Search For Pulsations Research Articles

RADIO PULSAR BINARIES IN GLOBULAR CLUSTERS: THEIR ORBITAL ECCENTRICITIES AND STELLAR INTERACTIONS

High sensitivity searches of globular clusters (GC) for radio pulsars by improved pulsar search algorithms and sustained pulsar timing observations have so far yielded some 140 pulsars in more than two dozen GCs. The observed distribution of orbital eccentricity and period of binary radio pulsars in GCs have imprints of the past interaction between single pulsars and binary systems or of binary pulsars and single passing non-compact stars. It is seen that GCs have different groups of pulsars. These may have arisen out of exchange or merger of a component of the binary with the incoming star or a "fly-by" in which the original binary remains intact but undergoes a change of eccentricity and orbital period. We consider the genesis of the distribution of pulsars using analytical and computational tools such as STARLAB, which performs numerical scattering experiments with direct N-body integration. Cluster pulsars with intermediate eccentricities can mostly be accounted for by fly-bys whereas those with high eccentricities are likely to be the result of exchanges and/or mergers of single stars with the binary companion of the pulsar, although there are a few objects which do not easily fit into this description. The corresponding distribution for galactic field pulsars shows notable differences from the GC pulsar orbital period and eccentricity distribution. The long orbital period pulsars in the galactic field with frozen out low eccentricities are largely missing from the globular clusters, and we show that ionization of these systems in GCs cannot alone account for the peculiarities.

A LARGE-AREA SURVEY FOR RADIO PULSARS AT HIGH GALACTIC LATITUDES

We have completed a survey for pulsars at high Galactic latitudes with the 64 m Parkes radio telescope. Observing with the 13 beam multibeam receiver at a frequency of 1374 MHz, we covered ~4150 square degrees in the region –100° ≤ l ≤ 50°, 15° ≤ |b| ≤ 30° with 7232 pointings of 265 s each, thus extending the Swinburne Intermediate Latitude Pulsar Survey a further 15° on either side of the Galactic plane. The signal from each beam was processed by a 96 channel × 3 MHz × 2 polarization filterbank, with the detected power in the two polarizations of each frequency channel summed and digitized with 1 bit sampling every 125 μs, giving good sensitivity to millisecond pulsars with low or moderate dispersion measure. The resulting 2.4 TB data set was processed using standard pulsar search techniques with the workstation cluster at the Swinburne Centre for Astrophysics and Supercomputing. This survey resulted in the discovery of 26 new pulsars including seven binary and/or millisecond pulsars, and redetected 36 previously known pulsars. We describe the survey methodology and results, and present timing solutions for the 19 newly discovered slow pulsars, as well as for nine slow pulsars discovered the Swinburne Intermediate Latitude Pulsar Survey that had no previous timing solutions. Even with a small sampling interval, 1374 MHz center frequency, and a large mid-latitude survey volume we failed to detect any very rapidly spinning pulsars. Evidently, such submillisecond pulsars are rare.

Singing and dancing white dwarfs

Accreting white dwarfs have recently been shown to exhibit non-radial pulsations similar to their non-interacting counterparts. This allows us to probe the interior of the accreting white dwarf using seismology, and may be the only way to determine masses for non-eclipsing cataclysmic variables. Improving our understanding of accreting white dwarfs will have implications for models of supernovae Type Ia. Pulsating white dwarfs in cataclysmic variables are also useful in establishing the effects of accretion on pulsations.A search for nonradial pulsations among suitable candidates has led to the discovery of twelve such systems known to date. With the goal of establishing an instability strip (or strips) for these pulsating accretors, we acquired HST ultra-violet time-series spectroscopy of six pulsating white dwarfs in cataclysmic variables in 2007 and 2008. This approach enables us to measure the effective temperature of the white dwarf using the co-added spectrum, and to simultaneously characterize the pulsations. We also intended to constrain the pulsation mode identification by comparing the ultra-violet amplitudes to those from near-simultaneous ground-based photometry. Our preliminary results indicate a broad instability strip in the temperature range of 10500–15400 K.

