I am primarily interested in high-energy radiation from compact objects (neutron stars and black holes) within our own Milky Way galaxy. We are developing data analysis tools and methods (described below) to help locate undiscovered compact objects in X-ray source catalogs. I am also interested in finding and studying outlying sources and interesting phenomena associated with them, as they could represent classes of exciting new objects or teach us about fundamental physics processes. Below I discuss some of the projects I am involved in.
High-mass Gamma-ray Binaries
What are high-mass gamma-ray binaries (HMGBs) and why are they important? To answer the first part of the question it is easiest to break down the name and digest it piece by piece. Let’s start with binaries, this implies that there is one object orbiting another. In the case of HMGBs it is a compact object (neutron star or black hole) orbiting a companion star. The next part we’ll look at is high-mass, which is referring to the mass of the companion star in the binary. These stars are typically O/Be type stars and have masses >10 times the mass of our own sun! Due to the large masses and high temperatures of these stars, they typically have winds, which can interact with the compact object in the binary. These interactions allow HMGBs to produce radiation across a large portion of the electromagnetic spectrum. This leads us to the last part of the name, which is gamma-ray. These systems are unique in that they produce radiation up to TeV energies. These energies are similar to, and even larger, than those produced by the Large Hadron Collider at CERN! These systems are exceedingly rare, with only ~6 currently known in our entire Galaxy. It has also proven difficult to discover the nature of the compact object in these systems, with only LS 2882 known to host a neutron star.
These systems are important because they allow us to study an interesting time in the life of binaries hosting massive stars. At this stage, one star has undergone a supernova explosion and left behind a compact object. The interactions of these objects allows us to study the properties of compact objects such as wind-wind interactions (for a pulsar), accretion (possibly both neutron star or black hole), and jet formation (for a black hole). By finding more of these objects we can also constrain population synthesis models.
(see here for more details on the high-energy properties of HMGBs)
Peculiar emission from LS 2883/PSR B1259-63
As mentioned above, LS 2883 is the only HMGB in which the nature of the compact object is confidently know. At a distance of 2.3 kpc (~7500 lyr), LS 2883 hosts the 47.8 ms pulsar PSR B1259-63, with a characteristic age of 330 kyr. Using the Chandra X-ray Observatory (CXO), we have discovered a very puzzling extended X-ray feature near the binary. The extended feature, nicknamed the blob, is traveling with a very large velocity that is ~10% the speed of light and shows a possible hint of acceleration! By monitoring this blob we hope to probe the physics of the wind-wind interaction between the massive star and pulsar, and possibly the unshocked pulsar wind.
Over the years many fields have been observed in X-rays, and typically, the observations are taken to study one particular object. However, there are almost always other objects in these fields (i.e., serendipitous sources) that the primary study does not focus on. These sources remain unstudied and their parameters eventually get placed into X-ray catalogs. Over the long observing lifetimes of Chandra and XMM-Newton (~20 years each) the number of sources in these catalogs has built up to about 850,000 sources, with a large fraction (>90%) of them, especially the faint sources, remaining unstudied. We are currently building a multi-wavelength pipeline (nicknamed MUWCLASS) that uses machine learning algorithms to classify these sources and search for unique outliers. Often times these faint sources cannot be classified with just the X-ray data alone, so we cross-match our X-ray sources with data at other wavelengths (e.g., optical, near-infrared, infrared) to aid in the classification process.
The code will eventually be made public and you can check for updates here. We have also used our code in several fields to search for X-ray counterparts to TeV gamma-ray sources including HESS J1616-508, HESS J1741-302, and HESS J1809-193.
We are also working on several fields containing unidentified Fermi sources, that could potentially be neutron stars or X-ray binaries (more on this to come in the near future!).
Many interesting objects, such as low-mass X-ray binaries, form at higher rates in Galactic star clusters than they do elsewhere in the Galaxy. This has prompted us to search for and classify X-ray sources in middle aged Galactic clusters. This can oftentimes be more easily accomplished if there is multi-wavelength data (e.g., Hubble optical or infrared, Spitzer) accompanying the X-ray data. Above is an infrared image from Hubble of the Galactic cluster Glimpse-C01. We have used both the Hubble infrared and Chandra X-ray data to understand the nature of the X-ray sources in this cluster (see here for more details).
Isolated compact objects, such as black holes or old neutron stars, are typically very faint and difficult to detect with modern astrophysical observatories. These objects are useful to study because they can teach us how these compact objects evolve over time. Microlensing events happen when a massive object passes in front of a background star and causes the light from the background star to be magnified. This magnification can be detected and used to infer the mass and distance of the lens. There are several ground-based telescopes searching our galaxy for these microlensing events (e.g., OGLE). Once these objects are detected from the ground we can try to observe them in X-rays to learn more about their nature. We have followed up two massive lenses (>1 solar mass) reported on by OGLE, to search for X-ray emission from these objects. X-ray emission could be observed from these objects if they are able to accrete from the interstellar medium and could teach us about low-accretion rate physics.