Context.
Repeating fast radio bursts (FRBs) present excellent opportunities to identify FRB progenitors and host environments as well as to decipher the underlying emission mechanism. Detailed ...studies of repeating FRBs might also hold clues as to the origin of FRBs as a population.
Aims.
We aim to detect bursts from the first two repeating FRBs, FRB 121102 (R1) and FRB 180814.J0422+73 (R2), and to characterise their repeat statistics. We also want to significantly improve the sky localisation of R2 and identify its host galaxy.
Methods.
We used the Westerbork Synthesis Radio Telescope to conduct extensive follow-up of these two repeating FRBs. The new phased-array feed system, Apertif, allows one to cover the entire sky position uncertainty of R2 with fine spatial resolution in a single pointing. The data were searched for bursts around the known dispersion measures of the two sources. We characterise the energy distribution and the clustering of detected R1 bursts.
Results.
We detected 30 bursts from R1. The non-Poissonian nature is clearly evident from the burst arrival times, which is consistent with earlier claims. Our measurements indicate a dispersion measure (DM) of 563.5(2) pc cm
−3
, suggesting a significant increase in DM over the past few years. Assuming a constant position angle across the burst, we place an upper limit of 8% on the linear polarisation fraction for the brightest burst in our sample. We did not detect any bursts from R2.
Conclusions.
A single power-law might not fit the R1 burst energy distribution across the full energy range or widely separated detections. Our observations provide improved constraints on the clustering of R1 bursts. Our stringent upper limits on the linear polarisation fraction imply a significant depolarisation, either intrinsic to the emission mechanism or caused by the intervening medium at 1400 MHz, which is not observed at higher frequencies. The non-detection of any bursts from R2, despite nearly 300 h of observations, implies either a highly clustered nature of the bursts, a steep spectral index, or a combination of the two assuming that the source is still active. Another possibility is that R2 has turned off completely, either permanently or for an extended period of time.
ABSTRACT
We report the detection of a bright fast radio burst, FRB 191108, with Apertif on the Westerbork Synthesis Radio Telescope. The interferometer allows us to localize the FRB to a narrow 5 ...arcsec × 7 arcmin ellipse by employing both multibeam information within the Apertif phased-array feed beam pattern, and across different tied-array beams. The resulting sightline passes close to Local Group galaxy M33, with an impact parameter of only 18 kpc with respect to the core. It also traverses the much larger circumgalactic medium (CGM) of M31, the Andromeda Galaxy. We find that the shared plasma of the Local Group galaxies could contribute ∼10 per cent of its dispersion measure of 588 pc cm−3. FRB 191108 has a Faraday rotation measure (RM) of +474 $\pm \, 3$ rad m−2, which is too large to be explained by either the Milky Way or the intergalactic medium. Based on the more moderate RMs of other extragalactic sources that traverse the halo of M33, we conclude that the dense magnetized plasma resides in the host galaxy. The FRB exhibits frequency structure on two scales, one that is consistent with quenched Galactic scintillation and broader spectral structure with Δν ≈ 40 MHz. If the latter is due to scattering in the shared M33/M31 CGM, our results constrain the Local Group plasma environment. We found no accompanying persistent radio sources in the Apertif imaging survey data.
Mismatch between the refractive indexes of immersion media and glass coverslips introduces spherical aberrations in microscopes especially for high numerical aperture objectives. This contribution ...demonstrates an automated adjustment of the coverslip correction collar in scanning confocal microscopy to compensate for spherical aberrations due to coverslip thickness mismatch. With a motorized coverslip correction collar, the adjustment procedure consists of xz image scans, image processing, correction quality evaluation, the mismatch estimation, and eventually the optimal adjustment of the correction collar. For fast correction with less photodamage, coarse-fine Gaussian fitting algorithms are proposed and evaluated with various specimen for their estimation accuracy. The benefits of the proposed automated correction are demonstrated for various coverslips with biological specimens, showing the optimized resolution of the confocal microscope.
Context.
The characteristics of the polarised radio sky are a key ingredient in constraining evolutionary models of magnetic fields in the Universe and their role in feedback processes. The origin of ...the polarised emission and the characteristics of the intergalactic medium on the line of sight can be investigated using large samples of polarised sources. Ancillary infrared (IR) and optical data can be used to study the nature of the emitting objects.
