arXiv subjects

Bohdan Paczynski

Publications and source records attributed to Bohdan Paczynski.

At least 37 records · Page 2Linked to original sources

arXiv1999

Current Status of the Microlensing Surveys

The ongoing microlensing searches have generated more photometric measurements of pulsating stars than all previous observing projects combined. In particular, OGLE has made ~ 340,000 B, V, and I-band measurements of ~ 1,300 Cepheids in the Large Magellanic Clouds accessible over Internet. Microlensing searches contributed to the development of very efficient image subtraction software which works best in crowded fields. This suggests the use of a period - flux amplitude rather than period - luminosity relation for the Cepheids for distance determination, as the flux amplitude is directly measurable with the image subtraction, and it is not biased by crowding. Future projects will dramatically increase the data rate, will provide all-sky coverage and a complete census of variables, including pulsating stars, to the ever fainter limits. Time will show which approach, a small number of large teams or a large number of small teams, will be more productive.

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arXiv1999

Advection Dominated Accretion Flows. A Toy Disk Model

A toy model of a disk undergoing steady state accretion onto a black hole is presented. The disk is in a hydrostatic equilibrium for all radii r > r_{in}, with the inner disk radius located between the marginally stable and marginally bound orbits: r_{ms} > r_{in} > r_{mb}. Matter flows from the disk through a narrow cusp at r_{ms} and falls freely into the black hole, carrying with it no thermal energy. At radii larger than r_{out} the disk is assumed to radiate away all locally generated heat, and therefore the disk is geometrically thin for r > r_{out}. We assume that no heat generated in the inner disk, with r_{out} > r > r_{in} can be radiated away, i.e. the disk is 100% advective, and it becomes geometrically thick in this range of radii. All enthalpy of the thick disk is used up to press the inner disk radius towards the marginally bound orbit, and to lower the efficiency of conversion of accreted mass into radiation generated only for r > r_{out}, by assumption. Conservation laws of mass, angular momentum and energy make it possible to calculate the inner thick disk radius r_{in} for any specified value of its outer radius r_{out}. As the nature of disk viscosity is not known there is some freedom in choosing the shape of the thick disk, subject to several general conditions, which include the hydrostatic equilibrium everywhere for r > r_{in}. The main purpose of this toy model is to emphasize the effect the disk thickness has on lowering the energetic efficiency of a black hole accretion.

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arXiv1998

Metallicity of Red Clump Giants in Baade's Window

The red clump giants are potentially very useful as standard candles. There is some controversy about the stability of their I-band absolute magnitude, but it does not seem to be serious. No controversy was anticipated about their colors, with metal rich giants expected to be redder and cooler than the metal poor giants. The purpose of this paper is to point out that no such correlation is apparent between [Fe/H] and effective temperature as determined with Washington CCD photometry for the giants in Baade's Window. No explanation is offered for this surprising result. It is also unknown why the galactic bulge red clump giants are redder than the clump giants near the Sun by 0.2 mag in the (V-I) color.

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arXiv1997

GRBs as Hypernovae

A standard fireball/afterglow model of a gamma-ray burst relates the event to a merging neutron star binary, or a neutron star - black hole binary, which places the events far away from star forming regions, and is thought to have an energy of ~ 10^51 erg. A hypernova, the death of a massive and rapidly spinning star, may release ~ 10^54 erg of kinetic energy by tapping the rotational energy of a Kerr black hole formed in the core collapse. Only a small fraction of all energy is in the debris ejected with the largest Lorentz factors, those giving rise to the GRB itself, but all energy is available to power the afterglow for a long time. In this scenario GRBs should be found in star forming regions, the optical afterglows may be obscured by dust, and the early thermal emission of the massive ejecta may give rise to X-ray precursors, as observed by Ginga. The optical and X-ray afterglows of GRBs 970228, 97508, 97828 provide some evidence that these bursts were located in galaxies, most likely in dwarf galaxies, in or near star forming regions.

