arXiv subjects

Alan M. Levine

Publications and source records attributed to Alan M. Levine.

At least 55 records · Page 3Linked to original sources

arXiv2014

The Transiting Exoplanet Survey Satellite

The Transiting Exoplanet Survey Satellite (TESS) will search for planets transiting bright and nearby stars. TESS has been selected by NASA for launch in 2017 as an Astrophysics Explorer mission. The spacecraft will be placed into a highly elliptical 13.7-day orbit around the Earth. During its two-year mission, TESS will employ four wide-field optical CCD cameras to monitor at least 200,000 main-sequence dwarf stars with I = 4-13 for temporary drops in brightness caused by planetary transits. Each star will be observed for an interval ranging from one month to one year, depending mainly on the star's ecliptic latitude. The longest observing intervals will be for stars near the ecliptic poles, which are the optimal locations for follow-up observations with the James Webb Space Telescope. Brightness measurements of preselected target stars will be recorded every 2 min, and full frame images will be recorded every 30 min. TESS stars will be 10-100 times brighter than those surveyed by the pioneering Kepler mission. This will make TESS planets easier to characterize with follow-up observations. TESS is expected to find more than a thousand planets smaller than Neptune, including dozens that are comparable in size to the Earth. Public data releases will occur every four months, inviting immediate community-wide efforts to study the new planets. The TESS legacy will be a catalog of the nearest and brightest stars hosting transiting planets, which will endure as highly favorable targets for detailed investigations.

astro-ph.EP↗

arXiv2014

The Relation between the Transit Depths of KIC 12557548b & the Stellar Rotation Period

Kawahara and collaborators analyzed the transits of the candidate disintegrating Mercury-mass planet KIC 12557548b and suggested that the transit depths were correlated with the phase of the stellar rotation. We analyze the transit depths of KIC 12557548b and confirm that there is indeed a robust, statistically significant signal in the transit depths at the rotation period of the spotted host star. This signal is more prominent in the first-half of the Kepler data, and is not due to leakage of the rotating spot signal into our measurement of the transit depths, or due to unocculted starspots. We investigate the suggestion that this signal could be due to an active region on the star, emitting enhanced ultraviolet or X-ray radiation leading to an increased mass loss rate of the planet; we confirm that such a scenario could cause both modulation of the transit depths of KIC 12557548b, and small enough transit-timing variations that they might not be detected in the Kepler data. Our preferred explanation for the fact that the transit depths of KIC 12557548b are modulated with the stellar rotation phase is that the candidate transiting planet is occulting starspots on this highly spotted star; such a scenario could cause transit depth variations as large as have been observed, and cause transit-timing variations small enough that they are arguably consistent with the Kepler data.

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arXiv2014

A Study of the Shortest-Period Planets Found With Kepler

We present the results of a survey aimed at discovering and studying transiting planets with orbital periods shorter than one day (ultra--short-period, or USP, planets), using data from the {\em Kepler} spacecraft. We computed Fourier transforms of the photometric time series for all 200,000 target stars, and detected transit signals based on the presence of regularly spaced sharp peaks in the Fourier spectrum. We present a list of 106 USP candidates, of which 18 have not previously been described in the literature. In addition, among the objects we studied, there are 26 USP candidates that had been previously reported in the literature which do not pass our various tests. All 106 of our candidates have passed several standard tests to rule out false positives due to eclipsing stellar systems. A low false positive rate is also implied by the relatively high fraction of candidates for which more than one transiting planet signal was detected. By assuming these multi-transit candidates represent coplanar multi-planet systems, we are able to infer that the USP planets are typically accompanied by other planets with periods in the range 1-50 days, in contrast with hot Jupiters which very rarely have companions in that same period range. Another clear pattern is that almost all USP planets are smaller than 2 $R_\oplus$, possibly because gas giants in very tight orbits would lose their atmospheres by photoevaporation when subject to extremely strong stellar irradiation. Based on our survey statistics, USP planets exist around approximately $(0.51\pm 0.07)\%$ of G-dwarf stars, and $(0.83\pm 0.18)\%$ of K-dwarf stars.

