Discovery of a 168.8 s X-ray pulsar transiting in front of its Be companion star in the Large Magellanic Cloud. (arXiv:1302.4665v1 [astro-ph.HE]):
Aims: We report the discovery of LXP169, a new high-mass X-ray binary in the
Large Magellanic Cloud. The optical counterpart is identified and exhibits an
eclipsing light curve. We performed follow-up observations to clarify the
eclipsing nature of the system. Methods: Energy spectra and time series were
extracted from two XMM-Newton observations to search for pulsations,
characterise the spectrum and measure spectral and timing changes. Long-term
X-ray variability was studied using archival ROSAT data. The XMM-Newton
positions were used to identify the optical counterpart. We obtained
ultraviolet to near-infrared photometry to characterise the companion, along
with its 4000 d long I-band light curve. We observed LXP169 with Swift at two
predicted eclipse times. Results: We found a spin period of 168.8 s which did
not change between two XMM-Newton observations. The X-ray spectrum, well
characterised by a power-law, was harder when the source was brighter. The
X-ray flux of LXP169 is found to be variable by a factor of at least 10. The
counterpart is highly variable on short and long timescales, and its photometry
is that of an early-type star with a near-infrared excess. This classifies the
source as a Be/X-ray binary pulsar. We observed a transit in the ultraviolet,
thereby confirming that the companion star itself is eclipsed. We give an
ephemeris for the transit of MJD56203.877(-0.197,+0.934)+N*(24.329+/-0.008). We
propose and discuss the scenario where the matter captured from the companion's
equatorial disc creates an extended region of high density around the neutron
star, which partially eclipses the companion as the neutron star transits in
front of it. Conclusions: LXP169 is the first confirmed eclipsing Be/X-ray
binary. For the first time we observe the compact object in an X-ray binary
eclipsing its companion star, and much can be learned by studying this
important system.
Saturday, March 9, 2013
Monday, March 4, 2013
Quasar Absorption Lines in the Far Ultraviolet: An Untapped Gold Mine for Galaxy Evolution Studies. (arXiv:1303.0043v1 [astro-ph.CO])
Quasar Absorption Lines in the Far Ultraviolet: An Untapped Gold Mine for Galaxy Evolution Studies. (arXiv:1303.0043v1 [astro-ph.CO]):
This white paper emphasizes the potential of QSO absorption lines in the
rest-frame far/extreme UV at rest-frame wavelengths from ~500 to 2000 A. In
this wavelength range, species such as Ne VIII, Na IX, and Mg X can be
detected, providing diagnostics of gas with temperatures >> 10^{6} K, as well
as banks of adjacent ions such as O I, O II, O III, O IV, O V, and O VI (and
similarly N I - N V; S II - S VI; Ne II - Ne VIII, etc.), which constrain
physical conditions with unprecedented precision. A UV spectrograph with good
sensitivity down to observed wavelengths of 1000 A can detect these new probes
in absorption systems with redshift z(abs) > 0.3, and at these redshifts, the
detailed relationships between the absorbers and nearby galaxies and
large-scale environment can be studied from the ground. By observing QSOs at z
= 1.0 - 1.5, HST has started to exploit extreme-UV QSO absorption lines, but
HST can only reach a small number of these targets. A future, more sensitive UV
spectrograph could open up this new discovery space.
This white paper emphasizes the potential of QSO absorption lines in the
rest-frame far/extreme UV at rest-frame wavelengths from ~500 to 2000 A. In
this wavelength range, species such as Ne VIII, Na IX, and Mg X can be
detected, providing diagnostics of gas with temperatures >> 10^{6} K, as well
as banks of adjacent ions such as O I, O II, O III, O IV, O V, and O VI (and
similarly N I - N V; S II - S VI; Ne II - Ne VIII, etc.), which constrain
physical conditions with unprecedented precision. A UV spectrograph with good
sensitivity down to observed wavelengths of 1000 A can detect these new probes
in absorption systems with redshift z(abs) > 0.3, and at these redshifts, the
detailed relationships between the absorbers and nearby galaxies and
large-scale environment can be studied from the ground. By observing QSOs at z
= 1.0 - 1.5, HST has started to exploit extreme-UV QSO absorption lines, but
HST can only reach a small number of these targets. A future, more sensitive UV
spectrograph could open up this new discovery space.
