the protons can be also accelerated diffusively in
the extended region of MGRO J2019+37.

##  4 Summary

MGRO J2019+37 is a special TeV $\gamma$-ray source in the Northern sky. This
paper presents a collection of the multi-wavelength observations of MGRO
J2019+37 from X-ray to TeV $\gamma$-ray bands. The available archival data in
the direction of MGRO J2019+37 from XMM-Newton at soft X-ray band, INTEGRAL at
hard X-ray band and _Fermi_ -LAT at GeV $\gamma$-ray band are analyzed. There
is no corresponding extended signal in INTEGRAL and _Fermi_ -LAT data, and the
flux upper limits are obtained. In XMM-Newton data, emissions from the PWN
G75.2+0.1 and the HII region sh2-104 are found. Spectral analyses of G75.2+0.1
and sh2-104 are performed. The possible multi-wavelength radiation mechanism
of the source is discussed. It is shown that a leptonic scenario can
marginally reproduce the X-ray to TeV $\gamma$-ray data. The PWN G75.2+0.1 of
PSR J2021+3651 might be the acceleration source of the high energy electrons.
Although the hadronic scenario fits the data worse, it should not be excluded
due to the lack of high quality data. The diffuse particle acceleration from
e.g. the ensemble of OB associations can also explain the observational
results.

Due to the lack of detailed observations, no favored model can be obtained
right now. To further unveil the puzzle of the dark accelerator MGRO J2019+37,
observations with more sensitive instruments at various bands are needed. In
the nearly future, the new EAS experiments such as HAWC, Tibet+MD and LHAASO
are expected to be able to achieve a more accurate observation of $\gamma$-ray
sources from 40 GeV to 1 PeV. The $\gamma$-ray emission mechanism of this
source is expected to be uncovered in the new era of VHE $\gamma$-ray
astronomy.

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