arXiv subjects

Tal Alexander

Publications and source records attributed to Tal Alexander.

At least 55 records · Page 3Linked to original sources

arXiv2005

Stellar Processes Near the Massive Black Hole in the Galactic Center

A massive black hole resides in the center of most, perhaps all galaxies. The one in the center of our home galaxy, the Milky Way, provides a uniquely accessible laboratory for studying in detail the connections and interactions between a massive black hole and the stellar system in which it grows; for investigating the effects of extreme density, velocity and tidal fields on stars; and for using stars to probe the central dark mass and probe post-Newtonian gravity in the weak- and strong-field limits. Recent results, open questions and future prospects are reviewed in the wider context of the theoretical framework and physical processes that underlie them. Contents: [1] Introduction (1.1) Astrophysical context (1.2) Science questions (1.3) Scope and connections to related topics [2] Observational overview: Stars in the Galactic center (2.1) The central 100 parsecs (2.2) The central parsec [3] Stellar dynamics at extreme densities (3.1) Physical processes and scales (3.2) The stellar cusp in the Galactic center (3.3) Mass segregation (3.4) Stellar Collisions [4] Probing the dark mass with stellar dynamics (4.1) Weighing and pinpointing the dark mass (4.2) Constraints on non-BH dark mass alternatives (4.3) Limits on MBH binarity (4.4) High-velocity runaway stars [5] Probing post-Newtonian gravity near the MBH (5.1) Relativistic orbital effects (5.2) Gravitational lensing [6] Strong star-MBH interactions (6.1) Tidal disruption (6.2) Dissipative interactions with the MBH [7] The riddle of the young stars (7.1) The difficulties of forming or importing stars near a MBH (7.2) Proposed solutions (7.3) Feeding the MBH with stellar winds [8] Outlook (8.1) Progress report (8.2) Future directions

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arXiv2004

Orbital capture of stars by a massive black hole via exchanges with compact remnants

We propose a dynamical mechanism for capturing stars around a massive black hole (MBH), which is based on the accumulation there of a very dense cluster of compact stellar remnants. This study is motivated by the presence of ~10 young massive stars (M*~3-15 Mo, ~B9V--O8V) less than 0.04 pc from the MBH in the Galactic center (GC). Their existence in the extreme environment so close to a MBH is a challenge for theories of star formation and stellar dynamics. We show that young stars, which formed far from the MBH and were then scattered into eccentric orbits, repeatedly cross a cluster of stellar black holes (SBHs), where they may undergo rare direct 3-body exchanges with a MBH-SBH "binary". The interaction between two objects of similar mass ejects the SBH and captures the star on a tight orbit around the MBH. Such captures can naturally explain trends observed in the orbits of the young stars. We derive the capture cross-section, validate it by Monte-Carlo simulations, and calculate the number of captured stars in the GC using the currently uncertain estimates of the numbers of SBHs inside 0.04 pc and of young stars in the inner few pc of the GC. We find that under favorable conditions 3-body exchange can account for \~25% of the observed stars, mostly at the fainter end of the observed range. We discuss additional effects that possibly increase the capture efficiency. Future observations will establish whether there are enough SBHs and young stars for exchange captures to singly account for the central young stars. We estimate that there are also ~35 lower mass stars (M*~1-3 Mo, ~G2V--A0V) inside 0.04 pc similarly captured by exchanges with neutron stars. Ongoing replacement of compact remnants by main sequence stars may regulate the accumulation of compact remnants near the MBH.

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arXiv2004

Ultraluminous X-ray Sources as Intermediate Mass Black Holes Fed by Tidally Captured Stars

The nature of ultraluminous X-ray sources (ULXs) is presently unknown. A possible explanation is that they are accreting intermediate mass black holes (IBHs) that are fed by Roche lobe overflow from a tidally captured stellar companion. We show that a star can circularize around an IBH without being destroyed by tidal heating (in contrast to the case of M_bh> 10^6 M_sun massive black holes in galactic centers, where survival is unlikely). We find that the capture and circularization rate is of the order of 5 \times 10^-8 yr^-1, almost independently of the cluster's relaxation time. We follow the luminosity evolution of the binary system during the main sequence Roche lobe overflow phase and show it can maintain ULX-like luminosities for >10 Myr. In particular, we show that the ULX in the young cluster MGG-11 in star-burst galaxy M82, which possibly harbors an IBH, is well explained by this mechanism, and we predict that \gtrsim 10% of similar clusters with IBHs have a tidally captured circularized star. The cluster can evaporate on a time-scale shorter than the lifetime of the binary. This raises the possibility of a ULX that outlives its host cluster, or even lights up only after the cluster has evaporated, in agreement with observations of host-less ULXs.