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arXiv1997

Gravitational Microlensing with the Space Interferometry Mission

The Space Interferometry Mission (SIM), with its launch date planned for 2005, has as its goal astrometry with ~ 1 micro-arcsecond accuracy for stars as faint as 20th mag. If the SIM lives to expectations it can be used to measure astrometric displacements in the light centroid caused by gravitational microlensing in the events detected photometrically from the ground. The effect is typically ~ 0.1 mas, i.e. two orders of magnitude larger than planned SIM's accuracy. Therefore, it will be possible to determine the mass, the distance, and the proper motion of almost any MACHO capable of inducing a photometric microlensing event towards the galactic bulge or the Magellanic Clouds, even though no light from the MACHO has to be detected. For strong microlensing events in which the source is photometrically resolved, like the recent MACHO 95-30 event, SIM's astrometry combined with accurate ground based photometry will allow the determination of the angular stellar radii, and therefore the effective temperature of the source. The effective astrometric cross sections for gravitational lensing by nearby high proper motion stars and brown dwarfs are ~ (1'')^2 and the effective time scales are ~ 1 year. SIM will provide the only practical way to measure masses of single nearby objects with ~ 1% accuracy. The times of lensing events can be predicted years ahead of time.

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arXiv1997

Tidal Disruption Eddington Envelopes around Massive Black Holes

Optically-thick envelopes may form following the tidal disruption of a star by a massive black hole. Such envelopes would reprocess hard radiation from accretion close to the black hole into the UV and optical bands producing AGN-luminosity flares with duration ~1 year. We show that due to relativistic effects, the envelopes are convective. If convection is efficient, then the structure of the envelopes is similar to that described in previous work; however, the photospheric radius is shown to be very sensitive to the luminosity at the envelope base, suggesting that either the envelope collapses or the envelope expands to a maximum radius at which point a wind may set in. For an envelope without winds, we find a maximum photospheric radius of ~10^16 cm (i.e. minimum effective temperature ~6,000 K). The evolution of the envelopes is described based on simple energy arguments.

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arXiv1997

Binary Source Parallactic Effect in Gravitational Micro-lensing

The first micro-lensing event discovered towards the Small Magellanic Cloud by the MACHO collaboration (Alcock et al. 1997b) had a very long time scale, t_0 = 123 days. The EROS collaboration (Palanque-Delabrouille et al. 1997) discovered a 2.5% brightness variation with a period P = 5.1 days. The OGLE collaboration (Udalski et al. 1997) established that the variation persists while the micro-lensing event is over, and the variable star is the one which has been micro-lensed, not its blend. The simplest explanation of the periodic variability is in terms of a binary star with the orbital period P(orb) = 10.2 days, with its component(s) tidally distorted. Such objects are known as ellipsoidal variables. The binary nature should be verified spectroscopically. Binary motion of the source introduces a parallactic effect into micro-lensing light curve, and a few examples are shown. The effect is relatively strong if the light center and the mass center of a binary are well separated, i.e. if the binary has a large photometric dipole moment. The diversity of binary parameters is large, and the corresponding diversity of photometric effects is also large. The presence or absence of the effect may constrain the lens mass and its distance from the source.

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arXiv1997

Galactocentric Distance With the OGLE and Hipparcos Red Clump Stars

We compare red clump stars with parallaxes known to better than 10% in the Hipparcos catalog and corrected for the interstellar extinction, with the OGLE red clump stars in Baade's Window also corrected for the interstellar extinction. There are $\sim 600$ and $\sim 10,000$ such stars in the two data sets, respectively. We find empirically that the average I-band magnitude of red clump stars does not depend on their intrinsic color in the range $ 0.8 < (V-I)_0 < 1.4 $. The red clump luminosity function is well represented by a gaussian with the peak at $ M_{I_0,m} = -0.26 $, and the dispersion $ σ_{RC} \approx 0.2 $ mag. This allows a single step determination of the distance to the galactic center and gives $ R_0 = 8.4 \pm 0.4 $ kpc. The number of red clump stars is so large that a formal statistical error is only $\sim 1%$. The local stars are relatively blue and have a small color dispersion: $<(V-I) > = 1.01, σ_{(V-I)} = 0.08$, while for the bulge stars $<(V-I)_0 > = 1.22, σ_{(V-I)_0} = 0.14$. Presumably, the bulge population has a broader range and a higher average metallicity than the local disk population.

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arXiv1997

Are Gamma-Ray Bursts in Star Forming Regions?

