Abstract
We carried out new CO(
J
= 2–1) observations toward the mixed-morphology supernova remnant (SNR) W49B with the Atacama Large Millimeter/submillimeter Array. We found that CO clouds at ∼10 km ...s
−1
show a good spatial correspondence to the synchrotron radio continuum as well as to an X-ray deformed shell. The bulk mass of molecular clouds accounts for the western part of the shell, not the eastern shell, where near-infrared H
2
emission is detected. The molecular clouds at ∼10 km s
−1
show higher kinetic temperatures of ∼20–60 K, suggesting that modest shock heating occurred. The expanding motion of the clouds with Δ
V
∼ 6 km s
−1
was formed by strong winds from the progenitor system. We argue that the barrel-like structure of Fe-rich ejecta was possibly formed not only by an asymmetric explosion, but also by interactions with dense molecular clouds. We also found a negative correlation between the CO intensity and the electron temperature of recombining plasma, implying that the origin of the high-temperature recombining plasma in W49B can be understood to be the thermal conduction model. The total energy of accelerated cosmic-ray protons
W
p
is estimated to be ∼2 × 10
49
erg by adopting an averaged gas density of ∼650 ± 200 cm
−3
. The SNR age–
W
p
diagram indicates that W49B shows one of the highest in situ values of
W
p
among gamma-ray-bright SNRs.
Identification of the cosmic-ray (CR) “PeVatrons,” which are sources capable of accelerating particles to ∼1015 eV energies and higher, may lead to resolving the long-standing question of the origin ...of the spectral feature in the all-particle CR spectrum known as the “knee.” Because CRs with these energies are deflected by interstellar magnetic fields identification of individual sources and determination of their spectral characteristics is more likely via very high energy γ-ray emissions, which provide the necessary directional information. However, pair production on the interstellar radiation field (ISRF) and cosmic microwave background (CMB) leads to steepening of the high energy tails of γ-ray spectra, and should be corrected for to enable true properties of the spectrum at the source to be recovered. Employing recently developed three-dimensional ISRF models this paper quantifies the pair-absorption effect on spectra for sources in the Galactic center (GC) direction at 8.5 and 23.5 kpc distances, with the latter corresponding to the far side of the Galactic stellar disc where it is expected that discrimination of spectral features >10 TeV is possible by the forthcoming Cherenkov Telescope Array (CTA). The estimates made suggest spectral cutoffs could be underestimated by factors of a few in the energy range so far sampled by TeV γ-ray telescopes. As an example to illustrate this, the recent HESS measurements of diffuse γ-ray emissions possibly associated with injection of CRs nearby Sgr A* are ISRF corrected, and estimates of the spectral cutoff are reevaluated. It is found that it could be higher by up to a factor of ∼2, indicating that these emissions may be consistent with a CR accelerator with a spectral cutoff of at least 1 PeV at the 95% confidence level.
HESS J1825−137 is a pulsar wind nebula (PWN) whose TeV emission extends across ∼1
$\deg$
. Its large asymmetric shape indicates that its progenitor supernova interacted with a molecular cloud located ...in the north of the PWN as detected by previous CO Galactic survey (e.g. Lemiere, Terrier & Djannati-Ataï). Here, we provide a detailed picture of the interstellar medium (ISM) towards the region north of HESS J1825−137, with the analysis of the dense molecular gas from our 7 and 12 mm Mopra survey and the more diffuse molecular gas from the Nanten CO(1–0) and GRS 13CO(1–0) surveys. Our focus is the possible association between HESS J1825−137 and the unidentified TeV source to the north, HESS J1826−130. We report several dense molecular regions whose kinematic distance matched the dispersion measured distance of the pulsar. Among them, the dense molecular gas located at (RA, Dec.) = (18h421h,−13
$_{.}^{\circ}$
282) shows enhanced turbulence and we suggest that the velocity structure in this region may be explained by a cloud–cloud collision scenario. Furthermore, the presence of a H α rim may be the first evidence of the progenitor supernova remnant (SNR) of the pulsar PSR J1826−1334 as the distance between the H α rim and the TeV source matched with the predicted SNR radius R
SNR ∼ 120 pc. From our ISM study, we identify a few plausible origins of the HESS J1826−130 emission, including the progenitor SNR of PSR J1826−1334 and the PWN G018.5−0.4 powered by PSR J1826−1256. A deeper TeV study however, is required to fully identify the origin of this mysterious TeV source.
