Compact objects
Compact objects are dominated by gravitation, which overcomes all other interactions. These objects provide the best framework for the study of the consistency of general relativity as well as for the exploration of its theoretical predictions in the strong field regimes. The group is actively involved in the theoretical and observational study of compact objects, from neutron stars to wormholes. Here are some topics of our research.
[Black holes][Boson stars][Wormholes (theoretical)][Wormholes (observational)][Microquasars]
Black holes are objects that have collapsed beyond their own Schwarzschild radius. They are separated from the rest of the universe by an event horizon: nothing, even light, can escape from them. Black holes, however, interact with their surroundings through their gravitational field. Matter accreting onto the hole tends to form an accretion disk, which emits thermal radiation. Magnetic fields anchored in the disk probably play an important role in the formation and confinement of relativistic jets. These jets are bipolar plasma ejections that transport mass and energy up to large distances to the hole. In the case of supermassive black holes in AGNs the jets can travel through distances of megaparsecs in the intergalactic medium.
|
|
|
Accretion power is fundamental in high-energy astrophysics. Collapsed objects are thought to be the primary engines of objects like quasars,
gamma-ray bursts and X-ray binaries. Our group has developed an intensive research activity in topics related to black holes. For instance, a binary
black hole model has been developed to explain the apparent jet precession of the famous quasar 3C273. Stability properties of relativistic jets have
been studied in a variety of ambient conditions, and significant research has been conducted on the subject of theoretical
modeling of the rapid variability presented by blazars.
Stellar size black holes can, in principle, be charged. Recent research by Brian Punsly has shown that certain configurations of Kerr-Newman black
holes plus magnetosphere can be stable and survive during relatively large time
scales. Our group has been involved in the analysis and search of possible observational signatures of such objects. As in other fields, we have
applied to this problem a multifrequency approach where experiment and observations are closely guided by original theoretical research in order
to obtain valuable results.
Maybe the closest theoretical construct to a black hole is a boson star. Boson stars are localized, asymptotically flat configurations of gravitationally bound zero temperature bosons. Mathematically, the boson field is described by a complex wave function whose Lagrangian possesses an internal U(1) symmetry that gives rise to a conserved charge N, interpreted as the total number of bosons. The time dependence of the boson field does not appear in the field equations, and the solutions describe bound eigenstates with a number of nodes that increases with the energy. The first boson star solutions were found by Kaup, and independently by Ruffini & Bonazzola, in the late 60's, who studied spherically symmetric stars in General Relativity (GR). They found that the solutions were qualitatively similar to those describing neutron stars and white dwarfs, although they were of much smaller mass. Colpi, Shapiro & Wasserman (1986) extended their work by examining GR boson stars whose matter Lagrangian includes a quartic self-interaction term. This additional term contributes to the pressure of the star, increasing both its mass and particle number. More general self-interaction terms can be considered, and a complete study on that were presented by some members of the group.
|
|
|
Left: Boson star configurations, see Boson stars in general scalar-tensor gravitation: Equilibrium configurations
by Torres in Phys. Rev. D56, 3478 (1997) for details. Right: A boson star
at the galactic center?, see the paper A supermassive scalar star at the galactic center?
by Torres, Capozziello and Lambiase in Physical Review D62, 104012 (2000) for
details.
Members of the group have also used boson stars as probes for different theories of gravity, in particular those in which the Newton constant may be subject to time variation. Interesting phenomena may appear in these situations, like a new kind of stellar evolution totally driven by the change in the gravitational theory. This kind of gravitational evolution may affect any other non-transient object too, like an ordinary white dwarf. A study of the evolution of white dwarfs within variable-G theories was also presented by members of GARRA.
The group is currently very interested in developing a program to search for observational signals of boson stars. Significant analogies with black holes has already been remarked, and then many of the astrophysical phenomena that are produced in black holes environments could also be find in boson star ones. For instance, very recently, Torres, Capozziello and Lambiase have been developing an alternative model for the galactic center (and those of other galaxies) using a supermassive boson star. This theoretical model seems to be a consistent alternative to the central black hole paradigm.