The optical afterglow of the gamma-ray burst GRB 970508 (z = 0.835) was a few hundred times more luminous than any supernova. Therefore, a name `hypernova' is proposed for the whole GRB/afterglow event. There is tentative evidence that the GRBs: 970228, 970508, and 970828 were close to star forming regions. If this case is strengthened with future afterglows then the popular model in which GRBs are caused be merging neutron stars will have to be abandoned, and a model linking GRBs to cataclysmic deaths of massive stars will be favored. The presence of X-ray precursors, first detected with Ginga, is easier to understand within a framework of a `dirty' rather than a `clean' fireball. A very energetic explosion of a massive star is likely to create a dirty fireball, rather than a clean one. A specific speculative example of such an explosion is proposed, a microquasar. Its geometrical structure is similar to the `failed supernova' of Woosley (1993a): the inner core of a massive, rapidly rotating star collapses into a ~ 10 solar mass Kerr black hole with ~ 5 x 10^54 erg of rotational energy, while the outer core forms a massive disk/torus. A superstrong ~ 10^15 G magnetic field is needed to make the object operate as a microquasar similar to the Blandford & Znajek (1977) model. Such events must be vary rare, 10^4 - 10^5 times less common than ordinary supernovae, if they are to account for the observed GRBs.

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arXiv1997

Microlensing of Blended Stellar Images

The current modelling of single microlensing light curves neglects the possibility that only a fraction of the light is due to the lensed star, the remaining being due to a close, unresolved blend, which may be related or unrelated to the lens. Unfortunately, the effects of blending are significant as all microlensing experiments choose very crowded fields as their targets. In this paper we point out a strong degeneracy of the fitting procedure which makes it practically impossible to detect the presence of a blend by purely photometric means, except in a small part of the parameter space. Some blends may be detected by astrometric means, but the majority have to be corrected for statistically. The luminosity function reaching well below the ground based detection limit (with the HST) would be very also important and this could be determined with the repeating microlensing events. If no correction is made then the event time scales, the lens masses, and the optical depth are all systematically underestimated.

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arXiv1996

The future of massive variability searches

This is a personal review of various issues related to massive photometric and astrometric searches. A complete inventory of variable stars down to almost any magnitude limit will improve our understanding of the stellar evolution and the galactic structure. A search for detached eclipsing binaries will improve the distance scale, the value of the Hubble constant, and the age of the oldest stars. A search for supernovae will help the determination of cosmological parameters Omega and Lambda. A search for microlensing events will provide insight into the stellar mass function, dark matter, and may lead to a discovery of earth-mass planets.

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arXiv1996

Detached eclipsing binaries as primary distance and age indicators

Detached eclipsing double line spectroscopic binaries offer an opportunity to measure directly stellar parameters: mass, luminosity, radius, as well as the distance. The only non-trivial step is the need to determine surface brightness of each component on the basis of something measurable, like the color or the line ratios. Modern model atmospheres provide a fairly good calibration of that relation, but empirical verification is possible, and it is needed to achieve the highest accuracy. When this approach is fully developed the detached eclipsing binaries should provide direct (single step) distances with about 1% accuracy to all galaxies in the Local Group. Recent discovery of numerous detached eclipsing binaries near the main sequence turn-off points in several globular clusters (Kaluzny et al. 1996a,b) will be followed by accurate determinations of distances, masses and luminosities. The empirical the mass - luminosity relation near the main sequence turn-off point will allow accurate age and helium determinations, and a check on the stellar evolution theory.

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arXiv1996

The Masses of Nearby Dwarfs and Brown Dwarfs with the HST

The known nearby stars, moving in front of the background of distant stars and galaxies, create 'weak gravitational lensing' variations in their positions. These variations may be measurable with the HST, and they may allow a direct mass determination for the nearby stars. The cross section for the HST measurable astrometric effect is much larger than the cross section for the photometric effect which is measurable from the ground. The mass determination will be easier for the fainter nearby lenses which will be discovered in future searches of faint high proper motion stars.

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arXiv1996

Gravitational Microlensing in the Local Group

The status of searches for gravitational microlensing events of the stars in our galaxy and in other galaxies of the Local Group, the interpretation of the results, some theory, and prospects for the future are reviewed. The searches have already unveiled about 100 events, at least two of them caused by binaries, and have already proven to be useful for studies of the Galactic structure. The events detected so far are probably attributable to the effects of ordinary stars, and possibly to sub-stellar brown dwarfs; however a firm conclusion cannot be reached yet because the analysis published to date is based on a total of only 16 events. The current searches, soon to be upgraded, will probably allow determination of the mass function of stars and brown dwarfs in the next few years; these efforts will also provide good statistical information about binary systems, in particular their mass ratios. They may also reveal the nature of dark matter and allow us to detect planets and planetary mass objects.

