267 research outputs found
First measurement of the total gravitational quadrupole moment of a black widow companion
We present the first measurement of the gravitational quadrupole moment of the companion star of a spider pulsar, namely the black widow PSR J2051â0827. To this end, we have re-analysed radio timing data using a new model that is able to account for periastron precession caused by tidal and centrifugal deformations of the star as well as by general relativity. The model allows for a time-varying component of the quadrupole moment, thus self-consistently accounting for the ill-understood orbital period variations observed in these systems. Our analysis results in the first detection of orbital precession in a spider system at ÏË=â68â.6+0â.9â0â.5 yrâ1 and the most accurate determination of orbital eccentricity for PSR J2051â0827 with e = (4.2 ± 0.1) Ă 10â5. We show that the variable quadrupole component is about 100 times smaller than the average quadrupole moment QÂŻ=â2.2+0.6â1Ă1041 kgm2â . We discuss how accurate modelling of high-precision optical light curves of the companion star will allow its apsidal motion constant to be derived from our results
Optical, X-ray, and Îł-ray observations of the candidate transitional millisecond pulsar 4FGL J0427.8-6704
We present an optical, X-ray, and Îł-ray study of 1SXPS J042749.2-670434, an eclipsing X-ray binary that has an associated Îł-ray counterpart, 4FGL J0427.8-6704. This association has led to the source being classified as a transitional millisecond pulsar (tMSP) in an accreting state. We analyse 10.5 yr of Fermi LAT data and detect a Îł-ray eclipse at the same phase as optical and X-ray eclipses at the >5âÏ level, a significant improvement on the 2.8âÏ level of the previous detection. The confirmation of this eclipse solidifies the association between the X-ray source and the Îł-ray source, strengthening the tMSP classification. However, analysis of several optical data sets and an X-ray observation do not reveal a change in the sourceâs median brightness over long time-scales or a bi-modality on short time-scales. Instead, the light curve is dominated by flickering, which has a correlation time of 2.6 min alongside a potential quasi-periodic oscillation at âŒ21 min. The mass of the primary and secondary stars is constrained to be M1=1.43+0.33â0.19 Mâ and M2=0.3+0.17â0.12 Mâ through modelling of the optical light curve. While this is still consistent with a white dwarf primary, we favour the tMSP in a low accretion state classification due to the significance of the Îł-ray eclipse detection
New methods to constrain the radio transient rate: results from a survey of four fields with LOFAR
We report on the results of a search for radio transients between 115 and 190 MHz with the LOw-Frequency ARray (LOFAR). Four fields have been monitored with cadences between 15 minutes and several months. A total of 151 images were obtained, giving a total survey area of 2275 deg2. We analysed our data using standard LOFAR tools and searched for radio transients using the LOFAR Transients Pipeline (TraP). No credible radio transient candidate has been detected; however, we are able to set upper limits on the surface density of radio transient sources at low radio frequencies. We also show that low-frequency radio surveys are more sensitive to steep-spectrum coherent transient sources than GHz radio surveys. We used two new statistical methods to determine the upper limits on the transient surface density. One is free of assumptions on the flux distribution of the sources, while the other assumes a power-law distribution in flux and sets more stringent constraints on the transient surface density. Both of these methods provide better constraints than the approach used in previous works. The best value for the upper limit we can set for the transient surface density, using the method assuming a power-law flux distribution, is 1.3 · 10-3 deg-2 for transients brighter than 0.3 Jy with a time-scale of 15 min, at a frequency of 150 MHz. We also calculated for the first time upper limits for the transient surface density for transients of different time-scales. We find that the results can differ by orders of magnitude from previously reported, simplified estimates
Enhanced ozone loss by active inorganic bromine chemistry in the tropical troposphere
Abstract Bromine chemistry, particularly in the tropics, has been suggested to play an important role in tropospheric ozone loss (Theys et al., 2011) although a lack of measurements of active bromine species impedes a quantitative understanding of its impacts. Recent modelling and measurements of bromine monoxide (BrO) by Wang et al. (2015) have shown current models under predict BrO concentrations over the Pacific Ocean and allude to a missing source of BrO. Here, we present the first simultaneous aircraft measurements of atmospheric bromine monoxide, BrO (a radical that along with atomic Br catalytically destroys ozone) and the inorganic Br precursor compounds HOBr, BrCl and Br2 over the Western Pacific Ocean from 0.5 to 7 km. The presence of 0.17-âŹâ1.64 pptv BrO and 3.6-8 pptv total inorganic Br from these four species throughout the troposphere causes 10-20% of total ozone loss, and confirms the importance of bromine chemistry in the tropical troposphere; contributing to a 6 ppb decrease in ozone levels due to halogen chemistry. Observations are compared with a global chemical transport model and find that the observed high levels of BrO, BrCl and HOBr can be reconciled by active multiphase oxidation of halide (Br- and Cl-Ëâ) by HOBr and ozone in cloud droplets and aerosols. Measurements indicate that 99% of the instantaneous free Br in the troposphere up to 8 km originates from inorganic halogen photolysis rather than from photolysis of organobromine species
A rapid optical and X-ray timing study of the neutron star X-ray binary Swift J1858.6â0814