Aims.
We analyse five early science datasets from the APERture Tile in Focus (Apertif) phased array feed system to verify the polarisation capabilities of Apertif in view of future larger data releases. We aim to characterise the source population of the polarised sky in the
L
-Band using polarised source information in combination with IR and optical data.
Methods.
We use automatic routines to generate full field-of-view Q- and U-cubes and perform rotation measure (RM)-Synthesis, source finding, and cross-matching with published radio, optical, and IR data to generate polarised source catalogues. All sources were inspected individually by eye for verification of their IR and optical counterparts. Spectral energy distribution (SED)-fitting routines were used to determine photometric redshifts, star-formation rates, and galaxy masses. IR colour information was used to classify sources as active galactic nuclei (AGN) or star-forming-dominated and early- or late-type.
Results.
We surveyed an area of 56 deg
2
and detected 1357 polarised source components in 1170 sources. The fraction of polarised sources is 10.57% with a median fractional polarisation of 4.70 ± 0.14%. We confirmed the reliability of the Apertif measurements by comparing them with polarised cross-identified NVSS sources. Average RMs of the individual fields lie within the error of the best Milky Way foreground measurements. All of our polarised sources were found to be dominated by AGN activity in the radio regime with most of them being radio-loud (79%) and of the Fanaroff-Riley (FR)II class (87%). The host galaxies of our polarised source sample are dominated by intermediate disc and star-forming disc galaxies. The contribution of star formation to the radio emission is on the order of a few percent for ≈10% of the polarised sources while for ≈90% it is completely dominated by the AGN. We do not see any change in fractional polarisation for different star-formation rates of the AGN host galaxies.
Conclusions.
The Apertif system is suitable for large-area high-sensitivity polarised sky surveys. The data products of the polarisation analysis pipeline can be used to investigate the Milky Way magnetic field on projected scales of several arcminutes as well as the origin of the polarised emission in AGN and the properties of their host galaxies.
We present a new image of a 26.5 square degrees region in the Boötes constellation obtained at 1.4 GHz using the Aperture Tile in Focus (Apertif) system on the Westerbork Synthesis Radio Telescope. ...We use a newly developed processing pipeline that includes direction-dependent self-calibration, which provides a significant improvement in the quality of the images compared to those released as part of the Apertif first data release. For the Boötes region, we mosaicked 187 Apertif images and extracted a source catalog. The mosaic image has an angular resolution of 27 × 11.5″ and a median background noise of 40 μJy beam
−1
. The catalog has 8994 sources and is complete down to the 0.3 mJy level. We combined the Apertif image with LOFAR images of the Boötes field at 54 and 150 MHz to study the spectral properties of the sources. We find a spectral flattening toward sources with a low flux density. Using the spectral index limits from Apertif nondetections, we derive that up to 9% of the sources have ultrasteep spectra with a slope below −1.2. A steepening of the spectral index with increasing redshift is also seen in the data, which shows a different dependence for the low-and high-frequency spectral index. The explanation probably is that a population of sources has concave radio spectra with a turnover frequency of about the LOFAR band. Additionally, we discuss cases of individual extended sources with an interesting resolved spectral structure. With the improved pipeline, we aim to continue to process data from the Apertif wide-area surveys and release the improved 1.4-GHz images of several well-known fields.
An optimal beamforming strategy is proposed for performing large-field surveys with dual-polarized phased-array-fed reflector antennas. This strategy uses signal-processing algorithms that maximize ...the beam sensitivity and the continuity of a field of view (FOV) that is formed by multiple closely overlapping beams. A mathematical framework and a newly developed numerical approach are described to analyze and optimize a phased array feed (PAF) system. The modeling approach has been applied to an experimental PAF system (APERTIF prototype) that is installed on the Westerbork Synthesis Radio Telescope. The resulting beam shapes, sensitivity, and polarization diversity characteristics (such as the beam orthogonality and the intrinsic cross-polarization ratio) are examined over a large FOV and frequency bandwidth. We consider weighting schemes to achieve a conjugate-field matched situation (max. received power), maximum signal-to-noise ratio (SNR), and a reduced SNR scenario but with constraints on the beam shape. The latter improves the rotational symmetry of the beam and reduces the sensitivity ripple, at a modest maximum sensitivity penalty. The obtained numerical results demonstrate a very good agreement with the measurements performed at the telescope.
