473 research outputs found
SN1991bg-like supernovae are a compelling source of most Galactic antimatter
The Milky Way Galaxy glows with the soft gamma ray emission resulting from
the annihilation of electron-positron pairs every
second. The origin of this vast quantity of antimatter and the peculiar
morphology of the 511keV gamma ray line resulting from this annihilation have
been the subject of debate for almost half a century. Most obvious positron
sources are associated with star forming regions and cannot explain the rate of
positron annihilation in the Galactic bulge, which last saw star formation some
ago, or else violate stringent constraints on the positron
injection energy. Radioactive decay of elements formed in core collapse
supernovae (CCSNe) and normal Type Ia supernovae (SNe Ia) could supply
positrons matching the injection energy constraints but the distribution of
such potential sources does not replicate the required morphology. We show that
a single class of peculiar thermonuclear supernova - SN1991bg-like supernovae
(SNe 91bg) - can supply the number and distribution of positrons we see
annihilating in the Galaxy through the decay of Ti synthesised in these
events. Such Ti production simultaneously addresses the observed
abundance of Ca, the Ti decay product, in solar system material.Comment: Accepted for publication in Proceedings of IAU Symposium 322: The
Multimessenger Astrophysics of the Galactic Center 4 page
SN1991bg-like supernovae are associated with old stellar populations
SN1991bg-like supernovae are a distinct subclass of thermonuclear supernovae
(SNe Ia). Their spectral and photometric peculiarities indicate their
progenitors and explosion mechanism differ from `normal' SNe Ia. One method of
determining information about supernova progenitors we cannot directly observe
is to observe the stellar population adjacent to the apparent supernova
explosion site to infer the distribution of stellar population ages and
metallicities. We obtain integral field observations and analyse the spectra
extracted from regions of projected radius about the
apparent SN explosion site for 11 91bg-like SNe in both early- and late-type
galaxies. We utilize full-spectrum spectral fitting to determine the ages and
metallicities of the stellar population within the aperture. We find that the
majority of the stellar populations that hosted 91bg-like supernovae have
little recent star formation. The ages of the stellar populations suggest that
that 91bg-like SN progenitors explode after delay times of ,
much longer than the typical delay time of normal SNe Ia, which peaks at .Comment: 12 pages, 3 figures, 3 tables, submitted to Publications of the
Astronomical Society of Australi
Effect of positron-alkali metal atom interactions in the diffuse interstellar medium
In the Milky Way galaxy, positrons, which are responsible for the diffuse 511 keV gamma ray emission observed by space-based gamma ray observatories, are thought to annihilate predominantly through charge exchange interactions with neutral hydrogen. These charge exchange interactions can only take place if positrons have energies greater than 6.8 eV, the minimum energy required to liberate the electron bound to the hydrogen atom and then form positronium, a short-lived bound state composed of a positron-electron pair. Here we demonstrate the importance of positron interactions with neutral alkali metals in the warm interstellar medium (ISM). Positrons may undergo charge exchange with these atoms at any energy. In particular, we show that including positron interactions with sodium at solar abundance in the warm ISM can significantly reduce the annihilation timescale of positrons with energies below 6.8 eV by at least an order of magnitude. We show that including these interactions in our understanding of positron annihilation in the Milky Way rules out the idea that the number of positrons in the Galactic ISM could be maintained in steady state by injection events occurring at a typical periodicity
>Myr
Diffuse Galactic antimatter from faint thermonuclear supernovae in old stellar populations
Our Galaxy hosts the annihilation of a few low-energy
positrons every second. Radioactive isotopes capable of supplying such
positrons are synthesised in stars, stellar remnants, and supernovae. For
decades, however, there has been no positive identification of a main stellar
positron source leading to suggestions that many positrons originate from
exotic sources like the Galaxy's central super-massive black hole or dark
matter annihilation. %, but such sources would not explain the
recently-detected positron signal from the extended Galactic disk. Here we show
that a single type of transient source, deriving from stellar populations of
age 3-6 Gyr and yielding ~0.03 of the positron emitter Ti, can
simultaneously explain the strength and morphology of the Galactic positron
annihilation signal and the solar system abundance of the Ti decay
product Ca. This transient is likely the merger of two low-mass white
dwarfs, observed in external galaxies as the sub-luminous, thermonuclear
