Dae-Sik Moon

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.

PSR B1951+32: A Bow Shock-confined X-Ray Nebula, a Synchrotron Knot, and an Optical Counterpart Candidate

The radio pulsar PSR B1951+32 and the supernova remnant CTB 80 provide a rich laboratory for the study of neutron stars and supernova remnants. Here, we present ground-based optical and near-infrared observations of them, along with X-ray observations with Chandra and a reanalysis of archival data obtained with the Hubble Space Telescope. The X-ray observations reveal a cometary pulsar wind nebula that appears to be confined by a bow shock produced by the high-velocity motion of the pulsar, making PSR B1951+32 a rare pulsar exhibiting both an Hα bow shock and a shocked X-ray pulsar wind nebula. The distribution of Hα and radio continuum emission is indicative of a contact discontinuity of the shocked pulsar winds and shocked ambient medium at ~0.05 pc. On the other hand, the optical synchrotron knot of PSR B1951+32 likely has a flat spectrum in the optical and near-infrared wave bands, and our astrometry is consistent with only one of the two reported optical counterpart candidates for the pulsar.

The Bell Laboratories 13CO Survey: Longitude-Velocity Maps

A survey is presented of the Galactic plane in the J = 1-0 transition of 13CO. About 73,000 spectra were obtained with the 7 m telescope at Bell Laboratories over a 10 yr period. The coverage of the survey is (l, b) = (-5° to 117°, -1° to 1°), or 244 deg2, with a grid spacing of 3 for |b| 05. The data presented here have been resampled onto a 3 grid. For 0.68 km s-1 channels, the rms noise level of the survey is 0.1 K on the T scale. The raw data have been transformed into FITS format, and all the reduction processes, such as correcting for emission in the reference positions, baseline removal, and interpolation, were conducted within IRAF using the FCRAO task package and additional programs. The reduced data are presented here in the form of longitude-velocity color maps at each latitude. These data allow identification and classification of molecular clouds with masses in excess of ~103 M☉ throughout the first quadrant of the Galaxy. Spiral structure is manifested by the locations of the largest and brightest molecular clouds.

The Evolution of LMC X-4 Flares: Evidence for Super-Eddington Radiation Oozing through Inhomogeneous Polar Cap Accretion Flows?

We present the results of two extensive Rossi X-Ray Timing Explorer observations of large X-ray flaring episodes from the high-mass X-ray binary pulsar LMC X-4. Light curves during the flaring episodes comprise bright peaks embedded in relatively fainter regions, with complex patterns of recurrence and clustering of flares. We identify precursors preceding the flaring activity. Pulse profiles during the flares appear to be simple sinusoids, and pulsed fractions are proportional to the flare intensities. We fit Gaussian functions to flare peaks to estimate the mean FWHM to be ~68 s. Significant rapid aperiodic variability exists up to a few hertz during the flares, which is related to the appearance of narrow, spiky peaks in the light curves. While spectral fits and softness ratios show overall spectral softening as the flare intensity increases, the narrow, spiky peaks do not follow this trend. The mean fluence of the flare peaks is (3.1 ± 2.9) × 1040 ergs in the 2.5-25 keV energy range, with its maximum at ~1.9 × 1041 ergs. The flare peak luminosity reaches up to (2.1 ± 0.2) × 1039 ergs s-1, far above the Eddington luminosity of a neutron star. We discuss possible origins of the flares, and we also propose that inhomogeneous accretion columns onto the neutron star polar caps are responsible for the observed properties.

Discovery of Coupling between Periodic and Aperiodic Variability and X-Ray Quasi-periodic Oscillations from Hercules X-1

We report the discovery of coupling between periodic and aperiodic variability and 12 mHz X-ray quasi-periodic oscillations (QPOs) from the X-ray binary pulsar Hercules X-1 using data from the Rossi X-Ray Timing Explorer. We found two different couplings, one during the preeclipse dips and the other during the normal state of the source, using a method that directly compares the low-frequency power-density spectra (PDSs) with those of the sidebands around the coherent pulse frequency. The preeclipse dip light curves show significant time variation of photon counts, and this variation appears in the PDSs as both strong millihertz powers and well-developed sidebands around the coherent pulse frequency. The linear correlation coefficients between the millihertz PDSs and the sideband PDSs obtained from two preeclipse dip data segments are 0.880 ? 0.003 and 0.982 ? 0.001, respectively. This very strong coupling demonstrates that the amplitudes of the coherent pulsations are almost exactly modulated by the aperiodic variabilities, suggesting that both the periodic and aperiodic variabilities are related to time variation of obscuration of X-rays from the central pulsar by an accretion disk during preeclipse dips. We also found weak coupling during the normal state of the source, together with 12 mHz QPOs. The normal state coupling seems to reconcile with the prediction that a significant fraction of the aperiodic variabilities from X-ray binary pulsars are due to time-varying accretion flows onto the pulsars magnetic poles. We discuss the possible origin of the 12 mHz QPOs.

SMC X-1 as an Intermediate-Stage Flaring X-Ray Pulsar

We present Rossi X-Ray Timing Explorer observations of the X-ray pulsar SMC X-1. The source is highly variable on short timescales (<1 hr), exhibiting apparent flares occupying a significant fraction (~3%) of the total observing time. The flares seem to occur over all binary orbital phases and correlate with the overall variability in the light curve. We find a total of 323 discrete flares having a mean FWHM of ~18 s. The detailed properties of SMC X-1 do not vary significantly between the flares and the normal state, suggesting that the flare may be an extension of the normal state persistent emission with increased accretion rates. The flares resemble type II X-ray bursts from GRO J1744-28. We discuss the origin of the SMC X-1 flares in terms of a viscous instability near the inner edge of the accretion disk around a weakly magnetized X-ray pulsar and find this is consistent with the interpretation that SMC X-1 is in fact an intermediate-stage source like GRO J1744-28.

A Next‐Generation High‐Speed Data Acquisition System for Multichannel Infrared and Optical Photometry

We report the design, operation, and performance of a next‐generation high‐speed data acquisition system for multichannel infrared and optical photometry based on the modern technologies of field programmable gate arrays, the Peripheral Component Interconnect bus, and the Global Positioning System. This system allows either direct recording of photon arrival times or binned photon counting with time resolution up to 1 μs precision in Universal Time, as well as real‐time data monitoring and analysis. The system also allows simultaneous recording of multichannel observations with very flexible, reconfigurable observational modes. We present successful 20 μs resolution simultaneous observations of the Crab Nebula pulsar in the infrared (H band) and optical (V band) wave bands obtained with this system and 100 μs resolution V‐band observations of the dwarf nova IY UMa with the 5 m Hale telescope at the Palomar Observatory.

An infrared photon-counting photometer based on the edge-illuminated solid-state photomultiplier

We present the design, construction, and test observations of a new infrared (IR) photon-counting photometer for astronomy based on the edge-illuminated solid-state photomultiplier (EISSPM). The EISSPM has a photon-counting capability over the 0.4-28 μm range with a nanosecond-scale intrinsic detector time resolution. Its quantum efficiency (QE) peaks greater than or equal to 30 % in the near-IR, which is much higher than the previous SSPM with back illumination. After characterizing the dark noise of the EISSPM at its operational temperature range, we develop an EISSPM-based IR photon-counting photometer for astronomical observations. This includes the design and construction of a full optical, cryo-mechanical, and electronics system as well as the software for operating the instrument on telescopes. We report the results of our test observations of the Crab Nebula pulsar using this new instrument on the Palomar Hale 5-m telescope with 10-μs time resolution.