A RADIO SEARCH FOR PULSAR COMPANIONS TO SLOAN DIGITAL SKY SURVEY LOW-MASS WHITE DWARFS

We have conducted a search for pulsar companions to 15 low-mass white dwarfs (LMWDs; M < 0.4 M_Sun) at 820 MHz with the NRAO Green Bank Telescope (GBT). These LMWDs were spectroscopically identified in the Sloan Digital Sky Survey (SDSS), and do not show the photometric excess or spectroscopic signature associated with a companion in their discovery data. However, LMWDs are believed to evolve in binary systems and to have either a more massive WD or a neutron star as a companion. Indeed, evolutionary models of low-mass X-ray binaries, the precursors of millisecond pulsars (MSPs), produce significant numbers of LMWDs (e.g., Benvenuto & De Vito 2005), suggesting that the SDSS LMWDs may have neutron star companions. No convincing pulsar signal is detected in our data. This is consistent with the findings of van Leeuwen et al. (2007), who conducted a GBT search for radio pulsations at 340 MHz from unseen companions to eight SDSS WDs (five are still considered LMWDs; the three others are now classified as "ordinary" WDs). We discuss the constraints our non-detections place on the probability P_MSP that the companion to a given LMWD is a radio pulsar in the context of the luminosity and acceleration limits of our search; we find that P_MSP < 10 +4 -2 %.

UPPER LIMITS ON PULSED RADIO EMISSION FROM THE 6.85 s X-RAY PULSAR XTE J0103–728 IN THE SMALL MAGELLANIC CLOUD

X-ray pulsations with a 6.85 s period were recently detected in the Small Magellanic Cloud (SMC) and were subsequently identified as originating from the Be/X-ray binary system XTE J0103–728. The recent localization of the source of the X-ray emission has made a targeted search for radio pulsations from this source possible. The detection of pulsed radio emission from XTE J0103–728 would make it only the second system after PSR B1259–63 that is both a Be/X-ray binary and a radio pulsar. We observed XTE J0103–728 in 2008 February with the Parkes 64 m radio telescope soon after the identification of the source of X-ray pulsations was reported in order to search for corresponding radio pulsations. We used a continuous 6.4 hr observation with a 256 MHz bandwidth centered at 1390 MHz using the center beam of the Parkes multibeam receiver. In the subsequent data analysis, which included a folding search, a Fourier search, a fast-folding algorithm search, and a single pulse search, no pulsed signals were found for trial dispersion measures (DMs) between 0 and 800 pc cm–3. This DM range easily encompasses the expected values for sources in the SMC. We place an upper limit of ~45 mJy kpc2 on the luminosity of periodic radio emission from XTE J0103–728 at the epoch of our observation, and we compare this limit to a range of luminosities measured for PSR B1259–63, the only Be/X-ray binary currently known to emit radio pulses. We also compare our limit to the radio luminosities of neutron stars having similarly long spin periods to XTE J0103–728. Since the radio pulses from PSR B1259–63 are eclipsed and undetectable during the portion of the orbit near periastron, repeated additional radio search observations of XTE J0103–728 may be valuable if it is undergoing similar eclipsing and if such observations are able to sample the orbital phase of this system well.

Efficient blind search: Optimal power of detection under computational cost constraints

Some astronomy projects require a blind search through a vast number of hypotheses to detect objects of interest. The number of hypotheses to test can be in the billions. A naive blind search over every single hypothesis would be far too costly computationally. We propose a hierarchical scheme for blind search, using various “resolution” levels. At lower resolution levels, “regions” of interest in the search space are singled out with a low computational cost. These regions are refined at intermediate resolution levels and only the most promising candidates are finally tested at the original fine resolution. The optimal search strategy is found by dynamic programming. We demonstrate the procedure for pulsar search from satellite gamma-ray observations and show that the power of the naive blind search can almost be matched with the hierarchical scheme while reducing the computational burden by more than three orders of magnitude.

