arXiv subjects

Patricia T. Boyd

Publications and source records attributed to Patricia T. Boyd.

28 records · Page 2Linked to original sources

arXiv2018

Evidence for an Optical Low-frequency Quasi-Periodic Oscillation in the Kepler Light Curve of an Active Galaxy

We report evidence for a quasi-periodic oscillation (QPO) in the optical light curve of KIC 9650712, a narrow-line Seyfert 1 galaxy in the original Kepler field. After the development and application of a pipeline for Kepler data specific to active galactic nuclei (AGN), one of our sample of 21 AGN selected by infrared photometry and X-ray flux demonstrates a peak in the power spectrum at log $ν$ = -6.58 Hz, corresponding to a temporal period of t=44 days. We note that although the power spectrum is well-fit by a model consisting of a Lorentzian and a single power law, alternative continuum models cannot be ruled out. From optical spectroscopy, we measure the black hole mass of this AGN as log $M_{\mathrm{BH}} / M_\odot$ = 8.17. We find that this frequency lies along a correlation between low-frequency QPOs and black hole mass from stellar and intermediate mass black holes to AGN, similar to the known correlation in high-frequency QPOs.

astro-ph.HE↗

arXiv2017

Evidence for Binarity and Possible Disk Obscuration in Kepler Observations of the Pulsating RV Tau Variable DF Cygni

The Kepler light curve of DF Cyg is unparalleled in precision and cadence for any RV Tau star to date spanning a baseline of $\sim$4 years and clearly displaying the signature pulsating behavior of alternating deep and shallow minima as well as the long-term trend indicative of an RVb-type variable. We measured DF Cyg's formal period (the time interval between two successive deep minima) to be $~49.84 \pm 0.02~$days. The trend in the arrival times emulates that of the long-term period. There appears to be precisely 16 deep+shallow minima cycles in one long-term cycle, suggesting a long-term cycle period of $\approx$ $795 \pm 5~$days. We argue that binarity may naturally explain the long-term periodicity in DF Cyg. The spectral energy distribution of DF Cyg features an infrared excess indicative of a disk possibly linked to a binary companion. From a recent Gaia parallax measurement for DF Cyg, we calculated that it has a distance of $990 \pm 372$pc and a physical radius of $R_\star = 10.3 \pm 3.8$ R$_\odot$. From kinematics and geometric arguments, we argue that the most likely interpretation for the decrease in flux from the long-period maximum to the long-period minimum, as well as the reduction of short-term pulsation amplitude, is caused by an occulting body such as a disk surrounding DF Cyg and its binary companion.

astro-ph.SR↗

arXiv2015

KSwAGS: A Swift X-ray and UV Survey of the Kepler Field. I

We introduce the first phase of the Kepler-Swift Active Galaxies and Stars survey (KSwAGS), a simultaneous X-ray and UV survey of ~6 square degrees of the Kepler field using the Swift XRT and UVOT. We detect 93 unique X-ray sources with S/N>3 with the XRT, of which 60 have observed UV counterparts. We use the Kepler Input Catalog (KIC) to obtain the optical counterparts of these sources, and construct the X-ray to optical flux ratio as a first approximation of the classification of the source. The survey produces a mixture of stellar sources, extragalactic sources, and sources which we are not able to classify with certainty. We have obtained optical spectra for thirty of these targets, and are conducting an ongoing observing campaign to fully identify the sample. For sources classified as stellar or AGN with certainty, we construct SEDs using the 2MASS, UBV and GALEX data supplied for their optical counterparts by the KIC, and show that the SEDs differ qualitatively between the source types, and so can offer a method of classification in absence of a spectrum. Future papers in this series will analyze the timing properties of the stars and AGN in our sample separately. Our survey provides the first X-ray and UV data for a number of known variable stellar sources, as well as a large number of new X-ray detections in this well-studied portion of the sky. The KSwAGS survey is currently ongoing in the K2 ecliptic plane fields.

