Stephen S. Eikenberry

FOURIER TECHNIQUES FOR VERY LONG ASTROPHYSICAL TIME-SERIES ANALYSIS

We present an assortment of both standard and advanced Fourier techniques that are useful in the analysis of astrophysical time series of very long duration—where the observation time is much greater than the time resolution of the individual data points. We begin by reviewing the operational characteristics of Fourier transforms of time-series data, including power-spectral statistics, discussing some of the differences between analyses of binned data, sampled data, and event data, and we briefly discuss algorithms for calculating discrete Fourier transforms (DFTs) of very long time series. We then discuss the response of DFTs to periodic signals and present techniques to recover Fourier amplitude lost during simple traditional analyses if the periodicities change frequency during the observation. These techniques include Fourier interpolation, which allows us to correct the response for signals that occur between Fourier frequency bins. We then present techniques for estimating additional signal properties such as the signals centroid and duration in time, the first and second derivatives of the frequency, the pulsed fraction, and an overall estimate of the significance of a detection. Finally, we present a recipe for a basic but thorough Fourier analysis of a time series for well-behaved pulsations.

A Wide-Field Infrared Camera for the Palomar 200-inch Telescope

The availability of both large aperture telescopes and large format near-infrared (NIR) detectors are making wide-field NIR imaging a reality. We describe the Wide-field Infrared Camera (WIRC), a newly commissioned instrument that provides the Palomar 200-inch telescope with such an imaging capability. WIRC features a field-of-view (FOV) of 4.33 arcminutes on a side with its currently installed 1024-square Rockwell Hawaii-I NIR detector. A 2048-square Rockwell Hawaii-II NIR detector will be installed and commissioned later this year, in collaboration with Caltech, to give WIRC an 8.7 arcminute FOV on a side. WIRC mounts at the telescopes f/3.3 prime focus. The instruments seeing-limited optical design, optimized for the JHK atmospheric bands, includes a 4-element refractive collimator, two 7-position filter wheels that straddle a Lyot stop, and a 5-element refractive f/3 camera. Typical seeing-limited point spread functions are slightly oversampled with a 0.25 arcsec per pixel plate scale at the detector. The entire optical train is contained within a cryogenic dewar with a 2.5 day hold-time. Entrance hatches at the top of the dewar allow access to the detector without disruption of the optics and optical alignment. The optical, mechanical, cryogenic, and electronic design of the instrument are described, a commissioning science image and performance analyses are presented.

Twenty Years of Timing SS 433

We present observations of the optical moving lines in spectra of the Galactic relativistic jet source SS 433 spread over a 20 year baseline from 1979 to 1999. The redshift/blueshift of the lines reveal the apparent precession of the jet axis in SS 433, and we present a new determination of the precession parameters based on these data. We investigate the amplitude and nature of time- and phase-dependent deviations from the kinematic model for the jet precession, including an upper limit on any precessional period derivative of < 5 × 10-5. We also discuss the implications of these results for the origins of the relativistic jets in SS 433.

INFRARED OBSERVATIONS OF THE CANDIDATE LBV 1806-20 AND NEARBY CLUSTER STARS

We report near-infrared photometry, spectroscopy, and speckle imaging of the hot, luminous star we identify as candidate LBV 1806-20. We also present photometry and spectroscopy of three nearby stars, which are members of the same star cluster containing LBV 1806-20 and SGR 1806-20. The spectroscopy and photometry show that LBV 1806-20 is similar in many respects to the luminous Pistol star, albeit with some important differences. They also provide estimates of the effective temperature and reddening of LBV 1806-20 and confirm distance estimates, leading to a best estimate for the luminosity of this star of greater than 5 ? 106 L?. The nearby cluster stars have spectral types and inferred absolute magnitudes that confirm the distance (and thus luminosity) estimate for LBV 1806-20. If we drop kinematic measurements of the distance (15.1 kpc), we have a lower limit on the distance of greater than 9.5 kpc and on the luminosity of greater than 2 ? 106 L?, based on the cluster stars. If we drop both the kinematic and cluster star indicators for distance, an ammonia absorption feature sets yet another lower limit to the distance of greater than 5.7 kpc, with a corresponding luminosity estimate of greater than 7 ? 105?L? for the candidate LBV 1806-20. Furthermore, on the absis of very high angular resolution speckle images, we determine that LBV 1806-20 is not a cluster of stars but is rather a single star or binary system. Simple arguments based on the Eddington luminosity lead to an estimate of the total mass of LBV 1806-20 (single or binary) exceeding 190 M?. We discuss the possible uncertainties in these results and their implications for the star formation history of this cluster.

Faint Infrared Flares from the Microquasar GRS 1915+105

We present simultaneous infrared and X-ray observations of the Galactic microquasar GRS 1915+105 using the Palomar 5 m telescope and Rossi X-Ray Timing Explorer on 1998 July 10 UT. Over the course of 5 hr, we observed six faint infrared (IR) flares with peak amplitudes of approximately 0.3-0.6 mJy and durations of approximately 500-600 s. These flares are associated with X-ray soft-dip/soft-flare cycles, as opposed to the brighter IR flares associated with X-ray hard-dip/soft-flare cycles seen in 1997 August by Eikenberry et al. Interestingly, the IR flares begin before the X-ray oscillations, implying an outside-in origin of the IR/X-ray cycle. We also show that the quasi-steady IR excess in 1997 August is due to the pileup of similar faint flares. We discuss the implications of this flaring behavior for understanding jet formation in microquasars.

