280 research outputs found
The Supernova Remnant W44: confirmations and challenges for cosmic-ray acceleration
The middle-aged supernova remnant (SNR) W44 has recently attracted attention
because of its relevance regarding the origin of Galactic cosmic-rays. The
gamma-ray missions AGILE and Fermi have established, for the first time for a
SNR, the spectral continuum below 200 MeV which can be attributed to neutral
pion emission. Confirming the hadronic origin of the gamma-ray emission near
100 MeV is then of the greatest importance. Our paper is focused on a global
re-assessment of all available data and models of particle acceleration in W44,
with the goal of determining on a firm ground the hadronic and leptonic
contributions to the overall spectrum. We also present new gamma-ray and CO
NANTEN2 data on W44, and compare them with recently published AGILE and Fermi
data. Our analysis strengthens previous studies and observations of the W44
complex environment and provides new information for a more detailed modeling.
In particular, we determine that the average gas density of the regions
emitting 100 MeV - 10 GeV gamma-rays is relatively high (n= 250 - 300 cm^-3).
The hadronic interpretation of the gamma-ray spectrum of W44 is viable, and
supported by strong evidence. It implies a relatively large value for the
average magnetic field (B > 10^2 microG) in the SNR surroundings, sign of field
amplification by shock-driven turbulence. Our new analysis establishes that the
spectral index of the proton energy distribution function is p1 = 2.2 +/- 0.1
at low energies and p2 = 3.2 +/- 0.1 at high energies. We critically discuss
hadronic versus leptonic-only models of emission taking into account
simultaneously radio and gamma-ray data. We find that the leptonic models are
disfavored by the combination of radio and gamma-ray data. Having determined
the hadronic nature of the gamma-ray emission on firm ground, a number of
theoretical challenges remains to be addressed.Comment: 13 pages, 11 figures, accepted by A&
Interstellar Gas and X-rays toward the Young Supernova Remnant RCW 86; Pursuit of the Origin of the Thermal and Non-Thermal X-ray
We have analyzed the atomic and molecular gas using the 21 cm HI and 2.6/1.3
mm CO emissions toward the young supernova remnant (SNR) RCW 86 in order to
identify the interstellar medium with which the shock waves of the SNR
interact. We have found an HI intensity depression in the velocity range
between and km s toward the SNR, suggesting a cavity in the
interstellar medium. The HI cavity coincides with the thermal and non-thermal
emitting X-ray shell. The thermal X-rays are coincident with the edge of the HI
distribution, which indicates a strong density gradient, while the non-thermal
X-rays are found toward the less dense, inner part of the HI cavity. The most
significant non-thermal X-rays are seen toward the southwestern part of the
shell where the HI gas traces the dense and cold component. We also identified
CO clouds which are likely interacting with the SNR shock waves in the same
velocity range as the HI, although the CO clouds are distributed only in a
limited part of the SNR shell. The most massive cloud is located in the
southeastern part of the shell, showing detailed correspondence with the
thermal X-rays. These CO clouds show an enhanced CO = 2-1/1-0 intensity
ratio, suggesting heating/compression by the shock front. We interpret that the
shock-cloud interaction enhances non-thermal X-rays in the southwest and the
thermal X-rays are emitted by the shock-heated gas of density 10-100 cm.
