Research

My current work focuses in characterizing absorption phenomena in both black hole and neutron star X-ray binaries. I study these systems using the High Energy Transmission Grating (or HETG) in the Chandra X-ray Observatory, which provides the high-resolution X-ray spectra required for this type of analysis.

Redshifted disk atmospheres in Ultra-Compact X-ray Binaries

Ultra-compact X-ray binaries are fascinating objects. Their small accretion disks make them excellent laboratories for probing phenomena that occur in the inner regions of the disk.

4U 1916-053 is a neutron star ultra-compact X-ray binary with a white dwarf companion orbiting with a period of only 50 minutes! In addition, its X-ray lightcurves display strong X-ray dips and type-I X-ray bursts.

In 2018 we performed a 250ks Chandra/HETG campaign on this system. At the time, it represented a fivefold increase on the total Chandra/HETG exposure on the system. Unsurprisingly, we found that most of the absorption was in the form of a disk atmosphere, as is the case in many of these systems. However, a closer look revealed that the absorption in all of our spectra, including a 2004 archival observation, is redshifted by ~260 km/s.

The significance of the redshift exceeds 5-sigma when combining all observations.

Si XIV absorption in 4U 1916-053

Fe XXV and XXVI absorption

The origin of the redshift is not immediately obvious. The redshift we measured is much larger compared to the expected radial velocity of the system (relative to earth) based on its location in the milky way. In order to rule out this possibility, we measured the velocity shift of absorption lines originating in the outer disk (during dipping events). These lines suggest the redshift is not due to the radial velocity of the system.

Given the constancy of the redshift, it is also unlikely it is produced by inflowing gas. Instead, we found the most likely explanation is that we are observing gravitationally redshifted absorption originating in a disk atmosphere located ~1200 GM/c^2. This interpretation is consistent with the radii suggested by the photoionization of the absorbing gas. Notably, we also found other additional sources with disk atmospheres redshifted by a similar or greater amount.

If this result holds, these would be the strongest gravitational redshifts ever detected in absorption. For more you can find my 2020 paper on 4U 1916-053 here.

Spectrum of the disk wind in 4U 1630-472

Disk Winds in Black Hole and Neutron Star X-Ray Binaries

We find transient disk winds in many accreting black holes and neutron stars within our galaxy. Disk winds are simply large outflows of gas originating from the surface of the disk which are expelled at 100s and even 1000s of km/s (or millions of miles per hour!). For the most part, we are only able to detect these winds via absorption. That is, when the wind is located between our line of sight and the central emitting regions of the disk.

The spectrum on the right shows the disk wind found in one of the Chandra/HETG spectra of the black hole candidate 4U 1630-472. Although the lines are all blue-shifted, the line profiles indicate that the wind is made up of multiple components originating in different parts of the disk, outflowing at different velocities, and displaying very different gas properties.

By what mechanism are winds launched from disk, especially at the velocities observe? It has been suggested X-rays originating in the regions of the disk closest to the black hole may heat the surface of the disk, providing enough thermal energy for a sizable portion of the gas to escape the system.

By modeling the photoionized absorption in systems known for their powerful winds (such as GRS 1915+105 and GRO J1655-40), we find that these winds are likely too fast, too dense, and launched too close to the black hole for thermal driving to be possible. Instead, it is likely these winds are the product of magnetic magnetic processes, such as the magnetorotational instability.

These results show how winds are a key observational signature for understanding the physics that mediate angular momentum and mass transfer within the disk. In my 2019 paper on the black hole candidate 4U 1630-472, we developed more sophisticated ways of testing models of magnetic accretion by probing the velocity and density structure of the disk wind across multiple observations. Our results are largely consistent with predictions of winds driven via MHD pressure.

We applied these same concepts on Swift data of GRO J1655-40 in a paper by UM graduate student Mayura Balakrishnan.

Wind Density Structure for 4U 1630-472

Past Work (Undergraduate)

Secular Dimming in KIC 8462852

This star generated a lot of interest in 2015 due to the dramatic dips in flux (~20%) observed in its IR lightcurves, the origin of which is still disputed. In addition to these unusual dips, the star also displays long-term dimming across multiple wavelengths. I am a co-author on this paper that shows that this secular dimming likely arises from circumstellar material which perhaps has the same origin as the material responsible for the dips. Click here for a JPL press release on the paper.

RR-Lyrae

RR-Lyrae variable stars are among the most powerful distance indicators we can observe. When observed in the mid-IR, their Period-Luminosity-Metallicity relation is well behaved and only moderately affected by extinction (compared to observing Cepheids in visible bands). They are also numerous, making them ideal for determining the distances to globular clusters.

As an undergraduate, I worked with Dr. Massimo Marengo and Dr. Jillian Neeley on Spitzer data obtained as part of the Carnegie RR-Lyrae Program. My work focused on nearby RR-Lyrae which we used as calibrators for the various Period-Luminosity-Metallicity relations.

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