The super-massive black holes residing at the centers of many galaxies emit jets of relativistic plasma moving near the speed of light, a process which can deposit enormous amounts of energy into the host galaxy and inter-galactic medium. Yet the detailed physics behind these jets, including how they are launched and collimated, how long they remain relativistic, and their total energy content remain poorly understood. Using over 13 years of archival Hubble Space Telescope (HST) observations of the relativistic jet in the archetypal radio galaxy M87, we have produced astrometric speed measurements of the optically bright synchrotron emitting plasma components in the jet with unprecedented accuracy. Building on previous work showing the superluminal nature of the jet in the optical, we have found that the jet motion is incredibly complex, with both transverse motions and flux variations which can be seen by eye in the time series of deep exposures. These observations of M87 provide us with a unique dataset with which to refine theoretical models of the large-scale jet structure, potentially addressing open questions such as the jet collimation mechanism, bulk acceleration and deceleration in the jet, and the presence of a helical structure. I will also present very recent results using data from the HST archive on the optical counterjet and nuclear regions of M87 and discuss the larger implications of these detailed studies of one of the most nearby AGN jets.

Location: Physics Bldg., Room 401

Date: Thursday, December 12, 2013
Time: 1:00 pm
Location: PHYS 401

TITLE:
Shedding New Light on Accreting Pulsars

ABSTRACT:
Neutron stars are remnants of stellar evolution, the collapsed cores of massive stars. They are extremely dense and sustained against further gravitational collapse by neutron degeneracy pressure. Many have the strongest magnetic fields found in the universe. Accreting X-ray pulsars are rotating neutron stars which show regular flashes of X-ray emission powered by the accretion of material from a stellar companion onto the magnetic poles of the neutron star. The processes that take place at the poles involve strong gravitational fields, high temperatures and the most extreme magnetic fields. These are conditions that cannot take place naturally on Earth and cannot be reproduced in laboratories.

In recent years, considerable progress has been made regarding the development of physical models describing the accretion process onto the magnetic poles such that these new models can now be tested for the first time. These new models can now provide the first direct connection between physical parameters of the accretion process (magnetic field strength, plasma temperature, plasma density, mass accretion rate) and X-ray spectral shape. The standard empirical and new physical spectral models will be systematically applied to a sample of pulsars. Most of the chosen pulsars show a “cyclotron line”, a spectral feature that allows for a direct measurement of their magnetic fields. This detailed spectral analysis will be mainly based on observations from the Japanese X-ray satellite, Suzaku, due to its instruments’ high sensitivity, spectral resolution, and broad-band coverage. (The project will also include additional satellite data of a Symbiotic X-ray Binary, a member of a small, under-studied subclass of X-ray pulsars accreting from a dense stellar companion wind. Its study will involve modeling the spectrum as well as characterizing the variability and comparing the results with more common X-ray binaries.)

The goal of this project is provide the first steps towards a much needed improvement of our theories and observational analysis of magnetically dominated accretion, which can only be achieved by fitting the best physical models to the best available data.