astro-ph.HE↗

arXiv2012

The Be/X-ray Binary Swift J1626.6-5156 as a Variable Cyclotron Line Source

Swift J1626.6-5156 is a Be/X-ray binary that was in outburst from December 2005 until November 2008. We have examined RXTE/PCA and HEXTE spectra of three long observations of this source taken early in its outburst, when the PCA 2-20 keV count rate was >70 counts/s/PCU, as well as several combined observations from different stages of the outburst. The spectra are best fit with an absorbed cutoff power law with a ~6.4 keV iron emission line and a Gaussian optical depth absorption line at ~10 keV. We present strong evidence that this absorption-like feature is a cyclotron resonance scattering feature, making Swift J1626.6-5156 a new candidate cyclotron line source. The redshifted energy of ~10 keV implies a magnetic field strength of ~8.6(1+z) x 10^11 G in the region of the accretion column close to the magnetic poles where the cyclotron line is produced. Analysis of phase averaged spectra spanning the duration of the outburst suggests a possible positive correlation between the fundamental cyclotron energy and source luminosity. Phase resolved spectroscopy from a long observation reveals a variable cyclotron line energy, with phase dependence similar to a variety of other pulsars, as well as the first harmonic of the fundamental cyclotron line.

astro-ph.HE↗

arXiv2012

Anomalous Low States and Long Term Variability in the Black Hole Binary LMC X-3

Rossi X-ray Timing Explorer observations of the black hole binary LMC X-3 reveal an extended very low X-ray state lasting from 2003 December 13 until 2004 March 18, unprecedented both in terms of its low luminosity (>15 times fainter than ever before seen in this source) and long duration (~3 times longer than a typical low/hard state excursion). During this event little to no source variability is observed on timescales of ~hours-weeks, and the X-ray spectrum implies an upper limit of 1.2x10^35 erg s^-1. Five years later another extended low state occurs, lasting from 2008 December 11 until 2009 June 17. This event lasts nearly twice as long as the first, and while significant variability is observed, the source remains reliably in the low/hard spectral state for the ~188 day duration. These episodes share some characteristics with the "anomalous low states" in the neutron star binary Her X-1. The average period and amplitude of the variability of LMC X-3 have different values between these episodes. We characterize the long-term variability of LMC X-3 before and after the two events using conventional and nonlinear time series analysis methods, and show that, as is the case in Her X-1, the characteristic amplitude of the variability is related to its characteristic timescale. Furthermore, the relation is in the same direction in both systems. This suggests that a similar mechanism gives rise to the long-term variability, which in the case of Her X-1 is reliably modeled with a tilted, warped precessing accretion disk.

astro-ph.HE↗

arXiv2007

GRB 060607A: A GRB with Bright Asynchronous Early $X$-ray and Optical Emissions

The early optical emission of the moderately high redshift ($z=3.08$) GRB 060607A shows a remarkable broad and strong peak with a rapid rise and a relatively slow power-law decay. It is not coincident with the strong early-time flares seen in the X-ray and gamma-ray energy bands. There is weak evidence for variability superposed on this dominant component in several optical bands that can be related to flares in high energy bands. While for a small number of GRBs, well-sampled optical flares have been observed simultaneously with X-ray and gamma ray pulses, GRB 060607A is one of the few cases where the early optical emission shows no significant evidence for correlation with the prompt emission. In this work we first report in detail the broad band observations of this burst by Swift. Then by applying a simple model for the dynamics and the synchrotron radiation of a relativistic shock, we show that the dominant component of the early emissions in optical wavelengths has the same origin as the tail emission produced after the main gamma ray activity. The most plausible explanation for the peak in the optical light curve seems to be the cooling of the prompt after the main collisions, shifting the characteristic synchrotron frequency to the optical bands. It seems that the cooling process requires a steepening of the electron energy distribution and/or a break in this distribution at high energies. The sharp break in the X-ray light curve at few thousands of seconds after the trigger, is not observed in the IR/optical/UV bands, and therefore can not be a jet break. Either the X-ray break is due to a change in the spectrum of the accelerated electrons or the lack of an optical break is due to the presence of a related delayed response component (Abbreviated).

astro-ph↗

arXiv2006

GRB 060313: A New Paradigm for Short-Hard Bursts?

We report the simultaneous observations of the prompt emission in the gamma-ray and hard X-ray bands by the Swift-BAT and the KONUS-Wind instruments of the short-hard burst, GRB 060313. The observations reveal multiple peaks in both the gamma-ray and hard X-ray bands suggesting a highly variable outflow from the central explosion. We also describe the early-time observations of the X-ray and UV/Optical afterglows by the Swift XRT and UVOT instruments. The combination of the X-ray and UV/Optical observations provide the most comprehensive lightcurves to date of a short-hard burst at such an early epoch. The afterglows exhibit complex structure with different decay indices and flaring. This behavior can be explained by the combination of a structured jet, radiative loss of energy, and decreasing microphysics parameters occurring in a circum-burst medium with densities varying by a factor of approximately two on a length scale of 10^17 cm. These density variations are normally associated with the environment of a massive star and inhomogeneities in its windy medium. However, the mean density of the observed medium (n approximately 10^−4 cm^3) is much less than that expected for a massive star. Although the collapse of a massive star as the origin of GRB 060313 is unlikely, the merger of a compact binary also poses problems for explaining the behavior of this burst. Two possible suggestions for explaining this scenario are: some short bursts may arise from a mechanism that does not invoke the conventional compact binary model, or soft late-time central engine activity is producing UV/optical but no X-ray flaring.

