arXiv subjects

Ph. Jetzer

Publications and source records attributed to Ph. Jetzer.

86 records · Page 5Linked to original sources

arXiv1997

Microlensing results and the galactic models

Microlensing results towards the LMC strongly depend on the properties of both the luminous and the dark matter distribution in the Galaxy. The two main sources of uncertainty come from the poor knowledge of the rotation curve at large galactocentric distance and from the possibility that MACHOs follow radial orbits in the outer Galaxy. Given these uncertainties MACHO mass values $ \sim 0.1~ M_{\odot}$ are still consistent with observations and thus brown dwarfs are to date viable candidates for MACHOs.

astro-ph↗

arXiv1997

X-ray emission from dark clusters of MACHOs

MACHOs (Massive Astrophysical Compact Halo Objects) - as discovered by microlensing experiments towards the LMC - provide a natural explanation for the galactic halo dark matter. A realistic possibility is that MACHOs are brown dwarfs of mass $\sim 0.1 M_{\odot}$. Various arguments suggest that brown dwarfs should have a coronal X-ray emission of $\sim 10^{27}$ erg s$^{-1}$. As MACHOs are presumably clumped into dark clusters (DCs), each DC is expected to have a total X-ray luminosity of $\sim 10^{29}-10^{32}$ erg s$^{-1}$. We discuss the possibility that dark clusters contribute to the diffuse X-ray background (XRB) or show up as discrete sources in very deep field X-ray satellite observations. Moreover, from the observed diffuse XRB we infer that the amount of virialized diffuse gas present in the galactic halo can at most make up 5% of the halo dark matter.

astro-ph↗

arXiv1997

Microlensing implications for halo dark matter

The observations of microlensing events in the Large Magellanic Cloud suggest that a sizable fraction ($\sim$ 50%) of the galactic halo is in the form of MACHOs (Massive Astrophysical Compact Halo Objects) with an average mass $\sim 0.27 M_{\odot}$, assuming a standard spherical halo model. We describe a scenario in which dark clusters of MACHOs and cold molecular clouds (mainly of $H_2$) naturally form in the halo at galactocentric distances larger than 10--20 kpc.

astro-ph↗

arXiv1997

Baryonic Dark Matter

Reasons supporting the idea that most of the dark matter in galaxies and clusters of galaxies is baryonic are discussed. Moreover, it is argued that most of the dark matter in galactic halos should be in the form of MACHOs and cold molecular clouds.

astro-ph↗

arXiv1997

Achromatic variability in the BL Lac object PKS 2155-304. A case for microlensing?

PKS 2155-304 is the only BL Lac object for which well-sampled multiwavelength light curves resolve the intraday variability at UV and X-ray wavelengths. In particular we focus on the multifrequency campaign of November 1991, which showed a rather exceptional behaviour of the source exhibiting substantial achromatic variability from optical to X-ray wavelengths. We suggest a scenario for this unique event, where the variability is due to microlensing. The relativistic motion of the source relative to the lens is taken into account. The lenses are proposed to be solar-mass stars in an intervening dwarf galaxy. The a priori probability of detecting a microlensing event, however, was only few percent, small but not negligible.

astro-ph↗

arXiv1997

Halo dark clusters of brown dwarfs and molecular clouds

The observations of microlensing events in the Large Magellanic Cloud suggest that a sizable fraction (\sim 50%) of the galactic halo is in the form of MACHOs (Massive Astrophysical Compact Halo Objects) with an average mass $\sim 0.27 M_{\odot}$, assuming a standard spherical halo model. We describe a scenario in which dark clusters of MACHOs and cold molecular clouds (mainly of H_2) naturally form in the halo at galactocentric distances larger than 10-20 kpc.

astro-ph↗

arXiv1996

Baryonic dark matter

Reasons supporting the idea that most of the dark matter in galaxies and clusters of galaxies is baryonic is discussed. Moreover it is argued that most of the dark matter in galactic halo should be in the form of MACHOs and cold molecular clouds.

astro-ph↗

arXiv1995

Microlensing rates from self-consistent galactic models

We study different models of dark matter distribution for the halo of our galaxy. In particular, we consider Eddington and King-Michie models, which [C[C, which include anisotropy in the velocity space, and compute in a self-consistent way the amount of dark matter present in the halo. Assuming that the dark matter is in form of MACHOs, we find for each model the expected number of microlensing events and their average time duration for an experiment monitoring stars in the LMC. The main effect of including anisotropy is to reduce the microlensing rate by about 30% and to increase, but only slightly, the mean event duration as compared to the standard halo model. Consideration of different luminous models for the visible part of the galaxy also induce variations in the microlensing results by roughly the same amount as mentioned above. The main uncertainty, in order to be able to discriminate between different dark matter distributions and to estimate the fraction of it in form of MACHOs is due to the poor knowledge of the rotation velocity at large galactocentric distances.