The NDL Equation of State for Supernova Simulations. (arXiv:1303.0064v1 [astro-ph.HE])
The NDL Equation of State for Supernova Simulations. (arXiv:1303.0064v1 [astro-ph.HE]):
We present an updated and improved equation of state (which we call the NDL
EoS) for use in neutron-star structure and supernova simulations. This EoS is
based upon a framework originally developed by Bowers & Wilson, but there are
numerous changes. Among them are: (1) a reformulation in the context of density
functional theory; (2) the possibility of the formation of material with a net
proton excess (Ye > 0.5); (3) an improved treatment of the nuclear statistical
equilibrium and the transition to heavy nuclei as the density approaches
nuclear matter density; (4) an improved treatment of the effects of pions in
the regime above nuclear matter density including the incorporation of all the
known mesonic and baryonic states at high temperature; (5) the effects of
3-body nuclear forces at high densities; and (6) the possibility of a
first-order or crossover transition to a QCD chiral symmetry restoration and
deconfinement phase at densities above nuclear matter density. This paper
details the physics of, and constraints on, this new EoS and describes its
implementation in numerical simulations. We show comparisons of this EoS with
other equations of state commonly used in supernova collapse simulations.
We present an updated and improved equation of state (which we call the NDL
EoS) for use in neutron-star structure and supernova simulations. This EoS is
based upon a framework originally developed by Bowers & Wilson, but there are
numerous changes. Among them are: (1) a reformulation in the context of density
functional theory; (2) the possibility of the formation of material with a net
proton excess (Ye > 0.5); (3) an improved treatment of the nuclear statistical
equilibrium and the transition to heavy nuclei as the density approaches
nuclear matter density; (4) an improved treatment of the effects of pions in
the regime above nuclear matter density including the incorporation of all the
known mesonic and baryonic states at high temperature; (5) the effects of
3-body nuclear forces at high densities; and (6) the possibility of a
first-order or crossover transition to a QCD chiral symmetry restoration and
deconfinement phase at densities above nuclear matter density. This paper
details the physics of, and constraints on, this new EoS and describes its
implementation in numerical simulations. We show comparisons of this EoS with
other equations of state commonly used in supernova collapse simulations.
Sunday, March 3, 2013
Conditions For Successful Helium Detonations In Astrophysical Environments. (arXiv:1302.6235v1 [astro-ph.HE])
Conditions For Successful Helium Detonations In Astrophysical Environments. (arXiv:1302.6235v1 [astro-ph.HE]):
Several models for type Ia-like supernovae events rely on the production of a
self-sustained detonation powered by nuclear reactions.In the absence of
hydrogen, the fuel that powers these detonations typically consists of either
pure helium (He) or a mixture of carbon and oxygen (C/O). Studies that
systematically determine the conditions required to initiate detonations in C/O
material exist, but until now no analogous investigation of degenerate He
matter has been conducted. We perform one-dimensional reactive hydrodynamical
simulations at a variety of initial density and temperature combinations and
find critical length scales for the initiation of He detonations that range
between 1 -- $10^{10}$ cm. These sizes are consistently smaller than the
corresponding Chapman-Jouguet (CJ) length scales by a factor of ~100, providing
opportunities for thermonuclear explosions in a wider range of low mass white
dwarfs (WDs) than previously thought possible. We find that virialized WDs with
as little mass as 0.24 $M_\odot$ can be detonated, and that even less massive
WDs can be detonated if a sizable fraction of their mass is raised to a higher
adiabat. That the initiation length is exceeded by the CJ length implies that
certain systems may not reach nuclear statistical equilibrium within the time
it takes a detonation to traverse the object. In support of this hypothesis, we
demonstrate that incomplete burning will occur in the majority of He WD
detonations and that $^{44}$Ti, rather than $^{56}$Ni, is the predominant
burning product for many of these events. We anticipate that a measure of the
quantity of $^{44}$Ti and $^{56}$Ni produced in a helium-rich thermonuclear
explosion can potentially be used to constrain the nature of the progenitor
system.
Several models for type Ia-like supernovae events rely on the production of a
self-sustained detonation powered by nuclear reactions.In the absence of
hydrogen, the fuel that powers these detonations typically consists of either
pure helium (He) or a mixture of carbon and oxygen (C/O). Studies that
systematically determine the conditions required to initiate detonations in C/O
material exist, but until now no analogous investigation of degenerate He
matter has been conducted. We perform one-dimensional reactive hydrodynamical
simulations at a variety of initial density and temperature combinations and
find critical length scales for the initiation of He detonations that range
between 1 -- $10^{10}$ cm. These sizes are consistently smaller than the
corresponding Chapman-Jouguet (CJ) length scales by a factor of ~100, providing
opportunities for thermonuclear explosions in a wider range of low mass white
dwarfs (WDs) than previously thought possible. We find that virialized WDs with
as little mass as 0.24 $M_\odot$ can be detonated, and that even less massive
WDs can be detonated if a sizable fraction of their mass is raised to a higher
adiabat. That the initiation length is exceeded by the CJ length implies that
certain systems may not reach nuclear statistical equilibrium within the time
it takes a detonation to traverse the object. In support of this hypothesis, we
demonstrate that incomplete burning will occur in the majority of He WD
detonations and that $^{44}$Ti, rather than $^{56}$Ni, is the predominant
burning product for many of these events. We anticipate that a measure of the
quantity of $^{44}$Ti and $^{56}$Ni produced in a helium-rich thermonuclear
explosion can potentially be used to constrain the nature of the progenitor
system.