An interference removal technique for radio pulsar searches

Searches for radio pulsars are becoming increasingly difficult because of a rise in impulsive man-made terrestrial radio-frequency interference. Here, we present a new technique, zero dispersion measure filtering, which can significantly reduce the effects of such signals in pulsar search data. The technique has already been applied to a small portion of the data from the Parkes multi-beam pulsar survey, resulting in the discovery of four new pulsars, so illustrating its efficacy.

Pulsar searches and timing with the square kilometre array

The Square Kilometre Array (SKA) is a planned multi purpose radio telescope with a collecting area approaching 1 million square metres. One of the key science objectives of the SKA is to provide exquisite strong-field tests of gravitational physics by finding and timing pulsars in extreme binary systems such as a pulsar-black hole binary. To find out how three preliminary SKA configurations will affect a pulsar survey, we have simulated SKA pulsar surveys for each configuration. We estimate that the total number of normal pulsars the SKA will detect, using only the 1-km core and 30 minutes integration time, is around 14000 normal pulsar and 6000 millisecond pulsars. We describe a simple strategy for follow-up timing observations and find that, depending on the configuration, it would take 1-6 days to obtain a single timing point for 14000 pulsars. Obtaining a single timing point for the high-precision timing projects of the SKA, will take less than 14 hours, 2 days, or 3 days, depending on the configuration. The presence of aperture arrays will be of great benefit here. We also study the computational requirements for beam forming and data analysis for a pulsar survey. Beam forming of the full field of view of the single-pixel feed 15-m dishes using the 1-km core of the SKA requires about 2.2*10^15 operations per second. The corresponding data rate from such a pulsar survey is about 4.7*10^11 bytes per second. The required computational power for a deep real time analysis is estimated to be 1.2*10^16 operations per second. For an aperture array or dishes equipped with phased array feeds, the survey can be performed faster, but the computational requirements and data rates will go up.

Pulsations of the Low-Mass ZZ Ceti Star HS 1824+6000

ABSTRACTMeasuring g-mode pulsations of isolated white dwarfs can reveal their interior properties to high precision. With a spectroscopic mass of ≈0.51 M⊙ (log g = 7.82), the DAV white dwarf HS 1824+6000 is near the transition between carbon/oxygen-core and helium-core white dwarfs, motivating our photometric search for additional pulsations from the Palomar 60-inch telescope. We confirmed (with much greater precision) the three frequencies: 2.751190 ± 0.000010 mHz (363.479 sec), 3.116709 ± 0.000006 mHz (320.851 sec), 3.495113 ± 0.000009 mHz (286.114 sec), previously found by B. Voss and collaborators, and found an additional pulsation at 4.443120 ± 0.000012 mHz (225.067 sec). These observed frequencies are similar to those found in other ZZ Ceti white dwarfs of comparable mass (e.g. log g < 8). We hope that future observations of much lower-mass ZZ Ceti stars (< 0.4 M⊙) will reveal pulsational differences attributable to a hydrogen covered helium core.

A high-frequency search for radio pulsars in three EGRET error boxes

We present a new survey for pulsars in the error boxes of the low-latitude EGRET sources 3EG J1027−5817, 3EG J1800−2338 and 3EG J1810−1032. Although all of these sources have been covered by previous pulsar surveys, the recent discovery of the young, energetic pulsar PSR J1410−6132 at 6.7 GHz has shown that pulsars of this type can be hidden from low-frequency surveys. Using an observing frequency of 3.1 GHz, we discovered a 91-ms pulsar, PSR J1028−5819, which observations made at the Parkes telescope and the Australia Telescope Compact Array have shown to be young and energetic. We believe this pulsar is likely to be powering the unidentified EGRET source 3EG J1027−5817. Like other energetic pulsars, PSR J1028−5819 is highly linearly polarized, but astonishingly has a pulse duty cycle of only 0.4 per cent, one of the smallest in the entire pulsar catalogue.