astro-ph↗

arXiv2006

Swift Observations of GRB 050603: An afterglow with a steep late time decay slope

We report the results of Swift observations of the Gamma Ray Burst GRB 050603. With a V magnitude V=18.2 about 10 hours after the burst the optical afterglow was the brightest so far detected by Swift and one of the brightest optical afterglows ever seen. The Burst Alert Telescope (BAT) light curves show three fast-rise-exponential-decay spikes with $T_{90}$=12s and a fluence of 7.6$\times 10^{-6}$ ergs cm$^{-2}$ in the 15-150 keV band. With an $E_{\rm γ, iso} = 1.26 \times 10^{54}$ ergs it was also one of the most energetic bursts of all times. The Swift spacecraft began observing of the afterglow with the narrow-field instruments about 10 hours after the detection of the burst. The burst was bright enough to be detected by the Swift UV/Optical telescope (UVOT) for almost 3 days and by the X-ray Telescope (XRT) for a week after the burst. The X-ray light curve shows a rapidly fading afterglow with a decay index $α$=1.76$^{+0.15}_{-0.07}$. The X-ray energy spectral index was $β_{\rm X}$=0.71\plm0.10 with the column density in agreement with the Galactic value. The spectral analysis does not show an obvious change in the X-ray spectral slope over time. The optical UVOT light curve decays with a slope of $α$=1.8\plm0.2. The steepness and the similarity of the optical and X-ray decay rates suggest that the afterglow was observed after the jet break. We estimate a jet opening angle of about 1-2$^{\circ}$

astro-ph↗

arXiv2001

X-Ray and UV Orbital Phase Dependence in LMC X-3

The black-hole binary LMC X-3 is known to be variable on time scales of days to years. We investigate X-ray and ultraviolet variability in the system as a function of the 1.7 day binary phase using a 6.4 day observation with the Rossi X-ray Timing Explorer (RXTE) from December 1998. An abrupt 14% flux decrease, lasting nearly an entire orbit, is followed by a return to previous flux levels. This behavior occurs twice, at nearly the same binary phase, but it is not present in consecutive orbits. When the X-ray flux is at lower intensity, a periodic amplitude modulation of 7% is evident in data folded modulo the orbital period. The higher intensity data show weaker correlation with phase. This is the first report of X-ray variability at the orbital period of LMC X-3. Archival RXTE observations of LMC X--3 during a high flux state in December 1996 show similar phase dependence. An ultraviolet light curve obtained with the High Speed Photometer aboard the Hubble Space Telescope shows orbital modulation consistent with that in the optical, caused by the ellipsoidal variation of the spatially deformed companion. The X-ray spectrum of LMC X-3 can be acceptably represented by a phenomenological disk-black-body plus a power law. Changes in the spectrum of LMC X-3 during our observations are compatible with earlier observations during which variations in the 2-10 keV flux are tracked closely by the disk geometry spectral model parameter.

astro-ph↗

arXiv2000

Canonical Timing and Spectral Behavior of LMC X-3 in the Low/Hard State

We present results from three observations with the Rossi X-ray Timing Explorer (RXTE) of LMC X-3, obtained while the source was in an extended 'low/hard' state. The data reveal a hard X-ray spectrum which is well fit by a pure power law with photon index Gamma=1.69+/-0.02, with a source luminosity at 50 kpc of 5-16x10^{36}erg/s (2--10 keV). Strong broad-band (0.01-100 Hz) time variability is also observed, with fractional rms amplitude 40+/-4%, plus a quasi-periodic oscillation (QPO) peak at 0.46+/-0.02 Hz with rms amplitude \~14%. This is the first reported observation in which the full canonical low/hard state behavior (pure hard power law spectrum combined with strong broad-band noise and QPO) for LMC X-3 is seen. We reanalyze several archival RXTE observations of LMC X-3 and derive consistent spectral and timing parameters, and determine the overall luminosity variation between high/soft and low/hard states. The timing and spectral properties of LMC X-3 during the recurrent low/hard states are quantitatively similar to that typically seen in the Galactic black hole candidates.

astro-ph↗