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arXiv2014

Multiwavelength Observations of the Candidate Disintegrating sub-Mercury KIC 12557548b

We present multiwavelength photometry, high angular resolution imaging, and radial velocities, of the unique and confounding disintegrating low-mass planet candidate KIC 12557548b. Our high angular resolution imaging, which includes spacebased HST/WFC3 observations in the optical, and groundbased Keck/NIRC2 observations in K'-band, allow us to rule-out background and foreground candidates at angular separations greater than 0.2 arcsec that are bright enough to be responsible for the transits we associate with KIC 12557548. Our radial velocity limit from Keck/HIRES allows us to rule-out bound, low-mass stellar companions to KIC 12557548 on orbits less than 10 years, as well as placing an upper-limit on the mass of the candidate planet of 1.2 Jupiter masses; therefore, the combination of our radial velocities, high angular-resolution imaging, and photometry are able to rule-out most false positive interpretations of the transits. Our precise multiwavelength photometry includes two simultaneous detections of the transit of KIC 12557548b using CFHT/WIRCam at 2.15 microns and the Kepler space telescope at 0.6 microns, as well as simultaneous null-detections of the transit by Kepler and HST/WFC3 at 1.4 microns. Our simultaneous HST/WFC3 and Kepler null-detections, provide no evidence for radically different transit depths at these wavelengths. Our simultaneous CFHT/WIRCam detections in the near-infrared and with Kepler in the optical reveal very similar transit depths (the average ratio of the transit depths at ~2.15 microns compared to ~0.6 microns is: 1.02 +/- 0.20). This suggests that if the transits we observe are due to scattering from single-size particles streaming from the planet in a comet-like tail, then the particles must be ~0.5 microns in radius or larger, which would favor that KIC 12557548b is a sub-Mercury, rather than super-Mercury, mass planet.

astro-ph.EP↗

arXiv2014

Overcoming real-world obstacles in 21 cm power spectrum estimation: A method demonstration and results from early Murchison Widefield Array data

We present techniques for bridging the gap between idealized inverse covariance weighted quadratic estimation of 21 cm power spectra and the real-world challenges presented universally by interferometric observation. By carefully evaluating various estimators and adapting our techniques for large but incomplete data sets, we develop a robust power spectrum estimation framework that preserves the so-called "EoR window" and keeps track of estimator errors and covariances. We apply our method to observations from the 32-tile prototype of the Murchinson Widefield Array to demonstrate the importance of a judicious analysis technique. Lastly, we apply our method to investigate the dependence of the clean EoR window on frequency--especially the frequency dependence of the so-called "wedge" feature--and establish upper limits on the power spectrum from z = 6.2 to z = 11.7. Our lowest limit is Delta(k) < 0.3 Kelvin at 95% confidence at a comoving scale k = 0.046 Mpc^-1 and z = 9.5.

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arXiv2013

Transits and Occultations of an Earth-Sized Planet in an 8.5-Hour Orbit

We report the discovery of an Earth-sized planet ($1.16\pm 0.19 R_\oplus$) in an 8.5-hour orbit around a late G-type star (KIC 8435766, Kepler-78). The object was identified in a search for short-period planets in the {\it Kepler} database and confirmed to be a transiting planet (as opposed to an eclipsing stellar system) through the absence of ellipsoidal light variations or substantial radial-velocity variations. The unusually short orbital period and the relative brightness of the host star ($m_{\rm Kep}$ = 11.5) enable robust detections of the changing illumination of the visible hemisphere of the planet, as well as the occultations of the planet by the star. We interpret these signals as representing a combination of reflected and reprocessed light, with the highest planet dayside temperature in the range of 2300 K to 3100 K. Follow-up spectroscopy combined with finer sampling photometric observations will further pin down the system parameters and may even yield the mass of the planet.

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arXiv2012

Low Frequency Imaging of Fields at High Galactic Latitude with the Murchison Widefield Array 32-Element Prototype

The Murchison Widefield Array (MWA) is a new low-frequency, wide field-of-view radio interferometer under development at the Murchison Radio-astronomy Observatory (MRO) in Western Australia. We have used a 32-element MWA prototype interferometer (MWA-32T) to observe two 50-degree diameter fields in the southern sky in the 110 MHz to 200 MHz band in order to evaluate the performance of the MWA-32T, to develop techniques for epoch of reionization experiments, and to make measurements of astronomical foregrounds. We developed a calibration and imaging pipeline for the MWA-32T, and used it to produce ~15' angular resolution maps of the two fields. We perform a blind source extraction using these confusion-limited images, and detect 655 sources at high significance with an additional 871 lower significance source candidates. We compare these sources with existing low-frequency radio surveys in order to assess the MWA-32T system performance, wide field analysis algorithms, and catalog quality. Our source catalog is found to agree well with existing low-frequency surveys in these regions of the sky and with statistical distributions of point sources derived from Northern Hemisphere surveys; it represents one of the deepest surveys to date of this sky field in the 110 MHz to 200 MHz band.