Is there potential complementarity between LISA and pulsar timing?

We open the discussion into how the Laser Interferometer Space Antenna (LISA) observations of supermassive black-hole (SMBH) mergers (in the mass range ∼ 106–108 M⊙) may be complementary to pulsar timing-based gravitational wave searches. We consider the toy model of determining pulsar distances by exploiting the fact that LISA SMBH inspiral observations can place tight parameter constraints on the signal present in pulsar timing observations. We also suggest, as a future path of research, the use of LISA ring-down observations from the most massive (≳ a few 107 M⊙) black-hole mergers, for which the inspiral stage will lie outside the LISA band, as both a trigger and constraint on searches within pulsar timing data for the inspiral stage of the merger.

A Search for Radio‐Quiet Gamma‐Ray Pulsars in EGRET Data Using a Time‐Differencing Technique

Most of the sources in the Galactic plane detected by the EGRET instrument on board the Compton Gamma Ray Observatory remain unidentified. Since the energy spectra of some of them strongly suggest that the gamma-ray emission originates from pulsars, it is interesting to look for pulsations in the data. However, detecting pulsations in gamma-ray emission by a blind search, that is, without prior knowledge of the pulsar spin parameters, is exceedingly difficult. Previous gamma-ray pulsar blind searches have relied on the execution of very long FFTs, requiring the use of supercomputers. We present a pulsar blind search, based on our time-differencing technique, that can be carried out more economically, in this case, on a standard PC. We present results from the application of this method to a periodicity search on all 271 sources from the third EGRET catalog. No statistically significant new detection was found, for a flux sensitivity of about 10−6 photons cm−2 s−1.

SDSS J142625.71+575218.3: A Prototype for a New Class of Variable White Dwarf

We present the results of a search for pulsations in six of the recently discovered carbon-atmosphere white dwarf (hot DQ) stars. Based on our theoretical calculations, the star SDSS J142625.71+575218.3 is the only object expected to pulsate. We observe this star to be variable, with significant power at 417.7 s and 208.8 s (first harmonic), making it a strong candidate as the first member of a new class of pulsating white dwarf stars, the DQVs. Its folded pulse shape, however, is quite different from that of other white dwarf variables, and shows similarities with that of the cataclysmic variable AM CVn, raising the possibility that this star may be a carbon-transferring analog of AM CVn stars. In either case, these observations represent the discovery of a new and exciting class of object.

A search for rapid pulsations among nine luminous Ap stars

The rapidly oscillating Ap (roAp) stars are of importance for studying the atmospheric structure of stars where the process of chemical element diffusion is significant. We have performed a survey for rapid oscillations in a sample of nine luminous Ap stars, selected from their location in the colour‐magnitude diagram as more evolved main-sequence Ap stars that are inside the instability strip for roAp stars. Until recently this region was devoid of stars with observed rapid pulsations. We used the Very Large Telescope UV‐Visual Echelle Spectrograph to obtain high time resolution spectroscopy to make the first systematic spectroscopic search for rapid oscillations in this region of the roAp instability strip. We report nine null detections with upper limits for radial velocity amplitudes of 20‐65 m s −1 and precisions of σ = 7‐ 20 m s −1 for combinations of Nd and Pr lines. Cross-correlations confirm these null results. At least six stars are magnetic and we provide magnetic field measurements for four of them, of which three are newly discovered magnetic stars. It is found that four stars have magnetic fields smaller than ∼2 kG, which according to theoretical predictions might be insufficient for suppressing envelope convection around the magnetic poles for more evolved Ap stars. Suppression of convection is expected to be essential for the opacity mechanism acting in the hydrogen ionization zone to drive the high-overtone roAp pulsations efficiently. Our null results suggest that the more evolved roAp stars may require particularly strong magnetic fields to pulsate. Three of the studied stars do, however, have magnetic fields stronger than 5 kG.

