Two Speakers: Mattia DiMauro (Stanford University) And Or Graur (Harvard-Smithsonian Center For Astrophysics)

Monday September 18, 2017 12:05 pm
Marlar Lounge 37-252

Characterizing the population of pulsars in the Galactic bulge with the Fermi Large Area Telescope

Mattia DiMauro
Stanford University

Several groups have demonstrated the existence of an excess in the gamma-ray emission around the Galactic Center (GC) with respect to the predictions from a variety of Galactic Interstellar Emission Models (GIEMs) and point source catalogs. The origin of this excess, peaked at a few GeV, is still under debate. A possible interpretation is that it comes from a population of unresolved Millisecond Pulsars (MSPs) in the Galactic bulge. We investigate the detection of point sources in the GC region using new tools which the Fermi-LAT Collaboration is developing in the context of searches for Dark Matter (DM) signals. These new tools perform very fast scans iteratively testing for additional point sources at each of the pixels of the region of interest. We show also how to discriminate between point sources and structural residuals from the GIEM. We apply these methods to the GC region considering different GIEMs and testing the DM and MSPs intepretations for the GC excess. Our analysis is capable of finding the characteristics of this putative population of MSPs in the Galactic bulge of our Galaxy. Additionally, we create a list of promising MSP candidates that could represent the brightest sources of a MSP bulge population.


The compact homes of tidal disruption events

Or Graur
Harvard-Smithsonian Center for Astrophysics

When a star strays too close to the super-massive black hole (SMBH) at the heart of a galaxy, it will be tidally disrupted. Half of it will be thrown out of the system, while the rest will fall into the black hole. The circularization of the disrupted matter is predicted to produce a bright flare, referred to as a tidal disruption event (TDE). In the last few years, dozens of TDE candidates have been discovered and the field has taken off. The TDE rate is predicted to depend on the stellar conditions of the sub-parsec-scale environment around the SMBH, which are very hard to measure. I will show that global galaxy properties, on kpc scales, which are easy to measure, can be used to constrain the TDE rate. I will describe the types of galaxies that host these TDE candidates, and show that they significantly prefer galaxies with high stellar surface mass densities. Finally, I will present a statistical framework that ties between the TDE rate and global galaxy properties. When applying this model to a sample of TDE host galaxies with SDSS data, the results are consistent with the TDE rate being driven by dynamical relaxation of the stars around the SMBH.