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arXiv2004

Intrinsic Absorption in the Spectrum of Mrk 279: Simultaneous Chandra, FUSE, and STIS Observations

We present a study of the intrinsic X-ray and far-ultraviolet absorption in the Seyfert 1.5 galaxy Markarian 279 using simultaneous observations from the Chandra X-ray Observatory, the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope, and the Far Ultraviolet Spectroscopic Explorer (FUSE). We also present FUSE observations made at three additional epochs. We detect the Fe K-alpha emission line in the Chandra spectrum, and its flux is consistent with the low X-ray continuum flux level of Mrk 279 at the time of the observation. Due to low signal-to-noise ratios in the Chandra spectrum, no O VII or O VIII absorption features are observable in the Chandra data, but the UV spectra reveal strong and complex absorption from HI and high-ionization species such as O VI, N V, and C IV, as well as from low-ionization species such as C III, N III, C II, and N II in some velocity components. The far-UV spectral coverage of the FUSE data provides information on high-order Lyman series absorption, which we use to calculate the optical depths and line and continuum covering fractions in the intrinsic HI absorbing gas in a self-consistent fashion. The UV continuum flux of Mrk 279 decreases by a factor of ~7.5 over the time spanning these observations and we discuss the implications of the response of the absorption features to this change. From arguments based on the velocities, profile shapes, covering fractions and variability of the UV absorption, we conclude that some of the absorption components, particularly those showing prominent low-ionization lines, are likely associated with the host galaxy of Mrk 279, and possibly with its interaction with a close companion galaxy, while the remainder arises in a nuclear outflow.

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arXiv2003

Squeezars: Tidally powered stars orbiting a massive black hole

We propose that there exists a class of transient sources, "squeezars", which are stars caught in highly eccentric orbits around a massive (m<10^8 Mo) black hole (MBH), whose atypically high luminosity (up to a significant fraction of their Eddington luminosity) is powered by tidal interactions with the MBH. Their existence follows from the presence of a mass sink, the MBH, in the galactic center, which drives a flow of stars into nearly radial orbits to replace those it has destroyed. We consider two limits for the stellar response to tidal heating: surface heating with radiative cooling ("hot squeezars") and bulk heating with adiabatic expansion ("cold squeezars"), and calculate the evolution of the squeezar orbit, size, luminosity and effective temperature. The squeezar formation rate is only ~0.05 that of tidal disruption flares, but squeezar lifetimes are many orders of magnitude longer, and so future observations of squeezars in nearby galaxies can probe the tidal process that feeds MBHs and the effects of extreme tides on stars. The mean number of squeezars orbiting the Galactic MBH is estimated at 0.1-1.

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arXiv2003

Orbital inspiral into a massive black hole in a galactic center

A massive black hole (MBH) in a galactic center drives a flow of stars into nearly radial orbits to replace those it destroyed. Stars whose orbits cross the event horizon r_s or the tidal disruption radius r_t are promptly destroyed in an orbital period P. Stars with orbital periapse r_p slightly larger than the sink radius q=max(r_s,r_t) may slowly spiral in due to dissipative interactions with the MBH, e.g. gravitational wave emission, tidal heating or accretion disk drag, with observable consequences and implications for the MBH growth rate. Unlike prompt destruction, the inspiral time is typically >>P. This time is limited by the same scattering process that initially deflected the star into its eccentric orbit, since it can deflect it again to a wider orbit where dissipation is inefficient. The ratio between slow and prompt event rates is therefore much smaller than that implied by the ratio of cross-sections, ~r_p/q, and so only prompt disruption contributes significantly to the mass of the MBH. Conversely, most stars that scatter off the MBH survive the extreme tidal interaction ("tidal scattering"). We derive general expressions for the inspiral event rate and the mean number of inspiraling stars, and show that the survival probability of tidally scattered stars is ~1, and that the number of tidally heated stars ("squeezars") and gravity wave emitting stars in the Galactic Center is ~0.1-1.

