2,104 research outputs found
A Chandra View Of Nonthermal Emission In The Northwestern Region Of Supernova Remnant RCW 86: Particle Acceleration And Magnetic Fields
The shocks of supernova remnants (SNRs) are believed to accelerate particles
to cosmic ray (CR) energies. The amplification of the magnetic field due to CRs
propagating in the shock region is expected to have an impact on both the
emission from the accelerated particle population, as well as the acceleration
process itself. Using a 95 ks observation with the Advanced CCD Imaging
Spectrometer (ACIS) onboard the Chandra X-ray Observatory, we map and
characterize the synchrotron emitting material in the northwestern region of
RCW 86. We model spectra from several different regions, filamentary and
diffuse alike, where emission appears dominated by synchrotron radiation. The
fine spatial resolution of Chandra allows us to obtain accurate emission
profiles across 3 different non-thermal rims in this region. The narrow width
(l = 10''-30'') of these filaments constrains the minimum magnetic field
strength at the post-shock region to be approximately 80 {\mu}G.Comment: 7 pages, 3 figures, submitted for publication at the Astrophysical
Journa
An Off-Axis Model for GRB 031203
The low luminosity radio emission of the unusually faint GRB 031203 has been
argued to support the idea of a class of intrinsically sub-energetic gamma-ray
bursts (GRBs), currently comprising two members. While low energy GRBs probably
exist, we show that the collective prompt and multiwavelength observations of
the afterglow of GRB 031203 do not necessarily require a sub-energetic nature
for that event. In fact, the data are more consistent with a typical, powerful
GRB seen at an angle of about twice the opening angle of the central jet. The
(redshift corrected) peak energy, E_p, of GRB 031203 then becomes ~ 2 MeV,
similar to many other GRBs.Comment: 4 pages, 3 figures; ApJL in pres
Gamma-ray bursts in normal and extreme star-forming galaxies
We discuss how gamma-ray burst (GRB) optical afterglows and multiwavelength
observations of their host galaxies can be used to obtain information about the
relative amounts of star formation happening in optical and submillimetre
galaxies. That such an analysis will be possible follows from the
currently-favoured idea that GRBs are closely linked with high-mass star
formation. Studying GRB host galaxies offers a method of finding low-luminosity
submillimetre galaxies, which cannot be identified either in optical Lyman
break surveys, because so much of their star formation is hidden by dust, or in
submillimetre surveys, because their submillimetre fluxes are close to or below
the confusion limit. Much of the star formation in the Universe could have
occurred in such objects, so searching for them is an important exercise. From
current observations, GRB host galaxies appear to be neither optically-luminous
Class-2 SCUBA galaxies like SMM J023990136 or SMM J14011+0252, nor galaxies
containing dense molecular cores like local ultraluminous infrared galaxies
(ULIGs), but rather some intermediate kind of galaxy. The host galaxy of GRB
980703 is a prototype of this kind of galaxy.Comment: 8 pages, 1 fig, TeX, MNRAS in pres
The Role of Stellar Feedback in the Dynamics of HII Regions
Stellar feedback is often cited as the biggest uncertainty in galaxy
formation models today. This uncertainty stems from a dearth of observational
constraints as well as the great dynamic range between the small scales (<1 pc)
where the feedback occurs and the large scales of galaxies (>1 kpc) that are
shaped by this feedback. To bridge this divide, in this paper we aim to assess
observationally the role of stellar feedback at the intermediate scales of HII
regions. In particular, we employ multiwavelength data to examine several
stellar feedback mechanisms in a sample of 32 HII regions in the Large and
Small Magellanic Clouds (LMC and SMC, respectively). Using optical, infrared,
radio, and X-ray images, we measure the pressures exerted on the shells from
the direct stellar radiation, the dust-processed radiation, the warm ionized
gas, and the hot X-ray emitting gas. We find that the warm ionized gas
dominates over the other terms in all of the sources, although two have
comparable dust-processed radiation pressures to their warm gas pressures. The
hot gas pressures are comparatively weak, while the direct radiation pressures