Eight New Millisecond Pulsars in NGC 6440 and NGC 6441

Motivated by the recent discovery of 30 new millisecond pulsars in Terzan 5, made using the Green Bank Telescope's S-band receiver and the Pulsar Spigot spectrometer, we have set out to use the same observing system in a systematic search for pulsars in other globular clusters. Here we report on the discovery of five new pulsars in NGC 6440 and three in NGC 6441; each cluster previously had one known pulsar. Using the most recent distance estimates to these clusters, we conclude that there are as many potentially observable pulsars in NGC 6440 and NGC 6441 as in Terzan 5. We present timing solutions for all of the pulsars in these globular clusters. Four of the new discoveries are in binary systems; one of them, PSR J1748–2021B (NGC 6440B), has a wide ( -->Pb = 20.5 days) and eccentric ( -->e = 0.57) orbit. This allowed a measurement of its rate of advance of periastron, = 0.00391(18)° yr−1$d{ω } = 0.00391(18) ° yr −1$ -->$d{ω } = 0.00391(18) ° yr t SUPgt −1t/SUPgt $ -->. If this is a purely relativistic effect, the total mass of this binary system is -->2.92 ± 0.20 -->M☉ (1 -->σ) implying a median pulsar mass of -->2.74 ± 0.21 -->M☉. There is a 1% probability that the inclination is low enough that pulsar mass is below 2 -->M☉, and 0.10% probability that it is between 1.20 and 1.44 -->M☉. If confirmed, this anomalously large mass would strongly constrain the equation of state for dense matter. The other highly eccentric binary, PSR J1750–37A, has -->e = 0.71, and = 0.0055(3)° yr−1$d{ω } = 0.0055(3) ° yr −1$ -->$d{ω } = 0.0055(3) ° yr t SUPgt −1t/SUPgt $ -->, implying a total system mass of -->1.97 ± 0.15 -->M☉ and, along with the mass function, maximum and median pulsar masses of 1.65 and 1.26 -->M☉, respectively.

The Field of the TeV Source HESS J1804−216 in X‐Rays and Other Wavelengths

The field of the extended TeV source HESS J1804-216 was serendipitously observed with the Chandra ACIS detector on 2005 May 4. The data reveal several X-ray sources within the bright part of HESS J1804-216. The brightest of these objects, CXOU J180432.4-214009, which has been also detected with Swift (2005 November 3) and Suzaku (2006 April 6), is consistent with being a pointlike source, with the 0.3-7 keV flux FX = (1.7 ± 0.2) × 10-13 ergs s-1 cm-2. Its hard and strongly absorbed spectrum can be fitted by the absorbed power-law model with the best-fit photon index Γ ≈ 0.45 and hydrogen column density nH ≈ 4 × 1022 cm-2, both with large uncertainties due to the strong correlation between these parameters. A search for pulsations resulted in a 106 s period candidate, which has a low significance of 97.9%, however. We found no infrared-optical counterparts for this source. The second brightest source, CXOU J180441.9-214224, which has been detected with Suzaku, is either extended or multiple, with the flux FX ~ 1 × 10-13 ergs cm-2 s-1. We found a nearby M dwarf within the X-ray source extension, which could contribute a fraction of the observed X-ray flux. The remaining sources are very faint (FX < 3 × 10-14 ergs cm-2 s-1), and at least some of them are likely associated with nearby stars. Although one or both of the two brighter X-ray sources could be faint accreting binaries or remote pulsars with pulsar wind nebulae (hence possible TeV sources), their relation to HESS J1804-216 remains elusive. The possibility that HESS J1804-216 is powered by the relativistic wind from the young pulsar B1800-21, located at a distance of ~10 pc from the TeV source, still remains a more plausible option.