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arXiv2010

An Extended and More Sensitive Search for Periodicities in RXTE/ASM X-ray Light Curves

We present the results of a systematic search in approximately 14 years of Rossi X-ray Timing Explorer All-Sky Monitor data for evidence of periodicities not reported by Wen et al. (2006). Two variations of the commonly used Fourier analysis search method have been employed to achieve significant improvements in sensitivity. The use of these methods and the accumulation of additional data have resulted in the detection of the signatures of the orbital periods of eight low-mass X-ray binary systems and of ten high-mass X-ray binaries not listed in the tables of Wen et al.

astro-ph.HE↗

arXiv2010

AXTAR: Mission Design Concept

The Advanced X-ray Timing Array (AXTAR) is a mission concept for X-ray timing of compact objects that combines very large collecting area, broadband spectral coverage, high time resolution, highly flexible scheduling, and an ability to respond promptly to time-critical targets of opportunity. It is optimized for submillisecond timing of bright Galactic X-ray sources in order to study phenomena at the natural time scales of neutron star surfaces and black hole event horizons, thus probing the physics of ultradense matter, strongly curved spacetimes, and intense magnetic fields. AXTAR's main instrument, the Large Area Timing Array (LATA) is a collimated instrument with 2-50 keV coverage and over 3 square meters effective area. The LATA is made up of an array of supermodules that house 2-mm thick silicon pixel detectors. AXTAR will provide a significant improvement in effective area (a factor of 7 at 4 keV and a factor of 36 at 30 keV) over the RXTE PCA. AXTAR will also carry a sensitive Sky Monitor (SM) that acts as a trigger for pointed observations of X-ray transients in addition to providing high duty cycle monitoring of the X-ray sky. We review the science goals and technical concept for AXTAR and present results from a preliminary mission design study.

astro-ph.IM↗

arXiv2010

Properties of the 24 Day Modulation in GX 13+1 from Near-Infrared and X-ray Observations

A 24 day period for the low-mass X-ray binary GX 13+1 was previously proposed on the basis of 7 years of RXTE ASM observations (Corbet 2003) and it was suggested that this was the orbital period of the system. This would make it the one of the longest known orbital periods for a Galactic low-mass X-ray binary powered by Roche lobe overflow. We present here the results of: (i) K-band photometry obtained with the SMARTS Consortium CTIO 1.3 telescope on 68 nights over a 10 month interval; (ii) Continued monitoring with the RXTE ASM, analyzed using a semi-weighted power spectrum instead of the data filtering technique previously used; and (iii) Swift BAT hard X-ray observations. Modulation near 24 days is seen in both the K-band and additional statistically independent ASM X-ray observations. However, the modulation in the ASM is not strictly periodic. The periodicity is also not detected in the Swift BAT observations, but modulation at the same relative level as seen with the ASM cannot be ruled out. If the 24 day period is the orbital period of system, this implies that the X-ray modulation is caused by structure that is not fixed in location. A possible mechanism for the X-ray modulation is the dipping behavior recently reported from XMM-Newton observations.

astro-ph.HE↗

arXiv2009

The 2003 Outburst of the X-ray Transient H 1743-322: Comparisons with the Black Hole Microquasar XTE J1550-564

The bright X-ray transient H 1743-322 was observed daily by the Rossi X-ray Timing Explorer (RXTE) during most of its 8-month outburst in 2003. We present a detailed spectral analysis and a supporting timing analysis of all of these data, and we discuss the behavior and evolution of the source in terms of the three principal X-ray states defined by Remillard and McClintock. These X-ray results are complemented by Very Large Array (VLA) data obtained at six frequencies that provide quite complete coverage of the entire outburst cycle at 4.860 GHz and 8.460 GHz. We also present photometric data and finding charts for the optical counterpart in both outburst and quiescence. We closely compare H 1743-322 to the well-studied black-hole X-ray transient XTE J1550-564 and find the behaviors of these systems to be very similar. As reported elsewhere, both H 1743-322 and XTE J1550-564 are relativistic jet sources and both exhibit a pair of high-frequency QPO oscillations with a 3:2 frequency ratio. The many striking similarities between these two sources argue strongly that H 1743-322 is a black hole binary, although presently no dynamical data exist to support this conclusion.