High Time Resolution Infrared Observations of the Crab Nebula Pulsar and the Pulsar Emission Mechanism

We present new, high signal-to-noise near-infrared observations of the Crab Nebula pulsar using the Solid State Photomultiplier instrument on the Multiple Mirror Telescope. Our observations cover the J (1.25 μm), H (1.65 μm), and K (2.2 μm) infrared wavebands and have 20 μs time resolution. Together with visible and UV observations made by the Hubble Space Telescope High-Speed Photometer, we have high time resolution observations covering over a decade in wavelength. We present the pulse profiles over this wavelength range, and we analyze the pulse shape as a function of wavelength, including the peak-to-peak phase separation, the peak full width half-maxima (FWHM), and the peak half-width half-maxima (HWHM). We also create both phase-averaged and phase-resolved color spectra of the pulsar emission. We find that the peak-to-peak phase separation shows a significant trend for an increase with wavelength, in rough agreement with models of the pulsar emission mechanism. The FWHM for peaks 1 and 2 also show a trend for increase with wavelength, again in qualitative agreement with the models. However, the HWHM for peaks 1 and 2 show significant differences in their wavelength dependences from the leading to trailing edges. This behavior is not predicted by current pulsar emission models, and the different wavelength dependences of the component HWHM values call into question the usefulness of FWHM measurements. Our spectral analyses show that the IR-UV dereddened phase-averaged color spectrum is essentially flat over more than a decade in frequency. This is in clear contrast to the X-ray and γ-ray regimes, where the spectrum is falling steeply. The color spectra of peaks 1 and 2 are also essentially flat, but the ratio of the two shows statistically significant variations from a constant value. Finally, the color spectra of peaks 1 and 2 show significant differences from the leading to trailing edges. As with the HWHM, this behavior is not predicted by current pulsar emission models.

Spectroscopy of Infrared Flares from the Microquasar GRS 1915+105

We present near-infrared medium-resolution (R~875) spectra of the microquasar GRS 1915+105 on 1997 August 13-15 UTC from the Hale 5 m telescope. The spectra show broad emission lines of He I (2.058 μm) and H I (2.166 μm; Brγ), which are consistent with previous work. On August 14 UTC, we took spectra with an ~6 minute time resolution during infrared flaring events similar to those reported in Eikenberry et al., which appear to reveal plasma ejection from the system. During the flares, the emission-line fluxes varied in approximately linear proportionality to the IR continuum flux, implying that the lines are radiatively pumped by the flares. We also detected a weak He II (2.189 μm) emission line on August 14 UTC. The nature of the line variability and the presence of the He II feature indicate that the emission lines in GRS 1915+105 arise in an accretion disk around the compact object rather than in the circumstellar disk of a proposed Oe/Be companion. The radiative line pumping also implies that the flare emission originates from ejecta that have moved out of the accretion disk plane.

ROSAT Timing of the LMC Pulsar 0540–69*

We present a timing study of the young rotation-powered pulsar 0540-69 in the Large Magellanic Cloud, based on 130 ks of archival ROSAT data spanning a ~3 yr period. We use f- techniques to measure the pulsar frequency as a function of frequency derivative at 17 independent epochs. From these measurements we derive a timing solution with a braking index n=2.5+ 0.6−0.7, and we compare this solution to previous timing studies of 0540-69. Using this frequency-based solution, we create 27 pulse profiles and perform a time-of-arrival (TOA) analysis to investigate further the pulsars timing behavior. While we can fit smooth spin-down models to subsets of the TOAs spanning up to 2 yr successfully, we are unable to obtain acceptable phase-coherent fits to the entire 3 yr set of TOAs. This behavior provides the first clear evidence for timing noise in 0540-69. We discuss the implications of these results for understanding previous studies of the timing behavior of 0540-69.

Large X-Ray Flares from LMC X-4: Discovery of Millihertz Quasi-periodic Oscillations and Quasi-periodic Oscillation-Modulated Pulsations

We report the discovery of millihertz (mHz) quasi-periodic oscillations (QPOs) and QPO-modulated pulsations during large X-ray flares from the high-mass X-ray binary pulsar LMC X-4 using data from the Rossi X-Ray Timing Explorer. The light curves of flares show that, in addition to ~74 mHz coherent pulsations, there exist two more time-varying temporal structures at frequencies of ~0.65-1.35 and ~2-20 mHz. These relatively long term structures appear in the power density spectra as mHz QPOs and also as well-developed sidebands around the coherent pulse frequency, indicating that the amplitudes of the coherent pulsation are modulated by those of the mHz QPOs. One interesting feature is that, while the first flare shows symmetric sidebands around the coherent pulse frequency, the second flare shows significant excess emission in the lower frequency sidebands due to the ~2-20 mHz QPOs. We discuss the origin of the QPOs using a combination of the beat-frequency model and a modified version of the Keplerian-frequency model. According to our discussion, it seems possible to attribute the origin of the ~0.65-1.35 and ~2-20 mHz QPOs to the beating between the rotational frequency of the neutron star and the Keplerian frequency of large accreting clumps near the corotation radius and to the orbital motion of clumps at Keplerian radii of (2-10) × 109 cm, respectively.

A Binary Millisecond Pulsar in Globular Cluster NGC 6544

We report the detection of a new 3.06 ms binary pulsar in the globular cluster NGC 6544 using a Fourier-domain acceleration search. With an implied companion mass of ~0.01 M☉ and an orbital period of only Pb ~ 1.7 hr, it displays orbital properties very similar to many pulsars that are eclipsed by their companion winds. The orbital period is the second shortest of known binary pulsars after 47 Tuc R. The measured flux density of 1.3 ± 0.4 mJy at 1332 MHz indicates that the pulsar is almost certainly the known steep-spectrum point source near the core of NGC 6544.