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arXiv2003

Stellar Tidal Processes Near Massive Black Holes

Close tidal interactions of stars with a central massive black hole (MBH) or with other stars in the high density cusp around it can affect a significant fraction of the stellar population within the MBH radius of influence. We consider three strong processes that have the potential of modifying stellar structure and evolution there. (1) Tidal spin-up by hyperbolic star-star encounters. (2) Tidal scattering of stars on the MBH. (3) Tidal heating of inspiraling stars - "squeezars" - that were tidally captured by the MBH. We discuss the implications for stellar populations near MBHs and for the growth of MBHs by tidal disruption of stars, and the possible observational signature of such processes near the MBH in the Galactic Center. We compare the event rates of prompt tidal encounters (tidal disruption and tidal scattering) and slow inspiral events (squeezars / tidal capture), and find that tidal capture is at least an order of magnitude less efficient than prompt disruption. This means that past studies, which assigned similar weights to prompt disruption and tidal capture, over-estimated the contribution of tidal disruption to the growth of the MBH by at least a factor of two.

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arXiv2002

Stars and singularities: Stellar phenomena near a massive black hole

This is a pedagogical review of recent results on the interactions of central massive black holes with stars very near them, focused on the black hole in the center of the Milky Way. Table of contents: [1] Introduction [2] Stellar dynamics near a black hole [2.1] Physical scales [2.2] A relaxed stellar system around a MBH [3] The stellar collider in the Galactic Center [3.1] The case for a dense stellar cusp in the Galactic Center [3.2] Tidal spin-up [3.3] Tidal scattering [4] The gravitational telescope in the Galactic Center [4.1] Gravitational lensing by a point mass [4.2] Pinpointing the MBH with lensed images [4.3] The detection of gravitational lensing [4.4] Magnification bias [4.5] Beyond the point mass lens approximation [5] Summary

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arXiv2001

Tidal scattering of stars on supermassive black holes in galactic centers

Some of the mass that feeds the growth of a massive black hole (BH) in a galactic center is supplied by tidal disruption of stars that approach it on unbound, low angular momentum orbits. For each star that is disrupted, others narrowly escape after being subjected to extreme tidal distortion, spin-up, mixing and mass-loss, which may affect their evolution and appearance. We show that it is likely that a significant fraction of the stars around massive BHs in galactic centers have undergone such extreme tidal interactions and survived subsequent total disruption, either by being deflected off their orbit or by missing the BH due to its Brownian motion. We discuss possible long-term observable consequences of this process, which may be relevant for understanding the nature of stars in galactic centers, and may provide a signature of the existence of massive BHs there.

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arXiv2001

The Cosmic Baryon Fraction and the Extragalactic Ionizing Background

We reassess constraints on the cosmological baryon density from observations of the mean decrement and power spectrum of the Lyman-alpha forest, taking into account uncertainties in all free parameters in the simplest gravitational instability model. The uncertainty is dominated by that of the photoionizing background, but incomplete knowledge of the thermal state of the intergalactic medium also contributes significantly to the error-budget. While current estimates of the baryon fraction from the forest do prefer values that are somewhat higher than the big bang nucleosynthesis value of Ω_b h^2 = 0.02 \pm 0.001, the discrepancy is at best about 3 σ. For instance, assuming the highest estimate of the ionizing background, as indicated by recent measurements of a large escape fraction from Lyman-break galaxies by Steidel, Pettini & Adelberger, we find Ω_b h^2 = 0.045 \pm 0.008. A recent measurement of the ionizing background from the proximity effect by Scott et al., on the other hand, implies Ω_b h^2 = 0.03 \pm 0.01. We provide an expression from which future likelihoods for Ω_b h^2 can be derived as measurements of the ionizing background improve -- consistency among constraints from the forest, nucleosynthesis and the microwave background will provide a powerful test of the gravitational instability model. We also develop a formalism which treats lower bounds on the baryon density in a statistical manner, which is appropriate if only a lower bound on the ionizing background is known. Finally, we discuss the implications of the escape fraction measurement for the age, structure and stellar content of Lyman-break galaxies.

