The image above (created for one of my press releases a few years ago) is an artist’s conception of an accreting stellar-mass black hole, the compact remnant of a massive star. In orbit around another star, the black hole slowly siphons material away from its companion. This material falls towards the black hole in the form of a disk (an “accretion disk”). I am interested in what happens next!

It turns out that black holes aren’t vacuum cleaners. Infalling material releases prodigious quantities of energy (think outshining the entire output of the sun by a factor of one million in X-rays alone), and some fraction of what falls in can be ejected (see the outflowing “wind” above). Broadly, I’m interested in using techniques from spectroscopy and variability to understand the physics of infalling and outflowing gas in these incredible systems! For more detail, see my publications.

Current Projects

Credit: NASA/CXC M. Weiss

Black hole astrophysics isn’t a lab experiment: we don’t get to pick what targets we can see, and we don’t get to control their behavior. This makes being an observer tricky, but also provides some fun detective work!

Last fall (2017), a new X-ray source appeared, called MAXI J1535-571. In Figure 1 at left, you can see its lightcurve as tracked by MAXI and Swift in soft and hard X-rays.

The vertical lines in Figure 1 represent a series of observations I collected with the hard X-ray telescope NuSTAR. I’ve color-coded these observations by time, spectral shape and flux; you can see their spectra in Figure 2 (below).

Figure 1: X-ray monitoring of the 2017 outburst of MAXI J1535-571. Top: the 2-20 keV lightcurve from MAXI on the ISS. This was an incredibly bright outburst; a typical bright black hole outburst stays below ~3 (roughly 1 “Crab,” or a flux of 2.4×10-8 erg/s). This outburst crossed 16! Middle: 15-50 keV lightcurve from Swift/BAT. Bottom: The hardness ratio BAT/MAXI. The spectrum softened somewhat during the outburst but stayed hard through its brightest phase (odd!).

As you can see from the high-energy portion of the spectra in Figure 2, many of the spectral parameters are relatively steady throughout the outburst.

The primary differences between spectra appear to be produced by changes in  the temperature of the disk and the fraction of photons intercepted by the scattering corona, as well as the fraction of scattered photons reflecting off the disk.

Figure 2: NuSTAR X-ray spectra of MAXI J1535-571, color coded by time/spectrum/flux. The grey overlapping lines represent a model composed of thermal emission plus relativistic and distant reflection from the accretion disk, all absorbed by cold gas in the interstellar medium.

NICER performed frequent monitoring of MAXI J1535-571, including some coordination with NuSTAR. In Figure 3 (left) you can see the benefit of NICER’S sensitive soft X-ray coverage. In a recent paper, Xu et al. (2018, ApJ, 852, 34) fit a NuSTAR observation of a spectrally hard state in 1535, and found a large interstellar column density NH~8×1022 cm-2. We find similar results for our hard state spectra, with NH≲10×1023 cm-2 when we fit NuSTAR alone. However, Figure 3 (left) clearly shows that this NuSTAR-only model isn’t consistent with the NICER data, which requires a much lower NH~3-4×1022 cm-2!

Figure 3: Coordinated NICER/NuSTAR X-ray spectra of MAXI J1535-571. In the top panel, the red line represents the best fit model to the NuSTAR data alone, with a large NH~10×1023 cm-2. This is clearly not consistent with the NICER data, here shown between 1-10 keV. The bottom panel shows my preliminary best fit to the joint dataset, which requires a much lower column density. Residuals are not shown due to some soon-to-be-resolved NICER calibration uncertainties.