arXiv subjects

P. Jetzer

Publications and source records attributed to P. Jetzer.

24 records · Page 2Linked to original sources

arXiv2008

On Gravitational Waves in Spacetimes with a Nonvanishing Cosmological Constant

We study the effect of a cosmological constant $Λ$ on the propagation and detection of gravitational waves. To this purpose we investigate the linearised Einstein's equations with terms up to linear order in $Λ$ in a de Sitter and an anti-de Sitter background spacetime. In this framework the cosmological term does not induce changes in the polarization states of the waves, whereas the amplitude gets modified with terms depending on $Λ$. Moreover, if a source emits a periodic waveform, its periodicity as measured by a distant observer gets modified. These effects are, however, extremely tiny and thus well below the detectability by some twenty orders of magnitude within present gravitational wave detectors such as LIGO or future planned ones such as LISA.

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arXiv2007

Q0045-3337: models including strong lensing by a spiral galaxy

Aims. Falomo et al. (2005) discovered a disk-like galaxy at ~ 1.2 arcsec from the QSO Q0045-3337 by means of ESO VLT adaptive optics. They estimated a galaxy Einstein radius (for a point mass) of comparable size, thus pointing up the existence of a new, rare, spiral lens candidate, despite no evident image splitting. Here we discuss the possible lensing effect of the galaxy in some more detail. Methods. We performed two dimensional surface photometry on the VLT image of the galaxy, confirming its spiral nature. We then verified if simple mass models, partially constrained by observational data, require unrealistic parameters to produce a still hidden second quasar image. We also evaluated the respective viability of an instrumental or a lensing origin of the observed QSO deformation. Results. After galaxy model subtraction, we found a residual image, likely not related to gravitational lensing. Existing data are not sufficient to assess the presence of image splitting, nor to constrain the number of images or discriminate between galaxy mass models. Conclusion. Further observations are mandatory to progress in the study of this remarkable system, that could shed more light on the lensing behaviour of spiral galaxies.

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arXiv2003

The Anomaly in the Candidate Microlensing Event PA-99-N2

The lightcurve of PA-99-N2, one of the recently announced microlensing candidates towards M31, shows small deviations from the standard Paczynski form. We explore a number of possible explanations, including correlations with the seeing, the parallax effect and a binary lens. We find that the observations are consistent with an unresolved RGB or AGB star in M31 being microlensed by a binary lens. We find that the best fit binary lens mass ratio is about one hundredth, which is one of most extreme values found for a binary lens so far. If both the source and lens lie in the M31 disk, then the standard M31 model predicts the probable mass range of the system to be 0.02-3.6 solar masses (95 % confidence limit). In this scenario, the mass of the secondary component is therefore likely to be below the hydrogen-burning limit. On the other hand, if a compact halo object in M31 is lensing a disk or spheroid source, then the total lens mass is likely to lie between 0.09-32 solar masses, which is consistent with the primary being a stellar remnant and the secondary a low mass star or brown dwarf. The optical depth (or alternatively the differential rate) along the line of sight toward the event indicates that a halo lens is more likely than a stellar lens provided that dark compact objects comprise no less than 15 per cent (or 5 per cent) of haloes.

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arXiv1999

Thermal equilibrium of molecular clouds in cooling flow clusters

In many clusters of galaxies there is evidence for cooling flows in the central regions. The ultimate fate of the gas which cools is still unknown. A possibility is that a fraction of the gas forms cold molecular clouds. We investigate the minimum temperature which can be reached by clouds in cooling flows by computing the cooling function due to H2, HD and CO molecules. As an example, we determine the minimum temperature achievable by clouds in the cooling flows of the Centaurus, Hydra and PKS 0745-191 clusters. Our results suggest that clouds can reach very low temperatures - less than 10 K - which would explain the non-detection of CO in these clusters.

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arXiv1998

Gravitational Microlensing by Globular Clusters

Stars in globular clusters can act either as sources for MACHOs (Massive Astrophysical Compact Halo Objects) located along the line of sight or as lenses for more distant background stars. Although the expected rate of microlensing events is small, such observations can lead to very useful results. In particular, one could get information on the shape of the galactic halo along different lines of sight, allowing to better constrain its total dark matter content. Moreover, on can also infer the total dark matter content of globular clusters, which is presently not well known. To this latter purpose, we analyse the microlensing events towards the galactic bulge, which lie close to the three globular clusters NGC 6522, NGC 6528 and NGC 6540. We find evidence that some microlensing events are indeed due to MACHOs located in the globular clusters, suggesting, therefore, that these clusters contain a significant amount of dark matter.

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arXiv1993

Charged Boson Stars and Vacuum Instabilities

We consider charged boson stars and study their effect on the structure of the vacuum. For very compact particle like ``stars", with constituent mass $m_{*}$ close to the Planck mass $m_{Pl}$, i.e. $m_{*}^{2} = {\cal O} (αm_{Pl}^{2})$, we argue that there is a limiting total electric charge $Z_c$, which, primarily, is due to the formation of a pion condensate ($Z_{c} \simeq 0.5α^{-1}e$, where $α$ is the fine structure constant and $e$ is the electric charge of the positron). If the charge of the ``star" is larger than $Z_c$ we find numerical evidence for a complete screening indicating a limiting charge for a very compact object. There is also a less efficient competing charge screening mechanism due to spontaneous electron-positron pair creation in which case $Z_{c} \simeq α^{-1}e$. Astrophysical and cosmological abundances of charged compact boson stars are briefly discussed in terms of dark matter.

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