ABSTRACT
Cosmic rays (CRs) interact with the diffuse gas, radiation, and magnetic fields in the interstellar medium (ISM) to produce electromagnetic emissions that are a significant component of the ...all-sky flux across a broad wavelength range. The Fermi–Large Area Telescope (LAT) has measured these emissions at GeV γ-ray energies with high statistics. Meanwhile, the high-energy stereoscopic system (H.E.S.S.) telescope array has observed large-scale Galactic diffuse emission in the TeV γ-ray energy range. The emissions observed at GeV and TeV energies are connected by the common origin of the CR particles injected by the sources, but the energy dependence of the mixture from the general ISM (true ‘diffuse’), those emanating from the relatively nearby interstellar space about the sources, and the sources themselves, is not well understood. In this paper, we investigate predictions of the broad-band emissions using the galprop code over a grid of steady-state 3D models that include variations over CR sources, and other ISM target distributions. We compare, in particular, the model predictions in the very-high energy ( VHE; ≳100 GeV) γ-ray range with the H.E.S.S. Galactic plane survey (HGPS) after carefully subtracting emission from catalogued γ-ray sources. Accounting for the unresolved source contribution, and the systematic uncertainty of the HGPS, we find that the galprop model predictions agree with lower estimates for the HGPS source-subtracted diffuse flux. We discuss the implications of the modelling results for interpretation of data from the next generation Cherenkov Telescope Array (CTA).
ABSTRACT
HESS J1825 − 137 is one of the most powerful and luminous TeV gamma-ray pulsar wind nebulae, making it an excellent laboratory to study particle transportation around pulsars. We present a ...model of the (diffusive and advective) transport and radiative losses of electrons from the pulsar PSR J1826 − 1334 powering HESS J1825 − 137 using interstellar medium gas (ISM) data, soft photon fields, and a spatially varying magnetic field. We find that for the characteristic age of $21\, \mathrm{k}\mathrm{yr}$, PSR J1826 − 1334 is unable to meet the energy requirements to match the observed X-ray and gamma-ray emission. An older age of $40\, \mathrm{k}\mathrm{yr}$, together with an electron conversion efficiency of 0.14 and advective flow of v = 0.002c, can reproduce the observed multiwavelength emission towards HESS J1825 − 137. A turbulent ISM with magnetic field of $B=20 \,{\rm to}\, 60 \,\mathrm{\mu }{\rm G}$ to the north of HESS J1825 − 137 (as suggested by ISM observations) is required to prevent significant gamma-ray contamination towards the northern $\mathrm{T}\mathrm{e\mathrm{V}}$ source HESS J1826 − 130.
Abstract
We have carried out a study of the interstellar medium (ISM) toward the shell-like supernova remnant (SNR) Puppis A using NANTEN CO and ATCA H
i
data. We synthesized a comprehensive picture ...of the SNR radiation by combining the ISM data with the gamma-ray and X-ray distributions. The ISM, both atomic and molecular gas, is dense and highly clumpy, and is distributed all around the SNR, but mainly in the northeast. The CO distribution revealed an enhanced line intensity ratio of CO(
J
= 2–1)/(
J
= 1–0) transitions as well as CO line broadening, which indicate shock heating/acceleration. The results support the assertion that Puppis A is located at 1.4 kpc, in the Local Arm. The ISM interacting with the SNR has a large mass of ∼10
4
M
⊙
, which is dominated by H
i
, showing good spatial correspondence with the Fermi-LAT gamma-ray image. This favors a hadronic origin of the gamma-rays, while an additional contribution from a leptonic component is not excluded. The distribution of the X-ray ionization timescales within the shell suggests that the shock front ionized various parts of the ISM at epochs ranging over a few to ten thousand years. We therefore suggest that the age of the SNR is around 10
4
yr as given by the largest ionization timescale. We estimate the total cosmic-ray energy
W
p
to be 10
47
erg, which is well placed in the cosmic-ray escaping phase of an age–
W
p
plot including more than ten SNRs.
ABSTRACT
HESS J1804−216 is one of the brightest yet most mysterious TeV γ-ray sources discovered to date. Previous arc-minute scale studies of the interstellar medium (ISM) surrounding this TeV γ-ray ...source revealed HESS J1804−216 is likely powered by a mature supernova remnant (SNR) or pulsar, hence its origin remains uncertain. In this paper, we focus on the diffusive escape of cosmic ray protons from potential SNR accelerators. These cosmic rays interact with the ISM to produce TeV γ-rays. We utilize the isotropic diffusion equation solution for particles escaping from a shell, to model the energy-dependent escape and propagation of protons into the ISM. This work is the first attempt at modelling the spatial morphology of γ-rays towards HESS J1804−216, using arc-minute ISM observations from both Mopra and the Southern Galactic Plane Survey. The spectral and spatial distributions of γ-rays for the two nearby potential SNR counterparts, SNR G8.7−0.1 and the progenitor SNR of PSR J1803−2137, are presented here. We vary the diffusion parameters and particle spectrum and use a grid search approach to find the best combination of model parameters. We conclude that moderately slow diffusion is required for both candidates. The most promising candidate to be powering the TeV γ-rays from HESS J1804−216 in a hadronic scenario is the progenitor SNR of PSR J1803−2137.
