Physics

Ph.D.
University of Maryland Baltimore County
2015

Applied Physics

M.S.
University of Maryland Baltimore County
2010

Physics Major; Mathematics Minor

B.S.
Wheeling Jesuit University
2007

Orbital Phase-resolved Analysis of X-Ray and Gamma-Ray Observations of the High-mass Gamma-Ray Binary 4FGL J1405.1‑6119

We present the results of multi-wavelength observations of the High-Mass Gamma-Ray Binary 4FGL J1405.1-6119. A pair of joint XMM-Newton and NuSTAR observations taken in 2019 (sampling the gamma-ray maximum and X-ray maximum) characterize the emission of soft and hard X-rays. We find variability of the hydrogen column density along our line of sight, NH, and photon index, Γ, and find no evidence of pulsations in X-rays. We also refine a new best-fit orbital period to P=13.7157±0.0014 days, the first orbital phase-resolved analysis based on nearly 16 years of Fermi--LAT observations of 4FGL J1405.1-6119 and the evolution of the spectral shape as a function of orbital phase. Finally, the X-ray and γ-ray spectra for the phases sampled in the new X-ray observations can be interpreted in the framework of the intrabinary shock model, previously applied to High-Mass Gamma-Ray binaries such as LS 5039.ADS Link: https://ui.adsabs.harvard.edu/abs/2025ApJ...986..181L/abstract

PHYS 010 General Astronomy

PHYS 210 Classical Mechanics

The study of Lagrangian and Hamiltonian Mechanics, central force problems, oscillations, rigid body motion and canonical transormations

PHYS 140 Introduction to Astrophysics

PHYS 241 Radiative Processes in Astrophysics

Orbital Phase-resolved Analysis of X-Ray and Gamma-Ray Observations of the High-mass Gamma-Ray Binary 4FGL J1405.1‑6119

We present the results of multi-wavelength observations of the High-Mass Gamma-Ray Binary 4FGL J1405.1-6119. A pair of joint XMM-Newton and NuSTAR observations taken in 2019 (sampling the gamma-ray maximum and X-ray maximum) characterize the emission of soft and hard X-rays. We find variability of the hydrogen column density along our line of sight, NH, and photon index, Γ, and find no evidence of pulsations in X-rays. We also refine a new best-fit orbital period to P=13.7157±0.0014 days, the first orbital phase-resolved analysis based on nearly 16 years of Fermi--LAT observations of 4FGL J1405.1-6119 and the evolution of the spectral shape as a function of orbital phase. Finally, the X-ray and γ-ray spectra for the phases sampled in the new X-ray observations can be interpreted in the framework of the intrabinary shock model, previously applied to High-Mass Gamma-Ray binaries such as LS 5039.ADS Link: https://ui.adsabs.harvard.edu/abs/2025ApJ...986..181L/abstract

Swift/XRT monitoring of the orbital and superorbital modulations in 4U 1909+07

We report on an observational campaign conducted with Swift/X-ray Telescope (XRT) on the wind-fed supergiant X-ray binary 4U 1909+07 to investigate the nature of the orbital and superorbital modulation of its X-ray emission. A total of 137 XRT observations were carried out, resulting in a total effective exposure time of 114 ks and covering 66 orbital and 19 superorbital cycles of the source. The XRT data folded on the orbital period of the source confirm and improve the previously reported variability in intensity and absorption column density, which can be ascribed to the neutron star accreting from the wind of its B supergiant companion across a nearly circular orbit. The XRT data folded on the superorbital period do not show significant variations in either the absorption column density and/or the power-law photon index. This may be due to a significant weakening of the superorbital modulation during the period of the XRT observations, as confirmed by the Burst Alert Telescope dynamic power spectrum. We discuss the implications of these findings within the corotating interaction region model proposed to interpret the superorbital variability in wind-fed supergiant X-ray binaries.

Simultaneous NICER and NuSTAR Observations of the Neutron Star Low Mass X-ray Binary Serpens X-1

We present the first contemporaneous NICER and NuSTAR analysis of the low-mass X-ray binary Serpens X-1 obtained in 2023 June, performing broadband X-ray spectral analysis modeling of the reprocessed emission with relxillNS from 0.4 to 30 keV. We test various continuum and background estimation models to ensure that our results do not hinge on the choice of model used, and found that the detection of reflection features is independent of the choice of both continuum and background model. The position of the inner accretion disk is consistent with the last stable circular orbit (R_in ≤ 1.2 R_ISCO) and a low inclination of i ≤ 8.3°. Additionally, we investigate the presence of the low-energy (∼1 keV) Fe L complex in the data from NICER and the Reflection Grating Spectrometer on XMM-Newton that was previously reported in the literature. We find that the line is at most a 2% feature relative to the reprocessed continuum, and are unable to claim a definitive detection for the current data set. However, we discuss plausible conditions and systems that would increase the likelihood of detecting this feature in the future.