We present a rapid timing analysis of optical (HiPERCAM and ULTRACAM) and X-ray (NICER) observations of the X-ray transient SwiftâJ1858.6â0814 during 2018 and 2019. The optical light curves show relatively slow, large amplitude (âŒ1âmag in gs) âblueâ flares (i.e. stronger at shorter wavelengths) on time-scales of âŒminutes as well as fast, small amplitude (âŒ0.1âmag in gs) âredâ flares (i.e. stronger at longer wavelengths) on time-scales of âŒseconds. The âblueâ and âredâ flares are consistent with X-ray reprocessing and optically thin synchrotron emission, respectively, similar to what is observed in other X-ray binaries. The simultaneous optical versus soft- and hard-band X-ray light curves show time- and energy-dependent correlations. The 2019 March 4 and parts of the June data show a nearly symmetric positive cross-correlations (CCFs) at positive lags consistent with simple X-ray disc reprocessing. The soft- and hard-band CCFs are similar and can be reproduced if disc reprocessing dominates in the optical and one component (disc or synchrotron Comptonization) dominates both the soft and hard X-rays. A part of the 2019 June data shows a very different CCFs. The observed positive correlation at negative lag in the soft band can be reproduced if the optical synchrotron emission is correlated with the hot flow X-ray emission. The observed timing properties are in qualitative agreement with the hybrid inner hot accretion flow model, where the relative role of the different X-ray and optical components that vary during the course of the outburst, as well as on shorter time-scales, govern the shape of the optical/X-ray CCFs
Rotation measure variations for 20 millisecond pulsars
We report on variations in the mean position angle of the 20 millisecond
pulsars being observed as part of the Parkes Pulsar Timing Array (PPTA)
project. It is found that the observed variations are dominated by changes in
the Faraday rotation occurring in the Earth's ionosphere. Two ionospheric
models are used to correct for the ionospheric contribution and it is found
that one based on the International Reference Ionosphere gave the best results.
Little or no significant long-term variation in interstellar RM was found with
limits typically about 0.1 rad m yr in absolute value. In a few
cases, apparently significant RM variations over timescales of a few 100 days
or more were seen. These are unlikely to be due to localised magnetised regions
crossing the line of sight since the implied magnetic fields are too high. Most
probably they are statistical fluctuations due to random spatial and temporal
variations in the interstellar electron density and magnetic field along the
line of sight.Comment: Accepted for publication in Astrophysics & Space Scienc
Measuring the mass of the black widow PSR J1555-2908
Accurate measurements of the masses of neutron stars are necessary to test
binary evolution models, and to constrain the neutron star equation of state.
In pulsar binaries with no measurable post-Keplerian parameters, this requires
an accurate estimate of the binary system's inclination and the radial velocity
of the companion star by other means than pulsar timing. In this paper, we
present the results of a new method for measuring this radial velocity using
the binary synthesis code Icarus. This method relies on constructing a model
spectrum of a tidally distorted, irradiated star as viewed for a given binary
configuration. This method is applied to optical spectra of the newly
discovered black widow PSR J1555-2908. By modelling the optical spectroscopy
alongside optical photometry, we find that the radial velocity of the companion
star is km s (errors quoted at 95\% confidence interval), as
well as a binary inclination of . Combined with -ray
pulsation timing information, this gives a neutron star mass of
1.67 M and a companion mass of
0.060 M, placing PSR J1555-2908 at the observed
upper limit of what is considered a black widow system.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society. 15 pages, 7 Figures. Underlying data available at
https://zenodo.org/record/565306
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Dispersion measure variability for 36 millisecond pulsars at 150MHz with LOFAR
Context. Radio pulses from pulsars are affected by plasma dispersion, which results in a frequency-dependent propagation delay. Variations in the magnitude of this effect lead to an additional source of red noise in pulsar timing experiments, including pulsar timing arrays (PTAs) that aim to detect nanohertz gravitational waves.
Aims. We aim to quantify the time-variable dispersion with much improved precision and characterise the spectrum of these variations.
Methods. We use the pulsar timing technique to obtain highly precise dispersion measure (DM) time series. Our dataset consists of observations of 36 millisecond pulsars, which were observed for up to 7.1 yr with the LOw Frequency ARray (LOFAR) telescope at a centre frequency of ~150 MHz. Seventeen of these sources were observed with a weekly cadence, while the rest were observed at monthly cadence.
Results. We achieve a median DM precision of the order of 10â5 cmâ3 pc for a significant fraction of our sources. We detect significant variations of the DM in all pulsars with a median DM uncertainty of less than 2 Ă 10â4 cmâ3 pc. The noise contribution to pulsar timing experiments at higher frequencies is calculated to be at a level of 0.1â10 ÎŒs at 1.4 GHz over a timespan of a few years, which is in many cases larger than the typical timing precision of 1 ÎŒs or better that PTAs aim for. We found no evidence for a dependence of DM on radio frequency for any of the sources in our sample.
Conclusions. The DM time series we obtained using LOFAR could in principle be used to correct higher-frequency data for the variations of the dispersive delay. However, there is currently the practical restriction that pulsars tend to provide either highly precise times of arrival (ToAs) at 1.4 GHz or a high DM precision at low frequencies, but not both, due to spectral properties. Combining the higher-frequency ToAs with those from LOFAR to measure the infinite-frequency ToA and DM would improve the result
The FRATS project:Real-time searches for fast radio bursts and other fast transients with LOFAR at 135 MHz
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