astro-ph↗

arXiv1995

MACHOs and molecular clouds in galactic halos

Recent observations of microlensing events in the Large Magellanic Cloud by the MACHO and EROS collaborations suggest that an important fraction of the galactic halo is in form of Massive Astrophysical Compact Halo Objects (MACHOs) with mass ~ 0.1 M_{\odot}. We outline a scenario in which dark clusters of MACHOs and molecular clouds form in the halo at galactocentric distances larger than ~ 10-20 kpc, provide baryons are a major constituent of the halo. Possible signatures of the presence of molecular clouds in our galaxy are discussed. We also discuss how molecular clouds as well as MACHOs can be observed directly in the nearby M31 galaxy.

hep-ph↗

arXiv1995

Is the galactic halo baryonic?

Recent observations of microlensing events in the Large Magellanic Cloud suggest that a sizeable fraction of the galactic halo is in form of MACHOs with mass less than abou 0.1 M_{\odot}. Here we argue that molecular clouds (mainly H_2) located in the galactic halo can contribute substantially to its total mass. We outline a scenario in which dark clusters of MACHOs and molecular clouds naturally form in the halo at large galactocentric distances. Possible ways of detecting MACHOs via infrared emission and molecular clouds via the induced gamma-ray flux are discussed. Molecular clouds located in the M31 dark halo could be discovered through cosmic background radiation (CBR) anisotropies or emission lines in the microwave band.

astro-ph↗

arXiv1995

Observing molecular hydrogen clouds and dark massive objects in galactic halos

Molecular hydrogen clouds can contribute substantially to the galactic halo< dark matter and may lead to the birth of massive halo objects (MHOs) observed indirectly by microlensing. We present a method to detect these molecular clouds in the halo of M31 using the Doppler shift effect. We also consider the possibility to directly observe MHOs in the halo of M31 via their infrared emission.

astro-ph↗

arXiv1994

A Scenario for a Baryonic Dark Halo

The recent observations of microlensing events in the LMC by the MACHO and EROS collaborations suggest that an important fraction of the galactic halo is in the form of Massive Halo Objects (MHO) of about 0.1 M_{\odot}. Here, we argue that the galactic halo is mainly baryonic and that besides MHO also H_2 molecular clouds may significantly contribute to it. We propose a scenario in which dark clusters of MHO and/or H_2 molecular clouds form in the halo at galactocentric distances larger than ~ 10-20 kpc, since there we expect less collisions among proto globular cluster clouds and a smaller UV radiation flux. Cosmic ray protons may induce a significant gamma-ray flux in H_2 molecular clouds. Our calculation gives an upper bound to this flux which is below present detectability.

astro-ph↗

arXiv1994

A case for a baryonic dark halo

Recent observations of microlensing events in the Large Magellanic Cloud by the MACHO and EROS collaborations suggest that an important fraction of the galactic halo is in the form of Massive Halo Objects (MHO) with mass $\sim 0.1 M_{\odot}$. We outline a scenario in which dark clusters of MHO with mass $\sim 0.1 M_{\odot}$ and $H_2$ molecular clouds form in the halo at galactocentric distances larger than $\sim$ 10 - 20 kpc, provided baryons are a major constituent of the halo. Possible signatures of our picture are discussed.

astro-ph↗

arXiv1994

On the Mass of the Dark Compact Objects in the Galactic Disk

Recently the Polish-American collaboration OGLE has reported the observation of four possible microlensing events by monitoring, over several months, the brightness of millions of stars in the region of the galactic bulge. If these events are due to microlensing, the most accurate way to get information on the mass of the dark compact objects, that acted as gravitational lenses, is to use the method of the mass moments. Here I apply this method to the analysis of the events detected by OGLE. The average mass turns out to be 0.28$M_{\odot}$, suggesting that the lens objects are faint disk stars. The same method applied to the five microlensing events detected so far by the EROS and MACHO collaborations, which monitor stars in the Large Magellanic Cloud, leads to an average value of 0.08$M_{\odot}$ for the dark compact halo objects.

astro-ph↗