ABSTRACT
We have used the central 44 antennas of the new 64-dish MeerKAT radio telescope array to conduct a deep search for new pulsars in the core of nine globular clusters (GCs). This has led to ...the discovery of eight new millisecond pulsars in six different clusters. Two new binaries, 47 Tuc ac and 47 Tuc ad, are eclipsing ‘spiders’, featuring compact orbits (≲0.32 d), very low mass companions, and regular occultations of their pulsed emission. The other three new binary pulsars (NGC 6624G, M62G, and Ter 5 an) are in wider (>0.7 d) orbits, with companions that are likely to be white dwarfs or neutron stars. NGC 6624G has a large eccentricity of e ≃ 0.38, which enabled us to detect the rate of advance of periastron. This suggests that the system is massive, with a total mass of Mtot = 2.65 ± 0.07 M⊙. Likewise, for Ter 5 an, with e ≃ 0.0066, we obtain Mtot = 2.97 ± 0.52 M⊙. The other three new discoveries (NGC 6522D, NGC 6624H, and NGC 6752F) are faint isolated pulsars. Finally, we have used the whole MeerKAT array and synthesized 288 beams, covering an area of ∼2 arcmin in radius around the centre of NGC 6624. This has allowed us to localize many of the pulsars in the cluster, demonstrating the beamforming capabilities of the TRAPUM software backend and paving the way for the upcoming MeerKAT GC pulsar survey.
Direct detection of low-frequency gravitational waves (GWs,
Hz) is the main goal of pulsar timing array (PTA) projects. One of the main targets for the PTAs is to measure the stochastic background of ...gravitational waves (GWB) whose characteristic strain is expected to approximately follow a power-law of the form
, where f is the GW frequency. In this paper we use the current data from the European PTA to determine an upper limit on the GWB amplitude A as a function of the unknown spectral slope α with a Bayesian algorithm, by modelling the GWB as a random Gaussian process. For the case α=−2/3, which is expected if the GWB is produced by supermassive black hole binaries, we obtain a 95 per cent confidence upper limit on A of 6 × 10−15, which is 1.8 times lower than the 95 per cent confidence GWB limit obtained by the Parkes PTA in 2006. Our approach to the data analysis incorporates the multitelescope nature of the European PTA and thus can serve as a useful template for future intercontinental PTA collaborations.
The New Magnetar SGR J1830−0645 in Outburst Zelati, F. Coti; Borghese, A.; Israel, G. L. ...
Astrophysical journal. Letters,
02/2021, Letnik:
907, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Abstract
The detection of a short hard X-ray burst and an associated bright soft X-ray source by the Swift satellite in 2020 October heralded a new magnetar in outburst, SGR J1830−0645. Pulsations at ...a period of ∼10.4 s were detected in prompt follow-up X-ray observations. We present here the analysis of the Swift/Burst Alert Telescope burst, of XMM-Newton and the Nuclear Spectroscopic Telescope Array observations performed at the outburst peak, and of a Swift/X-ray Telescope monitoring campaign over the subsequent month. The burst was single-peaked, lasted ∼6 ms, and released a fluence of ≈5 × 10
−9
erg cm
−2
(15–50 keV). The spectrum of the X-ray source at the outburst peak was well described by an absorbed double-blackbody model plus a power-law component detectable up to ∼25 keV. The unabsorbed X-ray flux decreased from ∼5 × 10
−11
to ∼2.5 × 10
−11
erg cm
−2
s
−1
one month later (0.3–10 keV). Based on our timing analysis, we estimate a dipolar magnetic field ≈5.5 × 10
14
G at pole, a spin-down luminosity ≈2.4 × 10
32
erg s
−1
, and a characteristic age ≈24 kyr. The spin modulation pattern appears highly pulsed in the soft X-ray band, and becomes smoother at higher energies. Several short X-ray bursts were detected during our campaign. No evidence for periodic or single-pulse emission was found at radio frequencies in observations performed with the Sardinia Radio Telescope and Parkes. According to magneto-thermal evolutionary models, the real age of SGR J1830−0645 is close to the characteristic age, and the dipolar magnetic field at birth was slightly larger, ∼10
15
G.