The AD775 cosmic event revisited: the Sun is to blame. (arXiv:1302.6897v1 [astro-ph.SR])
The AD775 cosmic event revisited: the Sun is to blame. (arXiv:1302.6897v1 [astro-ph.SR]):
Miyake et al. (henceforth M12) recently reported, based on 14C data, an
extreme cosmic event ca. AD775. Using a simple model, M12 claimed that the
event was too strong to be caused by a solar flare within the standard theory.
This implied a new paradigm of either an impossibly strong solar flare or a
very strong cosmic ray event of unknown origin occurred ca. AD775. We show that
the strength of the event was significantly overestimated by M12. Several
subsequent works have attempted to find a possible exotic source for such an
event, but they are all based on incorrect estimates by M12. We revisit this
event with analysis of new datasets and consistent theoretical modelling. We
verified the experimental result for the AD775 event using independent datasets
including 10Be series and newly measured 14C annual data. We surveyed available
historical chronicles for astronomical observations for the AD770s to identify
potential sightings of aurorae or supernovae. We interpreted the 14C
measurements using an appropriate carbon cycle model. We show that: (1) The
reality of the AD775 event is confirmed by new measurements of 14C; (2) by
using an inappropriate carbon cycle model, M12 strongly overestimated the
event's strength; (3) The revised magnitude of the event is consistent with
different independent datasets (14C, 10Be, 36Cl) and can be associated with a
strong, but not inexplicably strong, SEP event (or a sequence of events), and
provides the first evidence for an event of this magnitude (the fluence >30 MeV
was about 4.5*10^{10} /cm2) in multiple datasets; (4) This is in agreement with
increased auroral activity identified in historical chronicles. This point to
the likely solar origin of the event, which is the greatest solar event on a
multi-millennial time scale, placing a strong observational constraint on the
theory of explosive energy releases on the Sun and cool stars.
Miyake et al. (henceforth M12) recently reported, based on 14C data, an
extreme cosmic event ca. AD775. Using a simple model, M12 claimed that the
event was too strong to be caused by a solar flare within the standard theory.
This implied a new paradigm of either an impossibly strong solar flare or a
very strong cosmic ray event of unknown origin occurred ca. AD775. We show that
the strength of the event was significantly overestimated by M12. Several
subsequent works have attempted to find a possible exotic source for such an
event, but they are all based on incorrect estimates by M12. We revisit this
event with analysis of new datasets and consistent theoretical modelling. We
verified the experimental result for the AD775 event using independent datasets
including 10Be series and newly measured 14C annual data. We surveyed available
historical chronicles for astronomical observations for the AD770s to identify
potential sightings of aurorae or supernovae. We interpreted the 14C
measurements using an appropriate carbon cycle model. We show that: (1) The
reality of the AD775 event is confirmed by new measurements of 14C; (2) by
using an inappropriate carbon cycle model, M12 strongly overestimated the
event's strength; (3) The revised magnitude of the event is consistent with
different independent datasets (14C, 10Be, 36Cl) and can be associated with a
strong, but not inexplicably strong, SEP event (or a sequence of events), and
provides the first evidence for an event of this magnitude (the fluence >30 MeV
was about 4.5*10^{10} /cm2) in multiple datasets; (4) This is in agreement with
increased auroral activity identified in historical chronicles. This point to
the likely solar origin of the event, which is the greatest solar event on a
multi-millennial time scale, placing a strong observational constraint on the
theory of explosive energy releases on the Sun and cool stars.
Saturday, March 2, 2013
The progenitor of SN 2011ja: Clues from circumstellar interaction. (arXiv:1302.7067v1 [astro-ph.HE])
The progenitor of SN 2011ja: Clues from circumstellar interaction. (arXiv:1302.7067v1 [astro-ph.HE]):
Massive stars, possibly red supergiants, which retain extended hydrogen
envelopes until the time of core collapse produce Type IIP (Plateau)
supernovae. The ejecta from these explosions shock the circumstellar matter
originating from the mass loss of the progenitor during the final phases of its
life. This interaction accelerates particles to relativistic energies which
then lose energy via synchrotron radiation in the shock-amplified magnetic
fields and inverse Compton scattering against optical photons from the
supernova. These processes produce different signatures in the radio and X-ray
part of the electromagnetic spectrum. Observed together, they allow us to break
the degeneracy between shock acceleration and magnetic field amplification. In
this work we use X-rays observations from the Chandra and radio observations
from the ATCA to study the relative importance of particle acceleration and
magnetic fields in producing the non-thermal radiation from SN 2011ja. We use
radio observations to constrain the explosion date. Multiple Chandra
observations allow us to probe the history of variable mass loss from the
progenitor. The ejecta expands into a low density bubble followed by
interaction with a higher density wind from a red supergiant consistent with
M>16 solar masses. Our results suggest that a fraction of type IIP supernovae
may interact with circumstellar media set up by non-steady winds.