supernova known as SN1991bg-like.Comment: 28 pages main text with 4 figures in preprint style; 26 pages of
Supplementary Informatio
Prospects of direct detection of V gamma-rays from thermonuclear supernovae
Detection of gamma-rays emitted by radioactive isotopes synthesized in
stellar explosions can give important insights into the processes that power
transients such as supernovae, as well as providing a detailed census of the
abundance of different isotope species relevant to the chemical evolution of
the Universe. Observations of nearby supernovae have yielded observational
proof that Co powered the late-time evolution of SN1987A's lightcurve,
and conclusive evidence that Ni and its daughter nuclei power the light
curves of Type Ia supernovae. In this paper we describe the prospects for
detecting nuclear decay lines associated with the decay of V, the
daughter nucleus of Cr, which is expected to be synthesised in large
quantities - - in
transients initiated by explosive helium burning (-capture) of a thick
helium shell. We calculate emergent gamma-ray line fluxes for a simulated
explosion model of a thermonuclear explosion of carbon-oxygen white dwarf core
of mass surrounded by a thick helium layer of mass
. We present observational limits on the presence of V
in nearby SNe Ia 2014J using the \textit{INTEGRAL} space telescope, excluding a
Cr production on the surface of more than . We
find that the future gamma-ray mission AMEGO will have an approximately 5 per
cent chance of observing V gamma-rays from such events during the
currently-planned operational lifetime, based on our birthrate predictions of
faint thermonuclear transients. We describe the conditions for a
detection by the gamma-ray telescopes \textit{INTEGRAL}/SPI, COSI and AMEGO.Comment: 9 pages, 3 figures, submitted to MNRAS, minor revisions Sept 202
Chronic Stress Prevents Cortico-Accumbens Cue Encoding and Alters Conditioned Approach
Chronic stress impairs the function of multiple brain regions and causes severe hedonic and motivational deficits. One brain region known to be susceptible to these effects is the PFC. Neurons in this region, specifically neuronal projections from the prelimbic region (PL) to the nucleus accumbens core (NAcC), have a significant role in promoting motivated approach. However, little is known about how activity in this pathway changes during associative learning to encode cues that promote approach. Less is known about how activity in this pathway may be altered by stress. In this study, an intersectional fiber photometry approach was used in male Sprague Dawley rats engaged in a Pavlovian autoshaping design to characterize the involvement of the PL-NAcC pathway in the typical acquisition of learned approach (directed at both the predictive cue and the goal), and its potential alteration by stress. Specifically, the hypothesis that neural activity in PL-NAcC would encode a Pavlovian approach cue and that prior exposure to chronic stress would disrupt both the nature of conditioned approach and the encoding of a cue that promotes approach was tested. Results of the study demonstrated that the rapid acquisition of conditioned approach was associated with cue-induced PL-NAcC activity. Prior stress both reduced cue-directed behavior and impaired the associated cortical activity. These findings demonstrate that prior stress diminishes the task-related activity of a brain pathway that regulates approach behavior. In addition, the results support the interpretation that stress disrupts reward processing by altering the incentive value of associated cues
Positron Annihilation in the Galaxy
The 511 keV line from positron annihilation in the Galaxy was the first γ-ray line detected to originate from outside our solar system. Going into the fifth decade since the discovery, the source of positrons is still unconfirmed and remains one of the enduring mysteries in γ-ray astronomy. With a large flux of ∼10−3 γ/cm2/s, after 15 years in operation INTEGRAL/SPI has detected the 511 keV line at >50σ and has performed high-resolution spectral studies which conclude that Galactic positrons predominantly annihilate at low energies in warm phases of the interstellar medium. The results from imaging are less certain, but show a spatial distribution with a strong concentration in the center of the Galaxy. The observed emission from the Galactic disk has low surface brightness and the scale height is poorly constrained, therefore, the shear number of annihilating positrons in our Galaxy is still not well know. Positrons produced in β+-decay of nucleosynthesis products, such as 26Al, can account for some of the annihilation emission in the disk, but the observed spatial distribution, in particular the excess in the Galactic bulge, remains difficult to explain. Additionally, one of the largest uncertainties in these studies is the unknown distance that positrons propagate before annihilation. In this paper, we will summarize the current knowledge base of Galactic positrons, and discuss how next-generation instruments could finally provide the answers.Non peer reviewedFinal Accepted Versio
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