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arXiv2009

Scientific Productivity with X-ray All-Sky Monitors

We outline scientific objectives for monitoring X-ray sources and transients with wide-angle, coded mask cameras. It is now possible to instantaneously view half of the sky over long time intervals, gaining access to events of extraordinary interest. Solid state detectors can raise the quality of data products for bright sources to levels associated with pointed instruments. There are diverse ways to advance high energy astrophysics and quantitative applications for general relativity.

astro-ph.HE↗

arXiv2009

A New Dynamical Model for the Black Hole Binary LMC X-1

We present a dynamical model of the high mass X-ray binary LMC X-1 based on high-resolution optical spectroscopy and extensive optical and near-infrared photometry. From our new optical data we find an orbital period of P=3.90917 +/- 0.00005 days. We present a refined analysis of the All Sky Monitor data from RXTE and find an X-ray period of P=3.9094 +/- 0.0008 days, which is consistent with the optical period. A simple model of Thomson scattering in the stellar wind can account for the modulation seen in the X-ray light curves. The V-K color of the star (1.17 +/- 0.05) implies A_V = 2.28 +/- 0.06, which is much larger than previously assumed. For the secondary star, we measure a radius of R_2 = 17.0 +/- 0.8 solar radii and a projected rotational velocity of V_rot*sin(i) = 129.9 +/- 2.2 km/s. Using these measured properties to constrain the dynamical model, we find an inclination of i = 36.38 +/- 1.92 deg, a secondary star mass of M_2 = 31.79 +/- 3.48 solar masses, and a black hole mass of 10.91 +/- 1.41 solar masses. The present location of the secondary star in a temperature-luminosity diagram is consistent with that of a star with an initial mass of 35 solar masses that is 5 Myr past the zero-age main sequence. The star nearly fills its Roche lobe (~90% or more), and owing to the rapid change in radius with time in its present evolutionary state, it will encounter its Roche lobe and begin rapid and possibly unstable mass transfer on a timescale of a few hundred thousand years.

astro-ph↗

arXiv2009

GX 9+9: Variability of the X-Ray Orbital Modulation

Results of observations of the Galactic bulge X-ray source GX 9+9 by the All-Sky Monitor (ASM) and Proportional Counter Array (PCA) onboard the Rossi X-ray Timing Explorer are presented. The ASM results show that the 4.19 hour X-ray periodicity first reported by Hertz and Wood in 1987 was weak or not detected for most of the mission prior to late 2004, but then became strong and remained strong for approximately 2 years after which it weakened considerably. When the modulation at the 4.19 hour period is strong, it appears in folded light curves as an intensity dip over slightly less than 30% of a cycle and is distinctly nonsinusoidal. A number of PCA observations of GX 9+9 were performed before the appearance of strong modulation; two were performed in 2006 during the epoch of strong modulation. Data obtained from the earlier PCA observations yield at best limited evidence of the presence of phase-dependent intensity changes, while the data from the later observations confirm the presence of flux minima with depths and phases compatible with those apparent in folded ASM light curves. Light curves from a Chandra observation of GX 9+9 performed in the year 2000 prior to the start of strong modulation show the possible presence of shallow dips at the predicted times. Optical observations performed in 2006 while the X-ray modulation was strong do not show an increase in the degree of modulation at the 4.19 hour period. Implications of the changes in modulation strength in X-rays and other observational results are considered.

astro-ph.HE↗

arXiv2005

A Systematic Search for Periodicities in RXTE/ASM Data

We present the results of a systematic search in 8.5 years of Rossi X-ray Timing Explorer All-Sky Monitor data for evidence of periodicities. The search was conducted by application of the Lomb-Scargle periodogram to the light curves of each of 458 actually or potentially detected sources in each of four energy bands (1.5-3 keV, 3-5 keV, 5-12 keV, and 1.5-12 keV). A whitening technique was applied to the periodograms before evaluation of the statistical significance of the powers. We discuss individual detections with focus on relatively new findings.

