13 research outputs found
Magnetar-like X-ray Bursts from an Anomalous X-ray Pulsar
Anomalous X-ray Pulsars (AXPs) are a class of rare X-ray pulsars whose energy
source has been perplexing for some 20 years. Unlike other, better understood
X-ray pulsars, AXPs cannot be powered by rotation or by accretion from a binary
companion, hence the designation ``anomalous.'' AXP rotational and radiative
properties are strikingly similar to those of another class of exotic objects,
the Soft Gamma Repeaters (SGRs). However, the defining property of SGRs, namely
their low-energy gamma-ray and X-ray bursts, have heretofore not been seen in
AXPs. SGRs are thought to be ``magnetars,'' young neutron stars powered by the
decay of an ultra-high magnetic field. The suggestion that AXPs are magnetars
has been controversial. Here we report the discovery, from the direction of AXP
1E 1048-5937, of two X-ray bursts that have many properties similar to those of
SGR bursts. These events imply a close relationship between AXPs and SGRs, with
both being magnetars.Comment: 14 pages, 2 figures, accepted for publication in Nature. Note: The
content of this paper is embargoed until 1900 hrs London time / 1400 US
Eastern Time on Sept 1
An Anti-Glitch in a Magnetar
Magnetars are neutron stars showing dramatic X-ray and soft -ray
outbursting behaviour that is thought to be powered by intense internal
magnetic fields. Like conventional young neutron stars in the form of radio
pulsars, magnetars exhibit "glitches" during which angular momentum is believed
to be transferred between the solid outer crust and the superfluid component of
the inner crust. Hitherto, the several hundred observed glitches in radio
pulsars and magnetars have involved a sudden spin-up of the star, due
presumably to the interior superfluid rotating faster than the crust. Here we
report on X-ray timing observations of the magnetar 1E 2259+586 which we show
exhibited a clear "anti-glitch" -- a sudden spin down. We show that this event,
like some previous magnetar spin-up glitches, was accompanied by multiple X-ray
radiative changes and a significant spin-down rate change. This event, if of
origin internal to the star, is unpredicted in models of neutron star spin-down
and is suggestive of differential rotation in the neutron star, further
supporting the need for a rethinking of glitch theory for all neutron stars
X-ray emission from isolated neutron stars
X-ray emission is a common feature of all varieties of isolated neutron stars
(INS) and, thanks to the advent of sensitive instruments with good
spectroscopic, timing, and imaging capabilities, X-ray observations have become
an essential tool in the study of these objects. Non-thermal X-rays from young,
energetic radio pulsars have been detected since the beginning of X-ray
astronomy, and the long-sought thermal emission from cooling neutron star's
surfaces can now be studied in detail in many pulsars spanning different ages,
magnetic fields, and, possibly, surface compositions. In addition, other
different manifestations of INS have been discovered with X-ray observations.
These new classes of high-energy sources, comprising the nearby X-ray Dim
Isolated Neutron Stars, the Central Compact Objects in supernova remnants, the
Anomalous X-ray Pulsars, and the Soft Gamma-ray Repeaters, now add up to
several tens of confirmed members, plus many candidates, and allow us to study
a variety of phenomena unobservable in "standard'' radio pulsars.Comment: Chapter to be published in the book of proceedings of the 1st Sant
Cugat Forum on Astrophysics, "ICREA Workshop on the high-energy emission from
pulsars and their systems", held in April, 201
Mimicking human neuronal pathways in silico: an emergent model on the effective connectivity
International audienceWe present a novel computational model that detects temporal configurations of a given human neuronal pathway and constructs its artificial replication. This poses a great challenge since direct recordings from individual neurons are impossible in the human central nervous system and therefore the underlying neuronal pathway has to be considered as a black box. For tackling this challenge, we used a branch of complex systems modeling called artificial self-organization in which large sets of software entities interacting locally give rise to bottom-up collective behaviors. The result is an emergent model where each software entity represents an integrate-and-fire neuron. We then applied the model to the reflex responses of single motor units obtained from conscious human subjects. Experimental results show that the model recovers functionality of real human neuronal pathways by comparing it to appropriate surrogate data. What makes the model promising is the fact that, to the best of our knowledge, it is the first realistic model to self-wire an artificial neuronal network by efficiently combining neuroscience with artificial self-organization. Although there is no evidence yet of the model's connectivity mapping onto the human connectivity, we anticipate this model will help neuroscientists to learn much more about human neuronal networks, and could also be used for predicting hypotheses to lead future experiments