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arXiv1995

Gravitational Microlensing, The Distance Scale, And The Ages

A high optical depth to gravitational microlensing towards the galactic bulge is consistent with current models of the galactic bar. The low optical depth towards the LMC can probably be accounted for by the ordinary stars in our galaxy and in the LMC itself. No conclusive evidence is available yet for the presence or absence of a large number of brown dwarfs or other non-stellar compact objects which might account for the dark matter. There is little doubt that the amount of mass in objects in the range 10^{-8} < M/M_sun < 10^6 will be determined within the next few years with the continuing and expanding searches. Billions of photometric measurements generated by the microlensing searches have lead to the discovery of about 10^5 variable stars. In particular, a number of detached eclipsing binaries were discovered in the galactic bulge, in the LMC, and in the globular cluster Omega Centauri. The follow-up observations of these binaries will allow the determination of accurate distances to all these objects, as well as robust age determination of globular clusters.

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arXiv1995

HOW FAR AWAY ARE GAMMA-RAY BURSTERS?

The positions of over 1000 gamma-ray bursts detected with the BATSE experiment on board of the Compton Gamma Ray Observatory are uniformly and randomly distributed in the sky, with no significant concentration to the galactic plane or to the galactic center. The strong gamma-ray bursts have an intensity distribution consistent with a number density independent of distance in Euclidean space. Weak gamma-ray bursts are relatively rare, indicating that either their number density is reduced at large distances or that the space in which they are distributed is non-Euclidean. In other words, we appear to be at the center of a spherical and bounded distribution of bursters. This is consistent with the distribution of all objects that are known to be at cosmological distances (like galaxies and quasars), but inconsistent with the distribution of any objects which are known to be in our galaxy (like stars and globular clusters). If the bursters are at cosmological distances then the weakest bursts should be redshifted, i.e. on average their durations should be longer and their spectra should be softer than the corresponding quantities for the strong bursts. There is some evidence for both effects in the BATSE data. At this time the cosmological distance scale is strongly favored over the galactic one, but is not proven. A definite proof (or dis-proof) could be provided with the results of a search for very weak bursts in the Andromeda galaxy (M31) with an instrument $ \sim 10 $ times more sensitive than BATSE. If the bursters are indeed at cosmological distances then they are the most luminous sources of electromagnetic radiation known in the universe. At this time we have no clue as to their nature, even though well over a hundred suggestions were published in the scientific journals. An experiment providing

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arXiv1995

On the Detectability of Very Massive Compact Objects with Gravitational Microlensing

If the dark halo of our galaxy is made of compact objects as massive as M = 10^6 solar masses, their detection by means of ordinary microlensing searches would take a very long time as the characteristic time scale of such a lensing event, t_0, is approximately 200 years. Fortunately, the very high magnification events of the numerous faint stars, which are normally well below the detection threshold, have short duration peaks with a characteristic time scale of only t_0 / A_{max}, where A_{max} >> 1 is the peak magnification factor. The two almost equally bright images are separated by approximately 2'' (M/10^6 solar masses )^{1/2}, and they rotate very rapidly around the lens with the relative proper motion enhanced by a factor 2 A_{max}. The same events will offer an opportunity to study spectroscopically stars which are normally far too faint to be reached.

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arXiv1994

Parameter degeneracy in models of the quadruple lens system Q2237+0305

The geometry of the quadruple lens system Q2237+0305 is modeled with a simple astigmatic lens: a power-law mass distribution: m \propto r^βwith an external shear γ. The image positions can be reproduced with an accuracy better than 0.01 arcsec for any 0.0 < β< 1.85 and the corresponding value of γ= 0.1385 - 0.0689 β. This is a factor of about 4 more precise than what can be achieved by the best constant M/L lens models. The image intensity ratios and the time delay ratios are almost constant along our one parameter family of models, but the total magnification varies from 8 to > 1000, and the maximum time delay (between leading image B and trailing image C) for H_0 of 75 km/sec/Mpc varies from more than 20 hours to about 1.5 hours, while βincreases from 0.0 to 1.85.

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