are 1-2 orders of magnitude below the other terms. We discuss the implications
of these results, particularly highlighting evidence for hot gas leakage from
the HII shells and regarding the momentum deposition from the dust-processed
radiation to the warm gas. Furthermore, we emphasize that similar observational
work should be done on very young HII regions to test whether direct radiation
pressure and hot gas can drive the dynamics at early times.Comment: 19 pages, 8 figures; accepted by Ap
Nonsymmetric elliptic operators with wentzell boundary conditions in general domains
We study nonsymmetric second order elliptic operators with Wentzell boundary conditions in general domains with sufficiently smooth boundary. The ambient space is a space of Lp-type, 1 †p †â. We prove the existence of analytic quasicontractive (C0)-semigroups generated by the closures of such operators, for any 1 †p †â. Moreover, we extend a previous result concerning the continuous dependence of these semigroups on the coefficients of the boundary condition. We also specify precisely the domains of the generators explicitly in the case of bounded domains and 1 †p †â, when all the ingredients of the problem, including the boundary of the domain, the coefficients, and the initial condition, are of class Câ
SN 2007gr: a Normal Type Ic Supernova with a Mildly Relativistic Radio Jet?
A nearby type Ic supernova, SN 2007gr was observed with the EVN in two epochs 60 days apart (second observation also included the Green Bank Telescope). In both cases one of the EVN stations was the Westerbork Synthesis Radio Telescope (WSRT), which recorded the observational data not only in the VLBI mode, but also in its normal interferometric mode. Thus it provided an important reference observation. In the first epoch the fluxes measured by the VLBI network and the WSRT alone match well. However in the second epoch the peak brightness observed in the VLBI experiment is much lower than the total flux recorded by the WSRT. There could be multiple reasons for this discrepancy: a resolution effect, coherence losses in VLBI, or extended emission contaminating the WSRT measurement. With new WSRT observations we costrain the level of background emission and find that there is still a difference between the corrected total flux density and the VLBI peak brightness. If one assumes that this is dominated by resolution, this would correspond to an average apparent expansion speed of ~0.4c
The Landscape of Galaxies Harboring Changing-Look Active Galactic Nuclei in the Local Universe
We study the properties of the host galaxies of Changing-Look Active Galactic
Nuclei (CL AGNs) with the aim of understanding the conditions responsible for
triggering CL activity. We find that CL AGN hosts primarily reside in the
so-called green valley that is located between spiral-like star-forming
galaxies and dead ellipticals, implying that CL AGNs are activated during
distinct periods of quenching and galaxy transformation processes. CL AGN hosts
have low galaxy asymmetry indicators, suggesting that secular evolutionary
processes (the influence of bars and spirals, and possibly minor mergers) might
be the primary mechanism for transporting gas to the vicinity of the
supermassive black hole (SMBH) rather than major mergers. Similar to tidal
disruption events (TDEs) and highly variable AGNs, we find that CL AGN hosts
are associated with SMBHs residing in high density pseudo-bulges and appear to
overlap most significantly with the population of low-ionization nuclear
emission-line region (LINER) galaxies. As such, CL AGN are likely fueled by
strong episodic bursts of accretion activity, which appear to take place
preferentially as the amount of material accessible for star formation and
accretion dwindles. We also identify that CL AGN hosts are characterized by
either large S\'ersic indices or high bulge fractions, which suggests a simple
metric for identifying candidates for spectroscopic follow-up observations in
forthcoming synoptic surveys.Comment: 17 pages, 9 figures. Accepted to ApJ Letters. Revised version
includes an expanded discussion on asymmetry measurements and galaxy
disturbance
Constraining Explosion Type of Young Supernova Remnants Using 24 Micron Emission Morphology
Determination of the explosion type of supernova remnants (SNRs) can be
challenging, as SNRs are hundreds to thousands of years old and supernovae
(SNe) are classified based on spectral properties days after explosion.