Analysis of noise lines in the Virgo C7 data

The paper presents a description of the work of detection and identification of frequency lines in the Virgo dark fringe data from run C7. A number of methods are highlighted by which noise frequency lines are detected by data analysis and measurements in the laboratory. In this paper we give a description of the list of noise line candidates provided by the pulsar search analysis, the investigation of 10 Hz (and harmonics) noise, violin modes, noise from the end station buildings' air conditioners, sidebands in calibration lines and aliasing in the 4 kHz reconstructed data.

Erratum: Optimal filtering of the LISA data [Phys. Rev. D70, 022003 (2004)

The LISA time-delay-interferometry responses to a gravitational-wave signal are rewritten in a form that accounts for the motion of the LISA constellation around the Sun; the responses are given in closed analytic forms valid for any frequency in the band accessible to LISA. We then present a complete procedure, based on the principle of maximum likelihood, to search for stellar-mass binary systems in the LISA data. We define the required optimal filters, the amplitude-maximized detection statistic (analogous to the F statistic used in pulsar searches with ground-based interferometers), and discuss the false-alarm and detection probabilities. We test the procedure in numerical simulations of gravitational-wave detection.

Discovery of the Putative Pulsar and Wind Nebula Associated with the TeV Gamma‐Ray Source HESS J1813−178

We present a Chandra X-ray observation of G12.82-0.02, a shell-like radio supernova remnant coincident with the TeV gamma-ray source HESS J1813-178. We resolve the X-ray emission from the colocated Advanced Satellite for Cosmology and Astrophysics (ASCA) source into a point source surrounded by structured diffuse emission that fills the interior of the radio shell. The morphology of the diffuse emission strongly resembles that of a pulsar wind nebula. The spectrum of the compact source is well characterized by a power law with index Γ ≈ 1.3, typical of young and energetic rotation-powered pulsars. For a distance of 4.5 kpc, consistent with the X-ray absorption and an association with the nearby star formation region W33, the 2-10 keV X-ray luminosities of the putative pulsar and nebula are LPSR = 3.2 × 1033 ergs s-1 and LPWN = 1.4 × 1034 ergs s-1, respectively. Both the flux ratio of LPWN/LPSR = 4.3 and the total luminosity of this system predict a pulsar spin-down power of Ė > 1037 ergs s-1, placing it among the 10 most energetic young pulsars in the Galaxy. A deep search for radio pulsations using the Parkes telescope sets an upper limit of ≈0.07 mJy at 1.4 GHz for periods 50 ms. We discuss the energetics of this source and consider briefly the proximity of bright H II regions to this and several other High Energy Stereoscopic System (H.E.S.S.) sources, which may produce their TeV emission via inverse Compton scattering.

The Next Geminga: Search for Radio and X‐Ray Pulsations from the Neutron Star Identified with 3EG J1835+5918

We report unsuccessful searches for pulsations from the neutron star RX J1836.2+5925, identified with the EGRET source 3EG J1835+5918. A 24 hr observation with the NRAO Green Bank Telescope at 820 MHz placed an upper limit on flux density of 17 μJy for P 10 ms, and gradually increasing limits for 1 P 10 ms. The equivalent luminosity is lower than that of any known pulsar, with the possible exception of the radio-quiet γ-ray pulsar Geminga. A set of observations with the Chandra X-ray Observatory HRC totaling 118 ks revealed no pulsar with 1 ms ≤ P ≤ 10 s. The upper limit on its pulsed fraction is 35% assuming a sinusoidal pulse shape. The position of RX J1836.2+5925 in Chandra observations separated by 3 years is unchanged within errors, leading to an upper limit on its proper motion of <0.14'' yr-1, or vt < 530 km s-1 at d = 800 pc, a maximum distance estimated from its thermal X-ray spectrum. With these null results, the properties of 3EG J1835+5918 and its X-ray counterpart RX J1836.2+5925 are consistent with a more distant or older version of Geminga, or perhaps a recycled pulsar. Having nearly exhausted the capabilities of current instrumentation at all wavelengths, it will likely fall to the Gamma-ray Large Area Space Telescope to discover pulsations from 3EG J1835+5918.