ADS Link: https://ui.adsabs.harvard.edu/abs/2025ApJ...980..234H/abstract

Constraining the Evolution of the Unstable Accretion Disk in SMC X-1 with NICER

Neutron star high mass X-ray binaries with superorbital modulations in luminosity host warped inner accretion disks that occult the neutron star during precession. In SMC X-1, the instability in the warped disk geometry causes superorbital period "excursions:" times of instability when the superorbital period decreases from its typical value of 55 days to ∼40 days. Disk instability makes SMC X-1 an ideal system in which to investigate the effects of variable disk geometry on the inner accretion flow. Using the high resolution spectral and timing capabilities of the Neutron Star Interior Composition Explorer (NICER) we examined the high state of four different superorbital cycles of SMC X-1 to search forchanges in spectral shape and connections to the unstable disk geometry. We performed pulse phase-averaged and phase-resolved spectroscopy to closely compare the changes in spectral shape and any cycle-to-cycle variations. While some parameters including the photon index and absorbing column density show slight variations with superorbital phase, these changes are most evident during the intermediate state of the supeorbital cycle. Few spectral changes are observed within the high state of the superorbital cycle, possibly indicating the disk instability does not significantly change SMC X-1's accretion process.

A Study of the 20 Day Superorbital Modulation in the High-Mass X-ray Binary IGR J16493-4348

We report on Nuclear Spectroscopic Telescope Array (NuSTAR), Neil Gehrels Swift Observatory (Swift) X-ray Telescope (XRT) and Swift Burst Alert Telescope (BAT) observations of IGR J16493-4348, a wind-fed Supergiant X-ray Binary (SGXB) showing significant superorbital variability. From a discrete Fourier transform of the BAT light curve, we refine its superorbital period to be 20.058 ± 0.007 days. The BAT dynamic power spectrum and a fractional root mean square analysis both show strong variations in the amplitude of the superorbital modulation, but no observed changes in the period were found. The superorbital modulation is significantly weaker between MJD 55,700 and MJD 56,300. The joint NuSTAR and XRT observations, which were performed near the minimum and maximum of one cycle of the 20 day superorbital modulation, show that the flux increases by more than a factor of two between superorbital minimum and maximum. We find no significant changes in the 3-50 keV pulse profiles between superorbital minimum and maximum, which suggests a similar accretion regime. Modeling the pulse-phase averaged spectra we find a possible Fe Kα emission line at 6.4 keV at superorbital maximum. The feature is not significant at superorbital minimum. While we do not observe any significant differences between the pulse-phase averaged spectral continua apart from the overall flux change, we find that the hardness ratio near the broad main peak of the pulse profile increases from superorbital minimum to maximum. This suggests the spectral shape hardens with increasing luminosity. We discuss different mechanisms that might drive the observed superorbital modulation.ADS Link: https://ui.adsabs.harvard.edu/abs/2019ApJ...879...34C/abstract

The Orbital Parameters of the Eclipsing High-mass X-Ray Binary Pulsar IGR J16493-4348 from Pulsar Timing

IGR J16493-4348 is an eclipsing supergiant high-mass X-ray binary (sgHMXB), where accretion onto the compact object occurs via the radially outflowing stellar wind of its early B-type companion. We present an analysis of the system’s X-ray variability and periodic modulation using pointed observations (2.5-25 keV) and Galactic bulge scans (2-10 keV) from the Rossi X-ray Timing Explorer (RXTE) Proportional Counter Array (PCA), along with Swift Burst Alert Telescope (BAT) 70-month snapshot (14-195 keV) and transient monitor (15-50 keV) observations. The orbital eclipse profiles from the PCA scan and BAT light curves are modeled using asymmetric and symmetric step and ramp functions. We obtain an improved orbital period measurement of 6.7828 ± 0.0004 days from an observed minus calculated (O-C) analysis of mid-eclipse times derived from the BAT transient monitor and PCA scan data. No evidence is found for the presence of a strong photoionization or accretion wake. We refine the superorbital period to 20.067 ± 0.009 days from the discrete Fourier transform (DFT) of the BAT transient monitor light curve. A pulse period of 1093.1036 ± 0.0004 s is measured from a pulsar timing analysis using pointed PCA observations spanning ˜1.4 binary orbits. We present pulse times of arrival (ToAs), circular and eccentric timing models, and calculations of the system’s Keplerian binary orbital parameters. We derive an X-ray mass function of {f}x(M)={13.2}-2.5+2.4 M ⊙ and find a spectral type of B0.5 Ia for the supergiant companion through constraints on the mass and radius of the donor. Measurements of the eclipse half-angle and additional parameters describing the system geometry are provided.ADS Link: http://adsabs.harvard.edu/abs/2019ApJ...873...86P

