MIT Astrophysics Colloquia - Spring 2003

MIT Weekly Astrophysics Colloquia - Spring 2003

MIT Astrophysics Colloquia - Spring 2003
Tuesdays at 4:00 PM in the Marlar Lounge, Room 37-252
MIT Center for Space Research, 70 Vassar St, Cambridge, MA
Refreshments are served at 3:45 PM.

Sponsored by the Astrophysics Division
of the MIT Department of Physics.

February 4:

Re'em Sari
Caltech

Formation of Binaries in the Kuiper Belt

The Kuiper Belt is a collection of icy objects in our solar system, external to the orbit of Neptune. Pluto and Charon are its largest known members. The recent discovery that a substantial fraction of its objects are in binaries with wide separations and mass ratios of order unity, came as a surprise. Collisions coupled with tidal evolution, mechanisms that may explain other solar system binaries, fail to account for the formation of the Kuiper belt binaries. Collisionless gravitational interactions are more promising. The first step is the formation of a transient binary when two large bodies penetrate each other's Hill spheres. Stabilization of a transient binary requires that it lose energy. Either dynamical friction due to small bodies or the scattering of a third large body can be responsible. We predict that most objects of size comparable to those currently observed in the Kuiper belt are members of multiple systems. We derive the probability that a large body is a member of a binary with a given separation. We find that outside of a critical separation of about 3'', the binary probability is about 0.3%; below that it increases inversely with separation and at separations of 0.2'', comparable to current resolving capabilities, it reaches about 5%, in agreement with results from the HST binary survey. Though finding the binaries was a surprise, we now predict that the majority of objects are in binaries which are too tight to resolve.
February 11:

Edward Baltz
Columbia University

Microlensing Probes of the Dark Universe

For several years there has been a reported excess of microlensing of Large Magellanic Cloud stars, possibly due to a new population of stellar-mass objects in the Milky Way Halo. This observational work is being extended to more distant galaxies, in particular the Andromeda Galaxy in the Local Group, and the giant elliptical M87 in the Virgo Cluster. Microlensing at cosmological distances may also provide insights. This mysterious population may be remnants from early star formation, or something even more exotic.
February 18:

C A N C E L E D - due to 28 inches of Snow

Dr. Kevin Hurley
UC Berkeley

Soft Gamma-ray Repeaters

Soft gamma repeaters are a rare type of neutron star which exhibit both sporadic bursting behavior as well as quiescent emission in the X- and gamma-ray ranges. At unpredictable intervals, they can produce bursts which are the most energetic cosmic X- and gamma-ray events at Earth, with easily measurable effects on the ionosphere. There is a growing body of evidence that these neutron stars are magnetars, posessing fields of 10^13 G and more, the strongest magnetic fields we know of in the universe. I will review the observational data on the four known SGRs, and sketch the history of the various theories that have been proposed to explain them since their discovery in the late 1970's.
February 25:

Dr. Peter Woods
USRA/NSSTC, Huntsville, AL

Soft Gamma Repeater Burst Activity in Anomalous X-ray Pulsars

Anomalous X-ray Pulsars (AXPs) are a small class of isolated pulsars (five known) where the luminosity in X-rays greatly exceeds the rotational energy loss rate of the putative neutron star. Soft Gamma Repeaters (SGRs) are a group of high energy transient (four known) that intermittently enter episodes of burst activity. During periods of quiescence, the underlying persistent X-ray sources of SGRs resemble in many ways the AXPs. Moreover, the several minute duration March 5, 1979 giant flare from SGR 0526-66 showed 8 second pulsations (Mazets et al. 1979) which fall within the range of observed pulse periods for the AXPs. In 1996, Thompson & Duncan proposed that SGRs and AXPs were each members of the same class of object, namely magnetars or neutron stars where magnetism is the largest source of free energy. They predicted that (i) the persistent emission of SGRs should also pulse (like AXPs) and (ii) that AXPs may one day be observed to burst like SGRs. The presistent emission of SGRs was found to pulsate (Kouveliotou et al. 1998) and the latter prediction was recently realized with the discovery of bursts from two separate AXPs, 1E 1048-5937 (Gavriil, Kaspi & Woods 2002) and 1E 2259+586 (Kaspi et al. 2002). Dr. Woods will present observations of the burst activity observed in these AXPs and draw comparisons to those seen in SGRs. He will also describe some of the effects seen on the persistent emission during/following the burst activity and compare with SGRs. Finally, he will discuss what implications these observations have on the burst mechanism in the context of the magnetar model.
March 4:

Prof. Bohdan Paczynski
Princeton University

Optical Gravitational Lensing Experiment (OGLE)