astro-ph↗

arXiv2004

The X-ray Outburst of H1743-322: High-Frequency QPOs with a 3:2 Frequency Ratio

We observed the 2003 X-ray outburst of H1743-322 in a series of 130 pointed observation with RXTE. We searched individual observations for high-frequency QPOs (HFQPOs) and found only weak or marginal detections near 240 and 160 Hz. We next grouped the observations in several different ways and computed the average power-density spectra (PDS) in a search for further evidence of HFQPOs. This effort yielded two significant results for those observations defined by the presence of low-frequency QPOs (0.1-20 Hz) and an absence of ``band-limited'' power continua: (1) The 9 time intervals with the highest 7-35 keV count rates yielded an average PDS with a QPO at $166 \pm 5$ Hz. ($4.1 σ$; 3--35 keV); and (2) a second group with lower 7-35 keV count rates (26 intervals) produced an average PDS with a QPO at $242 \pm 3$ Hz ($6.0 σ$; 7--35 keV). The ratio of these two frequencies is $1.46 \pm 0.05$. This finding is consistent with results obtained for three other black hole systems that exhibit commensurate HFQPOs in a 3:2 ratio. Furthermore, the occurrence of H1743-322's slower HFQPO at times of higher X-ray luminosity closely resembles the behavior of XTE J1550-564 and GRO J1655-40. We discuss our results in terms of a resonance model that invokes frequencies set by general relativity for orbital motions near a black-hole event horizon.

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arXiv2002

RXTE All-Sky Monitor Detection of the Orbital Period of Scorpius X-1

The orbital period of Scorpius X-1 has been accepted as 0.787313 d since its discovery in archival optical photometric data by Gottlieb, Wright, & Liller (1975). This period has been confirmed in both photometric and spectroscopic optical observations, though to date only marginal evidence has been reported for modulation of the X-ray intensity at that period. We have used data taken with the RXTE All Sky Monitor to search for such a modulation. A major difficulty in detecting the orbit in X-ray data is presented by the flaring behavior of Sco X-1, which contributes white noise to Fourier transforms of the intensity time series, and tends to obscure weak modulations. We present a new technique for substantially reducing the effects of the flaring behavior while retaining much of any periodic orbital modulation, provided only that the two temporal behaviors exhibit different spectral signatures. Through such a search, we have found evidence for orbital modulation at about the 1% level with a period of 0.78893 d, equal within our accuracy to a period which differs by 1 cycle per year from the accepted value. If we compare our results with the period of the 1 year sideband cited by Gottlieb et al. we conclude that the actual period may be 0.78901 d.

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arXiv2001

Observations of Gamma-ray Bursts with the Rossi X-ray Timing Explorer

The role of the Rossi X-ray Timing Explorer (RXTE) in the study of Gamma-ray Bursts (GRBs) is reviewed. Through April 2001, the All-Sky Monitor (ASM) and the Proportional Counter Array (PCA) instruments have detected 30 GRBs. In 16 cases, an early celestial position was released to the community, sometimes in conjunction with IPN results. The subsequent optical and radio searches led to the detection of 5 x-ray afterglows, to at least 6 optical or radio afterglows, to 3 of the 17 secure redshifts known at this writing, and to 2 other likely redshifts. The decay curves of early x-ray afterglows have been measured. The rapid determination of the location of GRB 970828 and the absence of optical afterglow at that position gave one of the first indications that GRBs occur in star-forming regions (Groot et al. 1998, ApJ 493, L27). The location of GRB 000301C led to the determination of a break in the optical decay rate (Rhoads and Fruchter 2001, ApJ 546, 117) which is evidence for a jet, and to variability in the optical light curve that could represent gravitational lensing (Garnavich, Loeb, and Stanek 2000, ApJ 544, L11). X-ray light curves of GRB from the ASM in conjunction with gamma-ray light curves exhibit striking differences in different bands and may reveal the commencement of the x-ray afterglow (Smith et al. 2001, ApJ submitted, astroph 0103357).

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