We present the discovery and follow-up observations of 142 pulsars found in the Parkes 20-cm multibeam pulsar survey of the Galactic plane. These new discoveries bring the total number of pulsars ...found by the survey to 742. In addition to tabulating spin and astrometric parameters, along with pulse width and flux density information, we present orbital characteristics for 13 binary pulsars which form part of the new sample. Combining these results from another recent Parkes multibeam survey at high Galactic latitudes, we have a sample of 1008 normal pulsars which we use to carry out a determination of their Galactic distribution and birth rate. We infer a total Galactic population of 30 000 ± 1100 potentially detectable pulsars (i.e. those beaming towards us) having 1.4-GHz luminosities above 0.1 mJy kpc2. Adopting the Tauris & Manchester beaming model, this translates to a total of 155 000 ± 6000 active radio pulsars in the Galaxy above this luminosity limit. Using a pulsar current analysis, we derive the birth rate of this population to be 1.4 ± 0.2 pulsars per century. An important conclusion from our work is that the inferred radial density function of pulsars depends strongly on the assumed distribution of free electrons in the Galaxy. As a result, any analyses using the most recent electron model of Cordes & Lazio predict a dearth of pulsars in the inner Galaxy. We show that this model can also bias the inferred pulsar scaleheight with respect to the Galactic plane. Combining our results with other Parkes multibeam surveys we find that the population is best described by an exponential distribution with a scaleheight of 330 pc. Surveys underway at Parkes and Arecibo are expected to improve the knowledge of the radial distribution outside the solar circle, and to discover several hundred new pulsars in the inner Galaxy.
We report on the lowest-frequency detection to date of three bursts from the fast radio burst FRB 180916.J0158+65, observed at 328 MHz with the Sardinia Radio Telescope (SRT). The SRT observed the ...periodic repeater FRB 180916.J0158+65 for five days from 2020 February 20 to 24 during a time interval of active radio bursting, and detected the three bursts during the first hour of observations; no more bursts were detected during the remaining ∼30 hr. Simultaneous SRT observations at 1548 MHz did not detect any bursts. Burst fluences are in the range 37 to 13 Jy ms. No relevant scattering is observed for these bursts. We also present the results of the multi-wavelength campaign we performed on FRB 180916.J0158+65, during the five days of the active window. Simultaneously with the SRT observations, others with different time spans were performed with the Northern Cross at 408 MHz, with XMM-Newton, NICER, INTEGRAL, AGILE, and with the TNG and two optical telescopes in Asiago, which are equipped with fast photometers. XMM-Newton obtained data simultaneously with the three bursts detected by the SRT, and determined a luminosity upper limit in the 0.3-10 keV energy range of ∼1045 erg s−1 for the burst emission. AGILE obtained data simultaneously with the first burst and determined a fluence upper limit in the MeV range for millisecond timescales of . Our results show that absorption from the circumburst medium does not significantly affect the emission from FRB 180916.J0158+65, thus limiting the possible presence of a superluminous supernova around the source, and indicate that a cutoff for the bursting mechanism, if present, must be at lower frequencies. Our multi-wavelength campaign sensitively constrains the broadband emission from FRB 180916.J0158+65, and provides the best limits so far for the electromagnetic response to the radio bursting of this remarkable source of fast radio bursts.
Abstract
Very long baseline interferometric (VLBI) localizations of repeating fast radio bursts (FRBs) have demonstrated a diversity of local environments: from nearby star-forming regions to ...globular clusters. Here we report the VLBI localization of FRB 20201124A using an ad hoc array of dishes that also participate in the European VLBI Network (EVN). In our campaign, we detected 18 bursts from FRB 20201124A at two separate epochs. By combining the visibilities from both epochs, we were able to localize FRB 20201124A with a 1
σ
uncertainty of 2.7 mas. We use the relatively large burst sample to investigate astrometric accuracy and find that for ≳20 baselines (≳7 dishes) we can robustly reach milliarcsecond precision even using single-burst data sets. Subarcsecond precision is still possible for single bursts, even when only ∼6 baselines (four dishes) are available. In such cases, the limited
uv
coverage for individual bursts results in very high side-lobe levels. Thus, in addition to the peak position from the dirty map, we also explore smoothing the structure in the dirty map by fitting Gaussian functions to the fringe pattern in order to constrain individual burst positions, which we find to be more reliable. Our VLBI work places FRB 20201124A 710 ± 30 mas (1
σ
uncertainty) from the optical center of the host galaxy, consistent with originating from within the recently discovered extended radio structure associated with star formation in the host galaxy. Future high-resolution optical observations, e.g., with Hubble Space Telescope, can determine the proximity of FRB 20201124A’s position to nearby knots of star formation.