N132D is the brightest gamma-ray supernova remnant (SNR) in the Large Magellanic Cloud (LMC). We carried out 12CO(J = 1-0, 3-2) observations toward the SNR using the Atacama Large ...Millimeter/submillimeter Array (ALMA) and Atacama Submillimeter Telescope Experiment. We find diffuse CO emission not only at the southern edge of the SNR as previously known, but also inside the X-ray shell. We spatially resolved nine molecular clouds using ALMA with an angular resolution of 5″, corresponding to a spatial resolution of ∼1 pc at the distance of the LMC. Typical cloud sizes and masses are ∼2.0 pc and ∼100 M , respectively. High intensity ratios of CO J = 3-2/1-0 > 1.5 are seen toward the molecular clouds, indicating that shock heating has occurred. Spatially resolved X-ray spectroscopy reveals that thermal X-rays in the center of N132D are produced not only behind a molecular cloud but also in front of it. Considering the absence of a thermal component associated with the forward shock toward one molecular cloud located along the line of sight to the center of the remnant, this suggests that this particular cloud is engulfed by shock waves and is positioned on the near side of the remnant. If the hadronic process is the dominant contributor to the gamma-ray emission, the shock-engulfed clouds play a role as targets for cosmic rays. We estimate the total energy of cosmic-ray protons accelerated in N132D to be ∼0.5-3.8 × 1049 erg as a conservative lower limit, which is similar to that observed in Galactic gamma-ray SNRs.
ABSTRACT
We present two new radio continuum images from the Australian Square Kilometre Array Pathfinder (ASKAP) survey in the direction of the Small Magellanic Cloud (SMC). These images are part of ...the Evolutionary Map of the Universe (EMU) Early Science Project (ESP) survey of the Small and Large Magellanic Clouds. The two new source lists produced from these images contain radio continuum sources observed at 960 MHz (4489 sources) and 1320 MHz (5954 sources) with a bandwidth of 192 MHz and beam sizes of 30.0 × 30.0 arcsec2 and 16.3 × 15.1 arcsec2, respectively. The median root mean square (RMS) noise values are 186 $\mu$Jy beam−1 (960 MHz) and 165 $\mu$Jy beam−1 (1320 MHz). To create point source catalogues, we use these two source lists, together with the previously published Molonglo Observatory Synthesis Telescope (MOST) and the Australia Telescope Compact Array (ATCA) point source catalogues to estimate spectral indices for the whole population of radio point sources found in the survey region. Combining our ASKAP catalogues with these radio continuum surveys, we found 7736 point-like sources in common over an area of 30 deg2. In addition, we report the detection of two new, low surface brightness supernova remnant candidates in the SMC. The high sensitivity of the new ASKAP ESP survey also enabled us to detect the bright end of the SMC planetary nebula sample, with 22 out of 102 optically known planetary nebulae showing point-like radio continuum emission. Lastly, we present several morphologically interesting background radio galaxies.
Abstract
We report new H
i
observations of the Type Ia supernova remnant (SNR) SN 1006 using the Australia Telescope Compact Array with an angular resolution of
4
.′
5
×
1
.′
4
(∼2 pc at the assumed ...SNR distance of 2.2 kpc). We find an expanding gas motion in position–velocity diagrams of H
i
with an expansion velocity of ∼4 km s
−1
and a mass of ∼1000
M
⊙
. The spatial extent of the expanding shell is roughly the same as that of SN 1006. We here propose a hypothesis that SN 1006 exploded inside the wind-blown bubble formed by accretion winds from the progenitor system consisting of a white dwarf and a companion star, and then the forward shock has already reached the wind wall. This scenario is consistent with the single-degenerate model. We also derived the total energy of cosmic-ray protons
W
p
to be only ∼1.2–2.0 × 10
47
erg by adopting the averaged interstellar proton density of ∼25 cm
−3
. The small value is compatible with the relation between the age and
W
p
of other gamma-ray SNRs with ages below ∼6 kyr. The
W
p
value in SN 1006 will possibly increase up to several 10
49
erg in the next ∼5 kyr via the cosmic-ray diffusion into the H
i
wind shell.
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