The Physics of Accretion Onto Highly Magnetized Neutron Stars

Studying physical processes occurring above the magnetic poles of strongly magnetized, accreting neutron stars provides us with a laboratory for studying high temperature plasmas exposed to extreme conditions. This white paper discusses the current theoretical and observational challenges, and the importance of addressing these challenges.ADS Link: https://ui.adsabs.harvard.edu/abs/2019BAAS...51c.386W

Discovery of the Galactic High-Mass Gamma-ray Binary 4FGL J1405.1-6119

We report the identification from multi-wavelength observations of the Fermi Large Area Telescope (LAT) source 4FGL J1405.1-6119 (= 3FGL J1405.4-6119) as a high-mass gamma-ray binary. Observations with the LAT show that gamma-ray emission from the system is modulated at a period of 13.7135 +/- 0.0019 days, with the presence of two maxima per orbit with different spectral properties. X-ray observations using the Neil Gehrels Swift Observatory X-ray Telescope (XRT) show that X-ray emission is also modulated at this period, but with a single maximum that is closer to the secondary lower-energy gamma-ray maximum. A radio source, coincident with the X-ray source, is also found from Australia Telescope Compact Array (ATCA) observations, and the radio emission is modulated on the gamma-ray period with similar phasing to the X-ray emission. A large degree of interstellar obscuration severely hampers optical observations, but a near-infrared counterpart is found. Near-infrared spectroscopy indicates an O6 III spectral classification. This is the third gamma-ray binary to be discovered with the Fermi LAT from periodic modulation of the gamma-ray emission, the other two sources also have early O star, rather than Be star, counterparts. We consider at what distances we can detect such modulated gamma-ray emission with the LAT, and examine constraints on the gamma-ray binary population of the Milky Way.ADS Link: https://ui.adsabs.harvard.edu/abs/2019ApJ...884...93C/abstract

Superorbital Modulation in the High-Mass X-ray Binary 4U 1538-52, and Possible Modulation in IGR J16393-4643

Hard X-ray observations with the Neil Gehrels Swift Observatory Burst Alert Telescope (BAT) reveal superorbital modulation in the wind-accreting supergiant high-mass X-ray binary (HMXB) 4U 1538-52 at a period of 14.9130 +/- 0.0026 days that is consistent with four times the 3.73 day orbital period. These periods agree with a previously suggested correlation between superorbital and orbital periods in similar HMXBs. During the ~14 years of observations the superorbital modulation changes amplitude, and since ~MJD 57,650 it was no longer detected in the power spectrum, although a peak near the second harmonic of this was present for some time. Measurements of the spin period of the neutron star in the system with the Fermi Gamma-ray Burst Monitor show a long-term spin-down trend which halted towards the end of the light curve, suggesting a connection between dP(spin)/dt and superorbital modulation, as proposed for 2S 0114+650. However, an earlier torque reversal from INTEGRAL observations was not associated with superorbital modulation changes. B and V band photometry from the Las Cumbres Observatory reveals orbital ellipsoidal photometric variability, but no superorbital optical modulation. However the photometry was obtained when the 14.9130 day period was no longer detected in the BAT power spectrum. We revisit possible superorbital modulation in BAT observations of IGR J16393-4643 but cannot conclusively determine whether this is present, although is not persistent. We consider superorbital modulation mechanisms, and suggest that the Corotating Interaction Region model, with small deviations from orbital synchronization, appears promising.ADS Link: https://ui.adsabs.harvard.edu/abs/2021ApJ...906...13C/abstract

The X-ray pulsar XTE J1858+034 observed with NuSTAR and Fermi/GBM: spectral and timing characterization plus a cyclotron line

Accreting X-ray pulsars (XRPs) undergo luminous X-ray outbursts during which the spectral andtiming behavior of the neutron star can be studied in detail. We analyze a NuSTAR observation ofthe XRP XTE J1858+034 during its outburst in 2019. The spectrum is fit with a phenomenological, a semi-empirical and a physical spectral model. A candidate cyclotron line is found at 48 keV, implying a magnetic field of 5.4×10^12 G at the site of emission. This is also supported by the physical best-fit model. We propose an orbital period of about 81 days based on the visual inspection of the X-ray outbursts recurrence time. Based on Fermi Gamma-ray Burst Monitor data, the standard diskaccretion-torque theory allowed us to infer a distance of 10.9±1.0 kpc. Pulse profiles are single-peaked and show a pulsed fraction that is strongly energy-dependent at least up to 40 keV.ADS Link: https://ui.adsabs.harvard.edu/abs/2021arXiv210107020M/abstract