Moreover, we can clearly see an HI envelope around the CO cloud, suggesting
that the progenitor had a weaker wind than the massive progenitor of the
core-collapse SNR RX J1713.73949. It seems likely that the progenitor of RCW
86 was a system consisting of a white dwarf and a low-mass star with
low-velocity accretion winds.Comment: 19 pages, 15 figures, 4 tables, accepted for publication in Journal
of High Energy Astrophysics (JHEAp
ALMA CO Observations of Supernova Remnant N63A in the Large Magellanic Cloud: Discovery of Dense Molecular Clouds Embedded within Shock-Ionized and Photoionized Nebulae
We carried out new CO( = 1-0, 3-2) observations of a N63A supernova
remnant (SNR) from the LMC using ALMA and ASTE. We find three giant molecular
clouds toward the northeast, east, and near the center of the SNR. Using the
ALMA data, we spatially resolved clumpy molecular clouds embedded within the
optical nebulae in both the shock-ionized and photoionized lobes discovered by
previous H and [S II] observations. The total mass of the molecular
clouds is for the shock-ionized region and
for the photoionized region. Spatially resolved X-ray spectroscopy
reveals that the absorbing column densities toward the molecular clouds are
- cm, which are - times less
than the averaged interstellar proton column densities for each region. This
means that the X-rays are produced not only behind the molecular clouds, but
also in front of them. We conclude that the dense molecular clouds have been
completely engulfed by the shock waves, but have still survived erosion owing
to their high-density and short interacting time. The X-ray spectrum toward the
gas clumps is well explained by an absorbed power-law or high-temperature
plasma models in addition to the thermal plasma components, implying that the
shock-cloud interaction is efficiently working for both the cases through the
shock ionization and magnetic field amplification. If the hadronic gamma-ray is
dominant in the GeV band, the total energy of cosmic-ray protons is calculated
to be - erg with the estimated ISM proton density
of cm, containing both the shock-ionized gas and
neutral atomic hydrogen.Comment: 18 pages, 4 tables, 8 figures, accepted for publication in The
Astrophysical Journal (ApJ
Molecular Clouds associated with the Type Ia SNR N103B in the Large Magellanic Cloud
N103B is a Type Ia supernova remnant (SNR) in the Large Magellanic Cloud
(LMC). We carried out new CO( = 3-2) and CO( = 1-0)
observations using ASTE and ALMA. We have confirmed the existence of a giant
molecular cloud (GMC) at 245 km s towards the
southeast of the SNR using ASTE CO( = 3-2) data at an angular
resolution of 25 (6 pc in the LMC). Using the ALMA CO(
= 1-0) data, we have spatially resolved CO clouds along the southeastern edge
of the SNR with an angular resolution of 1.8 (0.4 pc in the
LMC). The molecular clouds show an expanding gas motion in the
position-velocity diagram with an expansion velocity of km s.
The spatial extent of the expanding shell is roughly similar to that of the
SNR. We also find tiny molecular clumps in the directions of optical nebula
knots. We present a possible scenario that N103B exploded in the wind-bubble
formed by the accretion winds from the progenitor system, and is now
interacting with the dense gas wall. This is consistent with a
single-degenerate scenario.Comment: 12 pages, 1 table, 8 figures, accepted for publication in The
Astrophysical Journal (ApJ
Discovery of possible molecular counterparts to the infrared Double Helix Nebula in the Galactic center
We have discovered two molecular features at radial velocities of -35 km/s
and 0 km/s toward the infrared Double Helix Nebula (DHN) in the Galactic center
with NANTEN2. The two features show good spatial correspondence with the DHN.
We have also found two elongated molecular ridges at these two velocities
distributed vertically to the Galactic plane over 0.8 degree. The two ridges
are linked by broad features in velocity and are likely connected physically
with each other. The ratio between the 12CO J=2-1 and J=1-0 transitions is 0.8
in the ridges which is larger than the average value 0.5 in the foreground gas,
suggesting the two ridges are in the Galactic center. An examination of the K
band extinction reveals a good coincidence with the CO 0 km/s ridge and is
consistent with a distance of 8 +/-2 kpc. We discuss the possibility that the
DHN was created by a magnetic phenomenon incorporating torsional Alfv\'en waves
launched from the circumnuclear disk (Morris, Uchida & Do 2006) and present a
first estimate of the mass and energy involved in the DHN.Comment: 32 pages, 23 figures, Accepted by Ap
Neutral pion emission from accelerated protons in the supernova remnant W44
We present the AGILE gamma-ray observations in the energy range 50 MeV - 10
GeV of the supernova remnant (SNR) W44, one of the most interesting systems for
studying cosmic-ray production. W44 is an intermediate-age SNR (20, 000 years)
and its ejecta expand in a dense medium as shown by a prominent radio shell,
nearby molecular clouds, and bright [SII] emitting regions. We extend our
gamma-ray analysis to energies substantially lower than previous measurements
which could not conclusively establish the nature of the radiation. We find
that gamma-ray emission matches remarkably well both the position and shape of
the inner SNR shocked plasma. Furthermore, the gamma-ray spectrum shows a
prominent peak near 1 GeV with a clear decrement at energies below a few
hundreds of MeV as expected from neutral pion decay. Here we demonstrate that:
(1) hadron-dominated models are consistent with all W44 multiwavelength
constraints derived from radio, optical, X-ray, and gamma-ray observations; (2)
ad hoc lepton-dominated models fail to explain simultaneously the
well-constrained gamma-ray and radio spectra, and require a circumstellar
density much larger than the value derived from observations; (3) the hadron
energy spectrum is well described by a power-law (with index s = 3.0 \pm 0.1)
and a low-energy cut-off at Ec = 6 \pm 1 GeV. Direct evidence for pion emission
is then established in an SNR for the first time.Comment: accepted for publication on ApJ
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