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arXiv2001

Enhanced Microlensing by Stars Around the Black Hole in the Galactic Center

The effect of stars on the lensing properties of the supermassive black hole in the Galactic Center is similar to the effect of planets on microlensing by a star. We show that the dense stellar cluster around SgrA* increases by factors of a few the probability of high-magnification lensing events of a distant background source by the black hole. Conversely, the gravitational shear of the black hole changes and enhances the microlensing properties of the individual stars. The effect is largest when the source image lies near the Einstein radius of the black hole 1.75"+/-0.20" for a source at infinity). We estimate that the probability of observing at least one distant background star which is magnified by a factor >5 in any infrared snapshot of the inner ~ 2" of the Galactic Center is 1% with a K-band detection threshold of 20 mag. The largest source of uncertainty in this estimate is the luminosity function of the background stars. The gravitational shear of the black hole lengthens the duration of high-magnification events near the Einstein radius up to a few months, and introduces a large variety of lightcurve shapes that are different from those of isolated microlenses. Identification of such events by image subtraction can be used to probe the mass function, density and velocity distributions of faint stars near the black hole, which are not detectable otherwise.

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arXiv2001

Pinpointing the massive black hole in the Galactic Center with gravitationally lensed stars

A new statistical method for pinpointing the massive black hole (BH) in the Galactic Center on the IR grid is presented and applied to astrometric IR observations of stars close to the BH. This is of interest for measuring the IR emission from the BH, in order to constrain accretion models; for solving the orbits of stars near the BH, in order to measure the BH mass and to search for general relativistic effects; and for detecting the fluctuations of the BH away from the dynamical center of the stellar cluster, in order to study the stellar potential. The BH lies on the line connecting the two images of any background source it gravitationally lenses, and so the intersection of these lines fixes its position. A combined search for a lensing signal and for the BH shows that the most likely point of intersection coincides with the center of acceleration of stars orbiting the BH. This statistical detection of lensing by the BH has a random probability of ~0.01. It can be verified by deep IR stellar spectroscopy, which will determine whether the most likely lensed image pair candidates (listed here) have identical spectra.

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arXiv2000

Tidal spin-up of stars in dense stellar cusps around massive black holes

We show that main-sequence stars in dense stellar cusps around massive black holes are likely to rotate at a significant fraction of the centrifugal breakup velocity due to spin-up by hyperbolic tidal encounters. We use realistic stellar structure models to calculate analytically the tidal spin-up in soft encounters, and extrapolate these results to close and penetrating collisions using smoothed particle hydrodynamics simulations. We find that the spin-up falls off only slowly with distance from the black hole because the increased tidal coupling in slower collisions at larger distances compensates for the decrease in the stellar density. We apply our results to the stars near the massive black hole in the Galactic Center. Over their lifetime, ~1 Msol main sequence stars in the inner 0.3 pc of the Galactic Center are spun-up on average to ~10%--30% of the centrifugal breakup limit. Such rotation is ~20--60 times higher than is usual for such stars and may affect their subsequent evolution and their observed properties.

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arXiv2000

Infrared spectroscopy of NGC 1068: Probing the obscured ionizing AGN continuum

The ISO-SWS 2.5-45 um infrared spectroscopic observations of the nucleus of the Seyfert 2 galaxy NGC 1068 (see companion paper) are combined with a compilation of UV to IR narrow emission line data to determine the spectral energy distribution (SED) of the obscured extreme-UV continuum that photoionizes the narrow line emitting gas in the active galactic nucleus. We search a large grid of gas cloud models and SEDs for the combination that best reproduces the observed line fluxes and NLR geometry. Our best fit model reproduces the observed line fluxes to better than a factor of 2 on average and is in general agreement with the observed NLR geometry. It has two gas components that are consistent with a clumpy distribution of dense outflowing gas in the center and a more extended distribution of less dense and more clumpy gas farther out that has no net outflow. The best fit SED has a deep trough at ~4 Ryd, which is consistent with an intrinsic Big Blue Bump that is partially absorbed by ~6x10^19 cm^-2 of neutral hydrogen interior to the NLR.

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arXiv1999

The distribution of stars near the super-massive black hole in the Galactic Center

We analyze three sets of infrared star counts in the inner ~0.5 pc of the Galactic Center. We perform statistical tests on the star counts and model in detail the extinction field and the effects of dwarf-giant collisions on the luminosity function. We find that both the star counts and the depletion of the brightest stars in the inner ~0.05 pc can be explained by a r^-αstellar cusp with αin the range 3/2 to 7/4, in which the envelopes of the brightest giants are destroyed by stellar collisions. Such a cusp is consistent with the Bahcall-Wolf solution for the distribution of stars that have undergone two-body relaxation around a black hole. We show that systematic uncertainties due to variable extinction and unrelaxed stars are probably small, but deeper star counts are required to confirm these results. We estimate that the tidal disruption rate of cusp stars by the black hole is few x 10^-5 yr^-1.