Mycobacterium chimaera colonizes water-based heater–cooler units (HCUs), from which it can spread to patients during surgery. Vermamoeba vermiformis is a free-living waterborne amoeba, which was ...consistently present within HCUs.
To determine whether these amoebae can be involved in the persistent presence of M. chimaera.
An in-vitro disinfection model.
Increased survival of M. chimaera was observed after chlorine exposure in the presence of V. vermiformis. Confocal microscopy demonstrated the intracellular presence of M. chimaera in V. vermiformis.
In this way, V. vermiformis can contribute to the persistent presence of M. chimaera in HCUs. Cleaning and disinfection protocols should take this phenomenon into account.
Verona-Integron-encoded-Metallo-β-lactamase-positive Pseudomonas aeruginosa (VIM-PA) is a cause of hard-to-treat nosocomial infections, and can colonize hospital water networks alongside ...Acanthamoeba. We developed an in-vitro disinfection model to examine whether Acanthamoeba castellanii can harbour VIM-PA intracellularly, allowing VIM-PA to evade being killed by currently used hospital disinfectants. We observed that A. castellanii presence resulted in significantly increased survival of VIM-PA after exposure to chlorine for 30 s or for 2 min. This undesirable effect was not observed after disinfection by 70% alcohol or 24% acetic acid. Confocal microscopy confirmed the presence of VIM-PA within A. castellanii pseudocysts. Our data indicate that A. castellanii contributes to persistent VIM-PA colonization of water systems after chlorine treatment.
The focal location of atherosclerosis in the vascular tree is correlated with local variations in shear stress. We developed a method to induce defined variations in shear stress in a straight vessel ...segment of a mouse. To this end, a cylinder with a tapered lumen was placed around the carotid artery, inducing a high shear stress field. Concomitantly, regions of low shear stress and oscillatory shear stress were created upstream and down-stream of the device, respectively. This device was used in mice transgenic for an eNOS3GFP fusion gene. We observed a strong induction of endothelial nitric oxide synthase–green fluorescent protein (eNOS-GFP) mRNA expression in the high shear stress region compared with the other regions (P < .05). Quantification of eNOS-GFP fluorescence or of immunoreactivity to the Golgi complex or to platelet endothelial cell adhesion molecule 1 (PECAM-1) showed an increase in the high shear stress region (P < .05) compared with nontreated carotid arteries. Colocalization of eNOS-GFP with either the Golgi complex or PECAM-1 also responded to alterations of shear stress. In conclusion, we showed a direct response of mRNA and protein expression in vivo to induced variations of shear stress. This model provides the opportunity to study the relationship between shear stress alterations, gene expression, and atherosclerosis.
Fragile X syndrome (FXS) is a single-gene disorder that is the most common heritable cause of intellectual disability and the most frequent monogenic cause of autism spectrum disorders (ASD). FXS is ...caused by an expansion of trinucleotide repeats in the promoter region of the fragile X mental retardation gene (Fmr1). This leads to a lack of fragile X mental retardation protein (FMRP), which regulates translation of a wide range of messenger RNAs (mRNAs). The extent of expression level alterations of synaptic proteins affected by FMRP loss and their consequences on synaptic dynamics in FXS has not been fully investigated.
Here, we used an Fmr1 knockout (KO) mouse model to investigate the molecular mechanisms underlying FXS by monitoring protein expression changes using shotgun label-free liquid-chromatography mass spectrometry (LC-MS(E)) in brain tissue and synaptosome fractions. FXS-associated candidate proteins were validated using selected reaction monitoring (SRM) in synaptosome fractions for targeted protein quantification. Furthermore, functional alterations in synaptic release and dynamics were evaluated using live-cell imaging, and interpretation of synaptic dynamics differences was investigated using electron microscopy.
Key findings relate to altered levels of proteins involved in GABA-signalling, especially in the cerebellum. Further exploration using microscopy studies found reduced synaptic vesicle unloading of hippocampal neurons and increased vesicle unloading in cerebellar neurons, which suggests a general decrease of synaptic transmission.
Our findings suggest that FMRP is a regulator of synaptic vesicle dynamics, which supports the role of FMRP in presynaptic functions. Taken together, these studies provide novel insights into the molecular changes associated with FXS.