| 1. |
- van Cappellen, W., et al.
(author)
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Apertif: Phased array feeds for the Westerbork Synthesis Radio Telescope: System overview and performance characteristics
- 2022
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 658
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Journal article (peer-reviewed)abstract
- We describe the APERture Tile In Focus (Apertif) system, a phased array feed (PAF) upgrade of the Westerbork Synthesis Radio Telescope that transforms this telescope into a high-sensitivity, wide-field-of-view L-band imaging and transient survey instrument. Using novel PAF technology, up to 40 partially overlapping beams are formed on the sky simultaneously, significantly increasing the survey speed of the telescope. With this upgraded instrument, an imaging survey covering an area of 2300 deg2 is being performed that will deliver both continuum and spectral line datasets, of which the first data have been publicly released. In addition, a time domain transient and pulsar survey covering 15 000 deg2 is in progress. An overview of the Apertif science drivers, hardware, and software of the upgraded telescope is presented, along with its key performance characteristics.
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| 2. |
- Oostrum, L. C., et al.
(author)
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Repeating fast radio bursts with WSRT/Apertif
- 2020
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 635
-
Journal article (peer-reviewed)abstract
- 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.
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| 3. |
- Pastor-Marazuela, Ines, et al.
(author)
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Chromatic periodic activity down to 120 megahertz in a fast radio burst
- 2021
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In: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 596:7873, s. 505-508
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Journal article (peer-reviewed)abstract
- Fast radio bursts (FRBs) are extragalactic astrophysical transients(1) whose brightness requires emitters that are highly energetic yet compact enough to produce the short, millisecond-duration bursts. FRBs have thus far been detected at frequencies from 8 gigahertz (ref. (2)) down to 300 megahertz (ref. (3)), but lower-frequency emission has remained elusive. Some FRBs repeat(4-6), and one of the most frequently detected, FRB 20180916B(7), has a periodicity cycle of 16.35 days (ref. (8)). Using simultaneous radio data spanning a wide range of wavelengths (a factor of more than 10), here we show that FRB 20180916B emits down to 120 megahertz, and that its activity window is frequency dependent (that is, chromatic). The window is both narrower and earlier at higher frequencies. Binary wind interaction models predict a wider window at higher frequencies, the opposite of our observations. Our full-cycle coverage shows that the 16.3-day periodicity is not aliased. We establish that low-frequency FRB emission can escape the local medium. For bursts of the same fluence, FRB 20180916B is more active below 200 megahertz than at 1.4 gigahertz. Combining our results with previous upper limits on the all-sky FRB rate at 150 megahertz, we find there are 3-450 FRBs in the sky per day above 50 Jy ms. Our chromatic results strongly disfavour scenarios in which absorption from strong stellar winds causes FRB periodicity. We demonstrate that some FRBs are found in 'clean' environments that do not absorb or scatter low-frequency radiation. The fast radio burst FRB 20180916B repeats with a periodicity of 16 days, and is now found to emit down to a frequency of 120 MHz, much lower than previously observed.
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| 4. |
- Pron, C. J., et al.
(author)
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Deciphering developmental stages of adult myelopoiesis
- 2008
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In: Cell Cycle. - 1538-4101 .- 1551-4005. ; 7:6, s. 706-713
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Journal article (peer-reviewed)abstract
- The ability to subfractionate minor cellular subsets by multiparameter flow cytometry and to evaluate such cells for functional properties has been used to ascertain lineal relationships and detail developmental hierarchies in the hematopoietic system for more than 20 years. However, steady advances in technology combined with the use of novel cell surface markers continues to redefine the developmental landscape as novel subpopulations are purified and characterized. We recently used such an approach to stage progenitor cell hierarchy involved in myeloid development with the use of two markers, Slamf1 and Endoglin that have recently been shown to be associated with hematopoietic stem cells. Here, we provide additional characterization of these cellular subsets to further refine their developmental potential. Little or no alterations in lineage potential were observed in these subsets when evaluated in a BCL2 transgenic setting or in response to various growth factor combinations, although BCL2 significantly enhanced their in vitro readout. Gene expression patterns of functionally opposing transcription factors that are known to play key roles for the appropriate development into separate myeloid lineages were associated with the functional activity of prospectively isolated subsets. Multiple genes traditionally associated with early lymphopoiesis were observed in early candidate granulocyte/monocyte, but not early megakaryocytic and/or erythroid progenitor cells. When functionally evaluated, such early granulocyte/monocyte precursors displayed a latent lymphoid activity, which was pronounced in subsets bearing high expression of the tyrosine kinase receptor FLT3.