Wormholes are non-trivial topological configurations of space-time that can be represented by solutions of Einstein field equations with stress-energy tensor fields that somewhere violate the so-called average null energy condition . Although microscopic violations of the energy conditions are well known (e.g. the Casimir effect), it is far from clear whether stable, macroscopic wormholes can naturally exist in the Universe. One of the ways in which one may obtain violations to the energy conditions is via a scalar fields coupled to gravity. Wormhole formation at a late cosmic time requires Lorentzian topology change in space, something that appears to be more than problematic to most physicists because it implies causality violations. However, if wormholes are created altogether with space-time and not formed by astrophysical processes, one could expect a cosmological population of these objects without the uncomfortable predictions of topology change theorems.
|
|
|
Left: Topologies of he space-time. For the Spanish reader, see our paper in Ciencia Hoy. Right: Problems with background time travel? Paradoxes are thoroughly analyzed in the recently submitted paper by Romero and Torres, Self-existing objects and auto-generated information in chronology-violating space-times.
To study wormhole existence any further we have first to see if there effectively are wormhole solutions of the relativistic field equations. We have been studying different situations, but specially in scalar tensor gravitation, where the scalar field of the theory can be itself the source of the violation of the energy conditions. We presented vacuum and non-vacuum wormhole solutions in that case.
We have shown that microlensing events produced by wormholes, if they exist at all, with active galactic nuclei (AGNs) as background sources, very much resemble certain types of GRBs; types that cannot be explained in standard models. We then used observational data on GRBs to determine an upper limit for the amount of wormhole-like objects in the universe: | \rho | < 9.05 x 10^{-36} g cm ^{-3}. Negative matter hardly would have any influence in cosmology.
Figure courtesy of New Scientists. See the complete explanation by P. Parsons,
New Scientist News and Views, March 28, 1998.
An unusual feature of the presented scenario is that, while GRB repetition has previously been seen as a strong evidence for non-cosmological origin, the microlensing model accepts it warmly: sources are cosmological and repetitions arise from different caustic crossings (recall the graphic explanation). Besides, this model implies that not only some bursting events must repeat, but also that they should do it with temporal profiles of specular character. This makes the model capable to be falsified. We expect that, with the improvement of the observational techniques and the increase of the GRB sample, more exact limits to the amount of the negative mass will be available. Forthcoming technologies and satellites such as the High Energy Transient Explorer (HETE), the next Gamma Ray Large Area Space Telescope (GLAST) and the current Beppo-SAX satellite will help to improve burst position measurements yielding light onto the repetition phenomenon.
Within the current observational capabilities (i.e. the extremely successful BATSE mission), we have made the first search for natural wormholes in astrophysical databases. Our search for natural wormholes through microlensing was sensitive to time-scales up to 3.5 years. Repetition of events over longer scales can not be ruled out. Our results show that, if repetition is associated to wormhole microlensing alone, it could reach, at most, a level 4% over time-scales larger than 3.5 years. At shorter scales, wormhole-induced repetitions are constrained at a level <0.2%. Since microlensing time-scale increases with larger masses of the lenses, the absence of clear detections in our search might be saying that wormholes, if they exist at all, have a mass distribution peaking far beyond the few tenths of solar masses required to produce typical microlensing events with time-scales of a few years.
Microquasars
are X-ray binary systems with non-thermal radio emission in the form of jet-like
features. These systems are formed by a compact object (black hole or neutron
star) and by a stellar companion that can be a high-mass and young star (in
which case we talk about `high-mass microquasars') or a low-mass and old star
(`low-mass microquasars´). These objects can present high-energy non-thermal
emission (X-rays, perhaps even gamma-rays) and superluminal motions, which are
indicative of relativistic bulk motions in the plasma ejected by the
compact
object.
Figure 1: Artistic view of microquasar.
Figure
2: X-ray binary system
Microquasars were discovered by Mirabel et al. in 1992 (Nature 358, 215, 1992).
In 1994 Mirabel and Rodriguez discovered the first superluminal source (the
microquasar GRS 1915+105) in the Galaxy (Nature 371, 46, 1994). In recent years
our group has been investigating the mechanisms that might produce high-energy
gamma-rays in microquasars as well as the possible connection between
microquasars and unidentified gamma-ray sources (e.g. Kaufman et al., Astron.
Astrophys. 385, L10, 2002; Romero et al. Astron. Astrophys. 393, L61, 2002).
Figure
3: Gamma-ray emission from the jet-wind interaction in a microquasar. From
Romero et al. (2003, A&A Lett, 410, L1). [BACK]
In addition to microquasars where the donor star has spherical wind we
have considered the case of systems where the wind is confined to the
equatorial plane of the star. In particular, we have studied the case of
the gamma-ray binary LSI +61 303, where the primary is a Be star (see
for instance Orellana & Romero, 2006). This is an
extraordinary object detected by COS-B, EGRET and MAGIC telescope, and
the nature of its high-energy emission is currently under debate.
