430 research outputs found
What Prevents DNA Replication between Meiosis I and II in Yeast?
During meiosis, a single round of DNA replication is followed by two consecutive rounds of chromosome segregation. While the suppression of DNA replication between meiosis I and –II is one of the defining features of meiosis, its mechanism has remained unclear. The control of DNA replication has been studied extensively in proliferating cells in which DNA replication during S phase strictly alternates with chromosome segregation at mitosis. The mechanism ensuring that each sequence is replicated only once per cell cycle is based on the dual function of Cdk1: low Cdk1 activity after mitosis allows the establishment of prereplicative complexes at replication origins (origin licensing). Activation of Cdk1 at the onset of S phase then initiates DNA replication (origin firing) by converting the pre-replicative complex to the post-replicative complex. Since high Cdk1 activity inhibits the reformation of pre-replicative complexes, the next round of DNA replication cannot occur until after Cdk1 has been inactivated during mitosis when replicated chromosomes segregate. However, applying this concept to meiosis would trigger an additional round of DNA replication because Cdk1 activity drops and then re-appears between meiosis I and –II. Two ideas have been proposed to solve this problem: in Xenopus eggs, Cdk1 activity is reduced rather than completely destroyed between meiosis I and –II, while in yeast, a Cdk1-related kinase, called Ime2, was thought to prevent origin relicensing at anaphase I. We have tested these ideas by artificially inactivating and then reactivating Cdk1 and Ime2 at anaphase I. Remarkably, DNA replication was not induced even when both kinases were simultaneously inhibited and re-activated at anaphase I. Thus, additional mechanisms must prevent DNA replication between meiosis I and –II
The bright optical/NIR afterglow of the faint GRB 080710 - Evidence for a jet viewed off axis
We investigate the optical/near-infrared light curve of the afterglow of GRB
080710 in the context of rising afterglows. Optical and near-infrared
photometry was performed using the seven channel imager GROND and the
Tautenburg Schmidt telescope. X-ray data were provided by the X-ray Telescope
onboard the Swift satellite. The optical/NIR light curve of the afterglow of
GRB 080710 is dominated by an initial increase in brightness, which smoothly
turns over into a shallow power law decay. The initially rising achromatic
light curve of the afterglow of GRB 080710 can be accounted for with a model of
a burst viewed off-axis or a single jet in its pre deceleration phase and in an
on-axis geometry. An unified picture of the afterglow light curve and prompt
emission properties can be obtained with an off-axis geometry, suggesting that
late and shallow rising optical light curves of GRB afterglows might be
produced by geometric effects.Comment: 9 pages, 4 figures, accepted by A and
The late-time afterglow of the extremely energetic short burst GRB 090510 revisited
The discovery of the short GRB 090510 has raised considerable attention
mainly because it had a bright optical afterglow and it is among the most
energetic events detected so far within the entire GRB population. The
afterglow was observed with swift/UVOT and swift/XRT and evidence of a jet
break around 1.5 ks after the burst has been reported in the literature,
implying that after this break the optical and X-ray light curve should fade
with the same decay slope. As noted by several authors, the post-break decay
slope seen in the UVOT data is much shallower than the steep decay in the X-ray
band, pointing to an excess of optical flux at late times. We reduced and
analyzed new afterglow light-curve data obtained with the multichannel imager
GROND. Based on the densely sampled data set obtained with GROND, we find that
the optical afterglow of GRB 090510 did indeed enter a steep decay phase
starting around 22 ks after the burst. During this time the GROND optical light
curve is achromatic, and its slope is identical to the slope of the X-ray data.
In combination with the UVOT data this implies that a second break must have
occurred in the optical light curve around 22 ks post burst, which, however,
has no obvious counterpart in the X-ray band, contradicting the interpretation
that this could be another jet break. The GROND data provide the missing piece
of evidence that the optical afterglow of GRB 090510 did follow a post-jet
break evolution at late times.Comment: submitted to Astronomy & Astrophysics, accepted for publication on
Dec 24, 201
The redshift and afterglow of the extremely energetic gamma-ray burst GRB 080916C
The detection of GeV photons from gamma-ray bursts (GRBs) has important
consequences for the interpretation and modelling of these most-energetic
cosmological explosions. The full exploitation of the high-energy measurements
relies, however, on the accurate knowledge of the distance to the events. Here
we report on the discovery of the afterglow and subsequent redshift
determination of GRB 080916C, the first GRB detected by the Fermi Gamma-Ray
Space Telescope with high significance detection of photons at >0.1 GeV.