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| 5. |
- Pronk, Cornelis J. H., et al.
(author)
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Elucidation of the phenotypic, functional, and molecular topography of a myeloerythroid progenitor cell hierarchy
- 2007
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In: Cell Stem Cell. - : Elsevier (Cell Press). - 1934-5909 .- 1875-9777. ; 1:4, s. 428-442
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Journal article (peer-reviewed)abstract
- The major myeloid blood cell lineages are generated from hematopoietic stem cells by differentiation through a series of increasingly committed progenitor cells. Precise characterization of intermediate progenitors is important for understanding fundamental differentiation processes and a variety of disease states, including leukemia. Here, we evaluated the functional in vitro and in vivo potentials of a range of prospectively isolated myeloid precursors with differential expression of CD150, Endoglin, and CD41. Our studies revealed a hierarchy of myeloerythroid progenitors with distinct lineage potentials. The global gene expression signatures of these subsets were consistent with their functional capacities, and hierarchical clustering analysis suggested likely lineage relationships. These studies provide valuable tools for understanding myeloid lineage commitment, including isolation of an early erythroid-restricted precursor, and add to existing models of hematopoietic differentiation by suggesting that progenitors of the innate and adaptive immune system can separate late, following the divergence of megakaryocytic/erythroid potential.
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| 6. |
- Attema, Joanne, et al.
(author)
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Hematopoietic stem cell ageing is uncoupled from p16 INK4A-mediated senescence
- 2009
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In: Oncogene. - : Nature Publishing Group. - 0950-9232 .- 1476-5594. ; 28:22, s. 2238-2243
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Journal article (peer-reviewed)abstract
- Somatic stem cells are ultimately responsible for mediating appropriate organ homeostasis and have therefore been proposed to represent a cellular origin of the ageing process-a state often characterized by inappropriate homeostasis. Specifically, it has been suggested that ageing stem cells might succumb to replicative senescence by a mechanism involving the cyclin-dependent kinase inhibitor p16(INK4A). Here, we tested multiple functional and molecular parameters indicative of p16(INK4A) activity in primary aged murine hematopoietic stem cells (HSCs). We found no evidence that replicative senescence accompanies stem cell ageing in vivo, and in line with p16(INK4A) being a critical determinant of such processes, most aged HSCs (>99%) failed to express p16(INK4A) at the mRNA level. Moreover, whereas loss of epigenetically guided repression of the INK4A/ARF locus accompanied replicative senescent murine embryonic fibroblasts, such repression was maintained in aged stem cells. Taken together, these studies indicate that increased senescence as mediated by the p16(INK4A) tumor suppressor has only a minor function as an intrinsic regulator of steady-state HSC ageing in vivo.
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| 7. |
- Santaguida, Marianne, et al.
(author)
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JunB Protects against Myeloid Malignancies by Limiting Hematopoietic Stem Cell Proliferation and Differentiation without Affecting Self-Renewal
- 2009
-
In: Cancer Cell. - : Elsevier BV. - 1878-3686 .- 1535-6108. ; 15:4, s. 341-352
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Journal article (peer-reviewed)abstract
- Loss of the JunB/AP-1 transcription factor induces a myeloproliferative disease (MPD) arising from the hematopoietic stem cell (HSC) compartment. Here, we show that junB inactivation deregulates the cell-cycle machinery and increases the proliferation of long-term repopulating HSCs (LT-HSCs) without impairing their self-renewal or regenerative potential in vivo. We found that JunB loss destabilizes a complex network of genes and pathways that normally limit myeloid differentiation, leading to impaired responsiveness to both Notch and TGF-beta signaling due in part to transcriptional deregulation of the Hes1 gene. These results demonstrate that LT-HSC proliferation and differentiation are uncoupled from self-renewal and establish some of the mechanisms by which JunB normally limits the production of myeloid progenitors, hence preventing initiation of myeloid malignancies.
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