ABSTRACT
The Fermi Large Area Telescope gamma-ray source 3FGL J2039.6–5618 contains a periodic optical and X-ray source that was predicted to be a ‘redback’ millisecond pulsar (MSP) binary system. ...However, the conclusive identification required the detection of pulsations from the putative MSP. To better constrain the orbital parameters for a directed search for gamma-ray pulsations, we obtained new optical light curves in 2017 and 2018, which revealed long-term variability from the companion star. The resulting orbital parameter constraints were used to perform a targeted gamma-ray pulsation search using the Einstein@Home-distributed volunteer computing system. This search discovered pulsations with a period of 2.65 ms, confirming the source as a binary MSP now known as PSR J2039–5617. Optical light-curve modelling is complicated, and likely biased, by asymmetric heating on the companion star and long-term variability, but we find an inclination i ≳ 60°, for a low pulsar mass between $1.1\, \mathrm{M}_{\odot } \lt M_{\rm psr} \lt $ 1.6 M⊙, and a companion mass of 0.15–$0.22\, \mathrm{M}_{\odot }$, confirming the redback classification. Timing the gamma-ray pulsations also revealed significant variability in the orbital period, which we find to be consistent with quadrupole moment variations in the companion star, suggestive of convective activity. We also find that the pulsed flux is modulated at the orbital period, potentially due to inverse Compton scattering between high-energy leptons in the pulsar wind and the companion star’s optical photon field.
We present timing observations of 4-ms pulsars discovered in the Parkes 20-cm multibeam pulsar survey of the Galactic plane. PSRs J1552−4937 and J1843−1448 are isolated objects with spin periods of ...6.28 and 5.47 ms, respectively. PSR J1727−2946 is in a 40-d binary orbit and has a spin period of 27 ms. The 4.43-ms pulsar J1813−2621 is in a circular 8.16-d binary orbit around a low-mass companion star with a minimum companion mass of 0.2 M⊙. Combining these results with detections from five other Parkes multibeam surveys, gives a well-defined sample of 56 pulsars with spin periods below 20 ms. We develop a likelihood analysis to constrain the functional form which best describes the underlying distribution of spin periods for millisecond pulsars. The best results were obtained with a lognormal distribution. A gamma distribution is less favoured, but still compatible with the observations. Uniform, power-law and Gaussian distributions are found to be inconsistent with the data. Galactic millisecond pulsars being found by current surveys appear to be in agreement with a lognormal distribution which allows for the existence of pulsars with periods below 1.5 ms.
ABSTRACT
The predicted nature of the candidate redback pulsar 3FGL J2039.6–5618 was recently confirmed by the discovery of γ-ray millisecond pulsations (Clark et al., hereafter Paper I), which ...identify this γ-ray source as PSR J2039–5617. We observed this object with the Parkes radio telescope in 2016 and 2019. We detect radio pulsations at 1.4 and 3.1 GHz, at the 2.6 ms period discovered in γ-rays, and also at 0.7 GHz in one 2015 archival observation. In all bands, the radio pulse profile is characterized by a single relatively broad peak which leads the main γ-ray peak. At 1.4 GHz, we found clear evidence of eclipses of the radio signal for about half of the orbit, a characteristic phenomenon in redback systems, which we associate with the presence of intra-binary gas. From the dispersion measure of 24.57 ± 0.03 pc cm−3, we derive a pulsar distance of 0.9 ± 0.2 or 1.7 ± 0.7 kpc, depending on the assumed Galactic electron density model. The modelling of the radio and γ-ray light curves leads to an independent determination of the orbital inclination, and to a determination of the pulsar mass, qualitatively consistent to the results in Paper I.
ABSTRACT
In this work, we study variations in the parabolic scintillation arcs of the binary millisecond pulsar PSR J1643−1224 over five years using the Large European Array for Pulsars (LEAP). The ...two-dimensional (2D) power spectrum of scintillation, called the secondary spectrum, often shows a parabolic distribution of power, where the arc curvature encodes the relative velocities and distances of the pulsar, ionized interstellar medium, and Earth. We observe a clear parabolic scintillation arc, which varies in curvature throughout the year. The distribution of power in the secondary spectra is inconsistent with a single scattering screen, which is fully 1D or entirely isotropic. We fit the observed arc curvature variations with two models: an isotropic scattering screen and a model with two independent 1D screens. We measure the distance to the scattering screen to be in the range 114–223 pc, depending on the model, consistent with the known distance of the foreground large-diameter H ii region Sh 2-27 (112 ± 17 pc), suggesting that it is the dominant source of scattering. We obtain only weak constraints on the pulsar’s orbital inclination and longitude of ascending node, since the scintillation pattern is not very sensitive to the pulsar’s motion and the screen is much closer to the Earth than the pulsar. More measurements of this kind – where scattering screens can be associated with foreground objects – will help to inform the origins and distribution of scattering screens within our galaxy.
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