Observations were done with 7-channel imager GROND at the 2.2m MPI/ESO
telescope, the SIRIUS instrument at the Nagoya-SAAO 1.4m telescope in South
Africa, and the GMOS instrument at Gemini-S. The afterglow photometric redshift
of z=4.35+-0.15, based on simultaneous 7-filter observations with the Gamma-Ray
Optical and Near-infrared Detector (GROND), places GRB 080916C among the top 5%
most distant GRBs, and makes it the most energetic GRB known to date. The
detection of GeV photons from such a distant event is rather surprising.
The observed gamma-ray variability in the prompt emission together with the
redshift suggests a lower limit for the Lorentz factor of the
ultra-relativistic ejecta of Gamma > 1090. This value rivals any previous
measurements of Gamma in GRBs and strengthens the extreme nature of GRB
080916C.Comment: 6 pages, 5 figures; subm. to A&
The fast transient sky with Gaia
The ESA Gaia satellite scans the whole sky with a temporal sampling ranging
from seconds and hours to months. Each time a source passes within the Gaia
field of view, it moves over 10 CCDs in 45 s and a lightcurve with 4.5 s
sampling (the crossing time per CCD) is registered. Given that the 4.5 s
sampling represents a virtually unexplored parameter space in optical time
domain astronomy, this data set potentially provides a unique opportunity to
open up the fast transient sky. We present a method to start mining the wealth
of information in the per CCD Gaia data. We perform extensive data filtering to
eliminate known on-board and data processing artefacts, and present a
statistical method to identify sources that show transient brightness
variations on ~2 hours timescales. We illustrate that by using the Gaia
photometric CCD measurements, we can detect transient brightness variations
down to an amplitude of 0.3 mag on timescales ranging from 15 seconds to
several hours. We search an area of ~23.5 square degrees on the sky, and find
four strong candidate fast transients. Two candidates are tentatively
classified as flares on M-dwarf stars, while one is probably a flare on a giant
star and one potentially a flare on a solar type star. These classifications
are based on archival data and the timescales involved. We argue that the
method presented here can be added to the existing Gaia Science Alerts
infrastructure for the near real-time public dissemination of fast transient
events.Comment: 10 pages, 5 figures and 5 tables; MNRAS in pres
PKS 0537-286, carrying the information of the environment of SMBHs in the early Universe
We present the results of a multifrequency campaign on the high-redshift (z =
3.1) blazar PKS 0537-286. The source was observed at different epochs from 2006
to 2008 with INTEGRAL and Swift, and nearly simultaneously with ground-based
near-IR/optical telescopes. The SEDs are compatible with a model based on
synchrotron radiation and external inverse Compton scattering. The campaign
gives an insight into the physical environment of the blazar.Comment: 8 pages, 5 figures, accepted for publication in A&
Evidence for Supernova-Synthesised Dust from the Rising Afterglow of GRB 071025 at z~5
We present observations and analysis of the broadband afterglow of Swift GRB
071025. Using optical and infrared (RIYJHK) photometry, we derive a photometric
redshift of 4.4 < z < 5.2; at this redshift our simultaneous multicolour
observations begin at ~30 s after the GRB trigger in the host frame and during
the initial rising phase of the afterglow. We associate the light curve peak at
580 s in the observer frame with the formation of the forward shock, giving an
estimate of the initial Lorentz factor Gamma_0 ~ 200. The red spectral energy
distribution (even in regions not affected by the Lyman-alpha break) provides
secure evidence of a large dust column. However, the inferred extinction curve
shows a prominent flat component between 2000-3000 Angstroms in the rest-frame,
inconsistent with any locally observed template but well-fit by models of dust
formed by supernovae. Time-dependent fits to the extinction profile reveal no
evidence of dust destruction and limit the decrease in the extinction column to
Delta A_3000 < 0.54 mag after t = 50 s in the rest frame. Our observations
provide evidence of a transition in dust properties at z~5, in agreement with
studies of high-z quasars, and suggest that SN-formed dust continues to
dominate the opacity of typical galaxies at this redshift.Comment: Resubmitted to MNRAS following referee report. Contains additional
figure and some extra analysis/discussio
Recommended from our members
