Tuesday, 19 November 2019

The Flaring X-Ray Universe

Speaker: Dr. Jimmy Irwin (UA)
Location: OPB 237, 2:50pm
Abstract: Violent (>100x amplitude), short-term (<1 minute rise time) X-ray flaring from a host of astrophysical objects have been increasingly detected in recent years from Milky Way stars, extragalactic X-ray binaries in nearby galaxies, and other transients at cosmological distances of uncertain origin. Our thorough search of the Chandra data archive for flaring objects has uncovered a number of such flares that have escaped notice until now. I present a summary of the most extreme X-ray flaring events from both our survey and the literature and discuss how such violent events further our understanding of stellar magnetic fields, compact object accretion, and compact object mergers.

Monday, 11 November 2019

The Transit of Mercury

Speaker: Dr. Jeremy Bailin (University of Alabama)
Location: University Greenway, 6:34 AM - 12:04 PM 
Abstract: This will be the last time a Transit of Mercury is visible from the United States until 2049.  Join the Astronomy Club to witness this historic event.  We will have telescopes set up for viewing on the Greenway.  Come join us bright and early!

Tuesday, 05 November 2019

A Plane Wave Model for Direct Simulation of Reflection and Transmission by Discretely Inhomogeneous Plane Parallel Media

Speaker: Dr. Daniel Mackowski (Auburn)

Location: OPB 234, 2:50pm
Abstract: A formulation is developed for numerically solving the frequency domain Maxwell's equations in plane parallel layers of inhomogeneous media.   As has been done in recent direct simulation models using the discrete dipole approximation (DDA), the plane parallel layer is modeled as an infinite square lattice of W  x W x H unit cells, with W being a sample width of the layer and H the layer thickness.    As opposed to the 3D volume integral/DDA formulation, the derivation begins with a Fourier expansion of the electric field amplitude in the lateral plane, and leads to a coupled system of 1D ordinary differential equations in the depth direction of the layer.   A 1-D dyadic Green's function is derived for this system and used to construct a set of coupled 1-D integral equations for the field expansion coefficients.  The resulting mathematical formulation is considerably simpler and more compact than that derived, for the same system, using the DDA applied to the periodic plane lattice.    Furthermore, the fundamental property variable appearing in the formulation is the Fourier transformed complex permittivity distribution in the unit cell, and the method obviates any need to define or calculate a dipole polarizability.   Although designed primarily for random media calculations, the method is also capable of predicting the single scattering properties of individual  particles; comparisons are presented with demonstrate that the method can accurately reproduce the polarimetric scattering properties of single and multiple spheres.  The derivation of the dyadic Green's function allows for an analytical preconditioning of the equations, and it is shown that this can result in significantly accelerated solution times when applied to densely-packed systems of particles.    Calculation results demonstrate that the method, when applied to inhomogeneous media, can predict coherent backscattering and polarization opposition effects.

Tuesday, 29 October 2019

Solar Wind Charge Exchange and Properties of the Local Hot Bubble from the DXL Mission

Speaker: Dr. Wenhao Liu (UAH)
Location: OPB 234, 2:50pm
Abstract: The diffuse X-ray background was discovered at the dawn of X-ray astronomy and has been extensively studied. It was believed that a significant fraction of the X-ray foreground emission in the 1/4 keV band originates  in an irregularly shaped cavity which contains the Sun and has been dubbed as local hot bubble (LHB). However, recent studies have shown that X-ray emission in this band can also originate within the solar system, via a process called Solar Wind Charge eXchange (SWCX).  Diffuse X-rays from the Local Galaxy (DXL) is a sounding rocket mission designed to quantify and characterize the contribution of  SWCX to the diffuse X-ray background and study the properties of the local hot bubble (LHB). The DXL mission measured the spatial  signature of SWCX emission due to the Helium focusing core. A direct comparison of DXL and Rosat All Sky Survey (RASS) data allowed  us to quantify the SWCX contribution to all 6 RASS bands. In this talk I will discuss the results from the DXL mission, SWCX contributions to the diffuse X-ray background, and the properties of LHB after removing the SWCX contamination.

Tuesday, 22 October 2019

Recent Developments in the Field of Gamma-ray Bursts, Nature's Brightest Explosions

Speaker: Dr. Peter Veres (UAH)
Location: OPB 234, 2:50pm
Abstract: Gamma-ray bursts (GRBs for short) were discovered by accident during the cold war by satellites looking for signatures of nuclear detonations in the atmosphere. These brief explosions occur at the farthest reaches of the Universe, but here at Earth they can outshine all the gamma-ray sources while they last. According to our current understanding, GRBs mark the death of massive stars (few times 10 Solar masses) or the merger of two neutron stars. The Gamma-ray Burst Monitor (GBM) instrument aboard the Fermi Space Telescope constantly monitors the gamma-ray sky looking for GRBs and detects a new burst almost every day.

On August 17, 2017, our team announced the detection of a run-of-the-mill GRB, named GRB 170817A after the date of the discovery. Only 2 seconds before the GRB, the LIGO and Virgo instruments detected the merger of two neutron stars through gravitational waves. The two signals came from the same location in the sky from a source 130 million lightyears away and marked the first object "seen" in both electromagnetic radiation and gravitational waves. It also confirmed the binary neutron star merger origin for some gamma-ray bursts.

I will discuss the discovery of GRB 170817A and its implications in addition to other interesting developments in the field of GRBs.