Massive stars, possibly red supergiants, which retain extended hydrogen
envelopes until the time of core collapse produce Type IIP (Plateau)
supernovae. The ejecta from these explosions shock the circumstellar matter
originating from the mass loss of the progenitor during the final phases of its
life. This interaction accelerates particles to relativistic energies which
then lose energy via synchrotron radiation in the shock-amplified magnetic
fields and inverse Compton scattering against optical photons from the
supernova. These processes produce different signatures in the radio and X-ray
part of the electromagnetic spectrum. Observed together, they allow us to break
the degeneracy between shock acceleration and magnetic field amplification. In
this work we use X-rays observations from the Chandra and radio observations
from the ATCA to study the relative importance of particle acceleration and
magnetic fields in producing the non-thermal radiation from SN 2011ja. We use
radio observations to constrain the explosion date. Multiple Chandra
observations allow us to probe the history of variable mass loss from the
progenitor. The ejecta expands into a low density bubble followed by
interaction with a higher density wind from a red supergiant consistent with
M>16 solar masses. Our results suggest that a fraction of type IIP supernovae
may interact with circumstellar media set up by non-steady winds.
Monday, February 25, 2013
The Bimodal Metallicity Distribution of the Cool Circumgalactic Medium at z<1. (arXiv:1302.5424v1 [astro-ph.CO])
The Bimodal Metallicity Distribution of the Cool Circumgalactic Medium at z<1. (arXiv:1302.5424v1 [astro-ph.CO]):
We assess the metal content of the circumgalactic medium (CGM) about galaxies
at z<1 using an HI-selected sample of 27 Lyman limit systems (LLS, defined here
as absorbers with 16.2\lesssimlog N(HI)\lesssim 18.5 observed in absorption
against background QSOs by the Cosmic Origins Spectrograph on-board the {Hubble
Space Telescope. The N(HI) selection avoids metallicity biases inherent in many
previous studies of the low-redshift CGM. We compare the column densities of
weakly ionized metal species (e.g., OII, CII, MgII) to N(HI) in the strongest
HI component of each absorber. We find that the metallicity distribution of the
LLS (and hence the cool CGM) is bimodal with metal-poor and metal-rich branches
peaking at [X/H]simeq1.6 and -0.4 (2.5% and 40% solar metallicities). The cool
CGM probed by these LLS is predominantly ionized. The metal-rich branch of the
population likely traces winds, recycled outflows, and tidally stripped gas;
the metal-poor branch has properties consistent with cold accretion streams
thought to be a major source of fresh gas for star forming galaxies. Both
branches have a nearly equal number of absorbers. Our results thus demonstrate
there is a significant mass of previously-undiscovered cold, metal-poor gas in
the CGM of z<1 galaxies.
We assess the metal content of the circumgalactic medium (CGM) about galaxies
at z<1 using an HI-selected sample of 27 Lyman limit systems (LLS, defined here
as absorbers with 16.2\lesssimlog N(HI)\lesssim 18.5 observed in absorption
against background QSOs by the Cosmic Origins Spectrograph on-board the {Hubble
Space Telescope. The N(HI) selection avoids metallicity biases inherent in many
previous studies of the low-redshift CGM. We compare the column densities of
weakly ionized metal species (e.g., OII, CII, MgII) to N(HI) in the strongest
HI component of each absorber. We find that the metallicity distribution of the
LLS (and hence the cool CGM) is bimodal with metal-poor and metal-rich branches
peaking at [X/H]simeq1.6 and -0.4 (2.5% and 40% solar metallicities). The cool
CGM probed by these LLS is predominantly ionized. The metal-rich branch of the
population likely traces winds, recycled outflows, and tidally stripped gas;
the metal-poor branch has properties consistent with cold accretion streams
thought to be a major source of fresh gas for star forming galaxies. Both
branches have a nearly equal number of absorbers. Our results thus demonstrate
there is a significant mass of previously-undiscovered cold, metal-poor gas in
the CGM of z<1 galaxies.
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