A Photometric Redshift of z ~ 9.4 for GRB 090429B
Gamma-ray bursts (GRBs) serve as powerful probes of the early universe, with their luminous afterglows revealing the locations and physical properties of star-forming galaxies at the highest redshifts, and potentially locating first-generation (Population III) stars. Since GRB afterglows have intrinsically very simple spectra, they allow robust redshifts from low signal-to-noise spectroscopy, or photometry. Here we present a photometric redshift of z ~ 9.4 for the Swift detected GRB 090429B based on deep observations with Gemini-North, the Very Large Telescope, and the GRB Optical and Near-infrared Detector. Assuming a Small Magellanic Cloud dust law (which has been found in a majority of GRB sight lines), the 90% likelihood range for the redshift is 9.06 7. The non-detection of the host galaxy to deep limits (Y(AB) ~ 28, which would correspond roughly to 0.001L* at z = 1) in our late-time optical and infrared observations with the Hubble Space Telescope strongly supports the extreme-redshift origin of GRB 090429B, since we would expect to have detected any low-z galaxy, even if it were highly dusty. Finally, the energetics of GRB 090429B are comparable to those of other GRBs and suggest that its progenitor is not greatly different from those of lower redshift bursts
Early Laparoscopic Cholecystectomy with Continuous Pressurized Irrigation and Dissection in Acute Cholecystitis
Background. The aim of this study was to evaluate the preliminary results of a new dissection technique in acute cholecystitis. Material and Method. One hundred and forty-nine consecutive patients with acute cholecystitis were operated on with continuous pressurized irrigation and dissection technique. The diagnosis of acute cholecystitis was based on clinical, laboratory, and radiological evidences. Age, gender, time from symptom onset to hospital admission, operative risk according to the American Society of Anesthesiologists (ASA) score, white blood cell count, C-reactive protein test levels, positive findings of radiologic evaluation of the patients, operation time, perioperative complications, mortality, and conversion to open surgery were prospectively recorded. Results. Of the 149 patients, 87 (58,4%) were female and 62 (41,6%) were male. The mean age was 46.3±6.7 years. The median time from symptom onset to hospital admission 3.2 days (range, 1–6). There were no major complications such as bile leak, common bile duct injury or bleeding. Subhepatic liquid collection occurred in 3 of the patients which was managed by percutaneous drainage. Conversion to open surgery was required in four (2,69%) patients. There was no mortality in the study group. Conclusion. Laparoscopic cholecystectomy with continuous pressurized irrigation and dissection technique in acute cholecystitis seems to be an effective and reliable procedure with low complication and conversion rates
Gaia transients in galactic nuclei
The high spatial resolution and precise astrometry and photometry of the Gaia
mission should make it particularly apt at discovering and resolving transients
occurring in, or near, the centres of galaxies. Indeed, some nuclear transients
are reported by the Gaia Science Alerts (GSA) team, but not a single confirmed
Tidal Disruption Event has been published. In order to explore the sensitivity
of GSA, we performed an independent and systematic search for nuclear
transients using Gaia observations. Our transient search is driven from an
input galaxy catalogue (derived from the Sloan Digital Sky Survey Release 12).
We present a candidate detection metric which is independent from the existing
GSA methodology, to see if Gaia Alerts are biased against the discovery of
nuclear transients, and in particular which steps may have an impact. Our
technique does require significant manual vetting of candidates, making
implementation in the GSA system impractical for daily operations, although it
could be run weekly, which for month-to-year long transients would make a
scientifically valuable addition. Our search yielded ~480 nuclear transients, 5
of which were alerted and published by GSA. The list of (in some cases ongoing)
transients includes candidates for events related to enhanced accretion onto a
super-massive black hole and TDEs. An implementation of the detection
methodology and criteria used in this paper as an extension of GSA could open
up the possibility for Gaia to fulfil the role as a main tool to find transient
nuclear activity as predicted in the literature.Comment: MNRAS accepted; full Table 2 attache
- …