46,728 research outputs found
Not enough stellar mass Machos in the Galactic halo
We present an update of results from the search for microlensing towards the
Large Magellanic Cloud (LMC) by EROS (Experience de Recherche d'Objets
Sombres). We have now monitored 25 million stars over three years. Because of
the small number of observed microlensing candidates (four), our results are
best presented as upper limits on the amount of dark compact objects in the
halo of our Galaxy. We discuss critically the candidates and the possible
location of the lenses, halo or LMC . We compare our results to those of the
MACHO group. Finally, we combine these new results with those from our search
towards the Small Magellanic Cloud as well as earlier ones from the EROS1 phase
of our survey. The combined data is sensitive to compact objects in the broad
mass range solar masses. The derived upper limit on the
abundance of stellar mass MACHOs rules out such objects as the dominant
component of the Galactic halo if their mass is smaller than 2 solar masses.Comment: 7 pages, 4 figures, presented at the XIX International Conference on
Neutrino Physics and Astrophysics, Sudbury, Canada, June 200
Reconstructing the Properties of Dark Energy using Standard Sirens
Future space-based gravity wave experiments such as the Big Bang Observatory
(BBO), with their excellent projected, one sigma angular resolution, will
measure the luminosity distance to a large number of gravity wave (GW) sources
to high precision, and the redshift of the single galaxies in the narrow solid
angles towards the sources will provide the redshifts of the gravity wave
sources. One sigma BBO beams contain the actual source only in 68 per cent
cases; the beams that do not contain the source may contain a spurious single
galaxy, leading to misidentification. To increase the probability of the source
falling within the beam, larger beams have to be considered, decreasing the
chances of finding single galaxies in the beams. Saini, Sethi and Sahni (2010)
argued, largely analytically, that identifying even a small number of GW source
galaxies furnishes a rough distance-redshift relation, which could be used to
further resolve sources that have multiple objects in the angular beam. In this
work we further develop this idea by introducing a self-calibrating iterative
scheme which works in conjunction with Monte-Carlo simulations to determine the
luminosity distance to GW sources with progressively greater accuracy. This
iterative scheme allows one to determine the equation of state of dark energy
to within an accuracy of a few percent for a gravity wave experiment possessing
a beam width an order of magnitude larger than BBO (and therefore having a far
poorer angular resolution). This is achieved with no prior information about
the nature of dark energy from other data sets such as SN Ia, BAO, CMB etc.Comment: 12 pages, 10 figures. Expanded discussion, additional references.
Main results unchanged. Matches published versio
Pulsar Timing Probes of Primordial Black Holes and Subhalos
Pulsars act as accurate clocks, sensitive to gravitational redshift and
acceleration induced by transiting clumps of matter. We study the sensitivity
of pulsar timing arrays (PTAs) to single transiting compact objects, focusing
on primordial black holes and compact subhalos in the mass range from to well above . We find that the Square Kilometer
Array can constrain such objects to be a subdominant component of the dark
matter over this entire mass range, with sensitivity to a dark matter
sub-component reaching the sub-percent level over significant parts of this
range. We also find that PTAs offer an opportunity to probe substantially less
dense objects than lensing because of the large effective radius over which
such objects can be observed, and we quantify the subhalo concentration
parameters which can be constrained.Comment: 18 pages, 6 figure
A New Channel for the Detection of Planetary Systems Through Microlensing: II. Repeating Events
In the companion paper we began the task of systematically studying the
detection of planets in wide orbits () via microlensing surveys.
In this paper we continue, focusing on repeating events. We find that, if all
planetary systems are similar to our own Solar System, reasonable extensions of
the present observing strategies would allow us to detect 3-6 repeating events
per year along the direction to the Bulge. Indeed, if planetary systems with
multiple planets are common, then future monitoring programs which lead to the
discovery of thousands of stellar-lens events will likely discover events in
which several different planets within a single system serve as lenses, with
light curves exhibiting multiple repetitions. In this paper we discuss
observing strategies to maximize the discovery of all wide-orbit planet-lens
events. We also compare the likely detection rates of planets in wide orbits to
those of planets located in the zone for resonant lensing. We find that,
depending on the values of the planet masses and stellar radii of the lensed
sources (which determine whether or not finite source size is important), and
also on the sensitivity of the photometry used by observers, the detection of
planets in wide orbits may be the primary route to the discovery of planets via
microlensing. We also discuss how the combination of resonant and wide-orbit
events can help us to learn about the distribution of planetary system
properties (S 6.1). In addition, by determining the fraction of short-duration
events due to planets, we indirectly derive information about the fraction of
all short-duration events that may be due to low-mass MACHOs (S 6.2).Comment: 51 pages, 7 figures. To be published in the Astrophysical Journal, 20
February 1999. This completes the introduction to the discovery of planets in
wide orbits begun in astro-ph/9808075, also to appear in ApJ on 20 February
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