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arXiv1999

Near-Infrared Microlensing of Stars by the Super-Massive Black Hole in the Galactic Center

We investigate microlensing amplification of faint stars in the dense stellar cluster in the Galactic Center (GC) by the super-massive black hole (BH). Such events would appear very close to the position of the radio source SgrA*, which is thought to coincide with the BH, and could be observed during the monitoring of stellar motions in the GC. We use the observed K-band (2.2 um) luminosity function (KLF) in the GC and in Baade's Window, as well as stellar population synthesis computations, to construct KLF models for the inner 300 pc of the Galaxy. These, and the observed dynamical properties of this region, are used to compute the rates of microlensing events, which amplify stars above specified detection thresholds. We present computations of the lensing rates and amplifications as functions of the event durations (weeks to years), for a range of detection thresholds. We find that short events dominate the total rate and that long events tend to have large amplifications. For the current detection limit of K=17 mag, the total microlensing rate is 0.003 1/yr, and the rate of events with durations >1 yr is 0.001 1/yr. Recent GC proper motion studies have revealed the possible presence of one or two variable K-band sources very close to SgrA* (Genzel et al 97; Ghez et al 98). These sources may have attained peak brightnesses of K~15 mag, about 1.5-2 mag above the observational detection limits, and appear to have varied on a timescale of ~1 yr. This behavior is consistent with long-duration microlensing of faint stars by the BH. However, we estimate that the probability that such an event could have been detected during the course of the recent proper motion studies is \~0.5%. A ten-fold improvement in the detection limit and 10 yr of monthly monitoring would increase the total detection probability to ~20%. (Abridged)

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arXiv1998

Infrared spectroscopy of NGC4151: Probing the obscured ionizing AGN continuum

The ISO-SWS infrared spectroscopic observations of the nucleus of Seyfert galaxy NGC4151, which are described in a companion paper, are used together with a compilation of UV to IR narrow emission line data to determine the spectral shape of the obscured extreme-UV continuum that photoionizes the narrow line emitting gas in the active galactic nucleus. We present a new method to determine the best fitting photoionizing continuum and emission line cloud model from a heterogeneous set of emission line data. For NGC4151, we find a best-fit model that reproduces the observed line fluxes to within a factor of 2 on average, and which is consistent with the observed geometry of the optical Narrow Line Region (NLR). Our model consists of a clumpy, optically thick (ionization bounded) gas distribution, with a hydrogen gas density of ~1000 cm^-3 and a volume filling factor of 6.5*10^-4. Our best fitting spectral energy distribution (SED) falls sharply beyond the Lyman limit and then rises sharply again towards 100 eV. In particular, it does not display a `Big Blue Bump' signature of a hot accretion disk. We find that this SED, which best reproduces the NLR line emission on the 100--500 pc scale, does not have enough UV photons to produce the observed BLR recombination emission from the inner 1 pc. This suggests that the BLR is photoionized by the intrinsic continuum source, which does have a strong UV component (perhaps a Big Blue Bump), but that this UV component is absorbed by material located between the NLR and BLR. Our analysis suggests that the absorber consists of \~5*10^19 cm^-2 of neutral hydrogen. Such an absorber was independently detected by UV absorption lines (Kriss et al. 1992, 1995).

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arXiv1998

On Uncertainties in Cross-Correlation Lags and the Reality of Wavelength-Dependent Continuum Lags in Active Galactic Nuclei

We describe a model-independent method of assessing the uncertainties in cross-correlation lags determined from AGN light curves, and use this method to investigate the reality of lags between UV and optical continuum variations in well-studied AGNs. Our results confirm the existence of such lags in NGC 7469. We find that the continuum variations at 1825 A, 4845 A, and 6962 A follow those at 1315A by 0.22^{+0.12}_{-0.13} days, 1.25^{+0.48}_{-0.35} days, and 1.84^{+0.93}_{-0.94} days, respectively, based on the centroids of the cross-correlation functions; the error intervals quoted correspond to 68% confidence levels, and each of these lags is greater than zero at no less than 97% confidence. We do not find statistically significant interband continuum lags in NGC 5548, NGC 3783, or Fairall 9. Wavelength-dependent continuum lags may be marginally detected in the case of NGC 4151. However, on the basis of theoretical considerations, wavelength-dependent continuum lags in sources other than NGC 7469 are not expected to have been detectable in previous experiments. We also confirm the existence of a statistically significant lag between X-ray and UV continuum variations in the blazar PKS 2155-304.

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