OGLE is doing a massive variability search, mainly towards the Galactic Bulge, and towards the Magellanic Clouds, using a dedicated 1.3 meter telescope located in Chile at the Las Campanas Observatory. Following a major upgrade in mid 2001: a mosaic 8K x 8K CCD camera, the project is now the largest ongoing search for gravitational microlensing, with almost 400 events detected in 2002. It is likely that some events were caused by stellar mass black holes, while others indicate the presence of planets. A search for photometric transits by planets has provided a number of promising candidates. One of them was confirmed spectroscopically to have a `hot Jupiter', with the shortest known orbital period. OGLE generated the largest ever catalog of variable stars, with about a quarter of a million variables, and hundreds of photometric measurements per object, with all data available over the Internet. A number of quasars was identified through their variability - these will provide an inertial reference frame for the Galactic Bulge, LMC and SMC. Proper motions were measured for approximately a quarter million stars, revealing rotation of the Galactic bar.
March 11:

Prof. Robert Caldwell
Dartmouth College

The Imprint of Dark Energy

Quintessence is a time-varying dark energy component that may account for the accelerated expansion of the universe. In this talk, Caldwell will describe how the equation of state and sound speed of the quintessence imprint features on the cosmic microwave background and large scale power spectrum. Observation of these features could be useful in discriminating amongst various models, and could reveal clues to the nature of dark energy.
March 18

Prof. Chuck Steidel
Caltech

Baryonic Structure in the High Redshift Universe

In the past several years, there has been considerable progress made in the discovery and observation of galaxies in the high redshift universe. An interesting question is "what do we do with them?" now that they are observationally accessible and large samples are feasible. Recent results based on ensemble statistics and on detailed studies of individual objects at redshifts z~2-4 will be summarized, to provide some background on the astrophysical context of the observed galaxies. By combining faint galaxy techniques and QSO absorption line studies probing the same volumes of space, it is now possible to explore the connection between diffuse baryons in the intergalactic medium and star forming galaxies. Example of questions that can be addressed are: What is the relative distribution of neutral hydrogen in the IGM and sites where galaxies have formed, and how does reality compare with theoretical expectations? How are metals in the IGM distributed relative to the locations of galaxies? What is the hydrodynamical effect of rapidly star forming galaxies on the local IGM? Answersi to these questions can test fundamental ideas about the physics of the IGM and, perhaps most importantly, the influence of supernova and AGN feedback on the galaxy formation process.
March 25:

Dr. Bhuvnesh Jain
University of Pennsylvania

Gravitational lensing and dark matter halos

Weak gravitational lensing is a way of directly mapping the mass distribution of halos and large-scale structure. This talk will review the state of large-scale measurements with weak lensing within the context of current CMB data. A new approach to modeling halos using correlation functions will be presented. Real space measurements of the 3-point correlation function on small scales are useful probes of halo as well as galaxy properties. Predictions from the halo model will be shown for measurements from galaxy and lensing surveys. It will be shown that with these techniques, forthcoming lensing surveys will yield precise measurements of halo properties, for halos ranging from those of large galaxies to galaxy cluster.
April 1:

Prof. Andrea Lommen
Franklin and Marshall College

Limits on Gravitational Radiation using Millisecond Pulsars

Pulsar timing is uniquely sensitive to gravitational radiation with periods near one year. The primary sources emitting at these frequencies are Massive Black Hole binaries throughout the universe that are on their way to coalescence. The final coalescence events of these binaries will be detected in the future by orbiting laser interferometer arrays. We present a new upper limit on the stochastic background of gravitational radiation using timing residuals from PSR B1855+09 (17 y) which improves on the work by Kaspi, Taylor, & Ryba (1994) and other authors. The results required combining data from 3 different observing projects: 2 with the Arecibo telescope and 1 with the 140ft telescope in Green Bank. This project represents early results from the `Pulsar Timing Array' which will soon be able detect the stochastic background from Massive Black Hole binaries.
April 8:

Prof. John Heise
SRON - Utrecht, The Netherlands

X-ray flashes and extragalactic transient X-ray sources

Giant stellar explosions dominate the sky in the optical (supernovae) and gamma ray range (Gamma Ray Bursts) such that they can be seen in distant galaxies. A similar type of explosion is seen in the X-ray range. Dr. Heise will summarize recent observational evidence on Fast X-ray Transients (FXTs), including data from BeppoSAX and a search for FXTs in XMM-data. He will also demonstrate that a subclass of FXTs, called X-ray Flashes, are of extragalactic orginin. At least some of them relate to extragalactic stellar explosions. For some others, a different association remains possible.
April 15:

Prof. David Merritt
Rutgers University

Feeding Black Holes

Feeding of stellar-mass things to supermassive black holes is important for a number of reasons, including the possible detection of flares from tidally disrupted stars, or gravitational radiation from compact objects that pass near the event horizon. Energy exchange with stars is probably also the primary mechanism by which binary supermassive black holes lose energy and decay down to separations at which coalescence can take place. I discuss the "loss cone" problem for single and binary black holes in galactic nuclei. Classical loss cone theory was worked out in the context of globular clusters, which are collisionally relaxed; however relaxation times in most galactic nuclei are longer than a Hubble time, and this fact implies that loss cone dynamics in nuclei are strongly dependent on the initial conditions, i.e. on the way in which the nucleus formed. I discuss formation scenarios in which feeding rates in galactic nuclei can be much higher, or much lower, than predicted by classical loss cone theory.
April 22 ===>NOTICE ROOM CHANGE TO 6-120<====:

Prof. David Hogg
New York University

Galaxies in the Sloan Digital Sky Survey

(1) The Sloan Digital Sky Survey is the best available data set for studying the statistical properties of galaxies. (2) Essentially all of the gross properties of galaxies are strongly correlated with one another, and the correlations are informative, non-linear and multi-valued. (3) All known relationships among galaxy properties are confirmed in the SDSS (but at much higher precision than in any previous survey) (4) For the bulk of galaxies, color (ie, stellar age) is much more strongly related to the overdensity of its local environment than its luminosity or stellar mass. (5) The majority of stars in the Universe are in old stellar populations.
April 29:

Dr. Tod Strohmayer
Goddard Space Flight Center

X-ray Timing Meaurements of Ultra-compact Binaries

The recent discoveries of two candidate double-degenerate binaries with orbital periods less than 10 minutes has opened up the possibility for X-ray timing to reveal the orbital evolution and explore the impact of gravitational radiation on binary evolution. The speaker will describe the results of a coherent timing study of the archival ROSAT and ASCA data for RX J1914.4+2456, a candidate double-degenerate binary (AM CVn) with a putative 569 s orbital period. The data suggest that the orbital frequency is increasing at a rate of (8 +/- 3)x10^-18 Hz/sec, consistent with the expected loss of angular momentum from the binary system via gravitational radiation. He will also describe preliminary results obtained from the first Chandra observations of this source as well as future efforts to combine X-ray and gravitational radiation measurements (with LISA) which can provide a new way of exploring compact binary evolution.
May 6:

Dr. Daniel Holz
Univ. of Chicago

Looking through a gravitational lens, darkly

The Universe we live in is lumpy, and as a consequence, all distant sources are gravitationally lensed. We quantify these effects on the brightness of high-redshift sources, and discuss the important qualitative features of the resulting amplification distributions. We then discuss the impact of lensing on high-redshift standard candles, with particular attention to current and proposed supernovae surveys. Lensing introduces a pernicious source of noise, but also offers the opportunity for independent measures of cosmological parameters.
May 13:

Prof. Meg Urry
Yale University

Grand Unification": The Co-Evolution of Galaxies and Black Holes

Recent data show evidence of a close connection between supermassive black holes and their host galaxies, as well as close similarities between active and normal galaxies. "Grand Unification" is the hypothesis that nuclear activity is a common phase in the evolution of all galaxies, meaning that AGN (active galactic nuclei) are intrinsically the same as non-active galaxies. I discuss the available evidence on Grand Unification, describe future tests of this hypothesis, and present a likely scenario for the co-evolution of black holes and galaxies.
May 20:

Dr. Robin Stebbins
Goddard Space Flight Center

Laser Interferometer Space Antenna (LISA): Exploring the Universe with Gravitational Waves Between 0.1 mHz and 1 Hz

The Laser Interferometer Space Antenna (LISA) is a joint ESA-NASA mission that will explore the Universe for gravitational wave sources between 0.1 mHz and 1 Hz. Anticipated sources of gravitational waves include: the inspiral of supermassive black holes resulting from galactic mergers; the inspiral of intermediate mass black holes; the inspiral of compact objects into supermassive black holes; thousands of close, compact binaries in our own Galaxy; and violent density fluctuations in the early universe if their (much more uncertain) amplitude permits. LISA consists of three spacecraft in a triangular formation separated by 5 million kilometers. Gravitational waves are detected by interferometrically monitoring the distances between free-falling reference masses within the spacecraft. LISA employs technology from "drag-free" control systems, spaceborne accelerometers, microthrusters, interferometric distance-ranging and precision measurements to measure strains of 10^-23 over very long baselines.
MIT Astrophysics Colloquia - updated 5/5/03
lewin@mit.edu, sahughes@mit.edu