Previous studies of thermal X-ray emission from Milky Way and Large Magellanic
Cloud (LMC) SNRs have shown that Type Ia and core-collapse (CC) SNRs have
statistically different symmetries, and thus these sources can be typed based
on their X-ray morphologies. In this paper, we extend the same technique, a
multipole expansion technique using power ratios, to infrared (IR) images of
SNRs to test whether they can be typed using the symmetry of their warm dust
emission as well. We analyzed archival Spitzer Space Telescope Multiband
Imaging Photometer (MIPS) 24 micron observations of the previously used X-ray
sample, and we find that the two classes of SNRs separate according to their IR
morphologies. The Type Ia SNRs are statistically more circular and mirror
symmetric than the CC SNRs, likely due to the different circumstellar
environments and explosion geometries of the progenitors. Broadly, our work
indicates that the IR emission retains information of the explosive origins of
the SNR and offers a new method to type SNRs based on IR morphology.Comment: 6 pages, 4 figures; accepted by ApJ
Deducing the Lifetime of Short Gamma-Ray Burst Progenitors from Host Galaxy Demography
The frequency of short gamma-ray bursts (GRBs) in galaxies with distinct star
formation histories can be used to constrain the lifetime of the progenitor
systems. As an illustration, we consider here the constraints that can be
derived from separating the host galaxies into early and late types. On
average, early-type galaxies have their stars formed earlier than late-type
galaxies, and this difference, together with the time delay between progenitor
formation and short GRB outburst, leads to different burst rates in the two
types of hosts. Presently available data suggest, but not yet prove, that the
local short GRB rate in early-type galaxies may be comparable to that in
late-type galaxies. This suggests that, unlike Type Ia supernovae, at least
half of the short GRB progenitors that can outburst within a Hubble time have
lifetimes greater than about 7 Gyr. Models of the probability distribution of
time delays, here parametrized as P(\tau) \propto \tau^n, with n > -1 are
favored. This apparent long time delay and the fact that early-type galaxies in
clusters make a substantial contribution to the local stellar mass inventory
can explain the observed preponderance of short GRBs in galaxy clusters.Comment: 8 pages, 4 figures. Accepted for publication in ApJ. Extended for
more investigations and discussions on systematic
Measurement of the core-collapse progenitor mass distribution of the Small Magellanic Cloud
The physics of core-collapse (CC) supernovae (SNe) and how the explosions
depend on progenitor properties are central questions in astronomy. For only a
handful of SNe, the progenitor star has been identified in pre-explosion
images. Supernova remnants (SNRs), which are observed long after the original
SN event, provide a unique opportunity to increase the number of progenitor
measurements. Here, we systematically examine the stellar populations in the
vicinities of 23 known SNRs in the Small Magellanic Cloud (SMC) using the star
formation history (SFH) maps of Harris & Zaritsky (2004). We combine the
results with constraints on the SNR metal abundances and environment from X-ray
and optical observations. We find that 22 SNRs in the SMC have local SFHs and
properties consistent with a CC explosion, several of which are likely to have
been high-mass progenitors. This result supports recent theoretical findings
that high-mass progenitors can produce successful explosions. We estimate the
mass distribution of the CC progenitors and find that this distribution is
similar to a Salpeter IMF (within the uncertainties), while this result is
shallower than the mass distribution found in M31 and M33 by Jennings et al.
(2014) and D\'{\i}az-Rodr\'{\i}guez et al. (2018) using a similar approach.
Additionally, we find that a number of the SMC SNRs exhibit a burst of star
formation between 50-200 Myr ago. As these sources are likely CC, this
signature may be indicative of massive stars undergoing delayed CC as a
consequence of binary interaction, rapid rotation, or low metallicity. In
addition, the lack of Type Ia SNRs in the SMC is possibly a result of the short
visibility times of these sources as they may fall below the sensitivity limits
of current radio observations.Comment: Accepted to ApJ, 24 pages, 7 Figures, 3 Table
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