91 research outputs found
Generation of defects and disorder from deeply quenching a liquid to form a solid
We show how deeply quenching a liquid to temperatures where it is linearly
unstable and the crystal is the equilibrium phase often produces crystalline
structures with defects and disorder. As the solid phase advances into the
liquid phase, the modulations in the density distribution created behind the
advancing solidification front do not necessarily have a wavelength that is the
same as the equilibrium crystal lattice spacing. This is because in a deep
enough quench the front propagation is governed by linear processes, but the
crystal lattice spacing is determined by nonlinear terms. The wavelength
mismatch can result in significant disorder behind the front that may or may
not persist in the latter stage dynamics. We support these observations by
presenting results from dynamical density functional theory calculations for
simple one- and two-component two-dimensional systems of soft core particles.Comment: 25 pages, 11 figure
The Northern Cross Fast Radio Burst project - II. Monitoring of repeating FRB 20180916B, 20181030A, 20200120E and 20201124A
In this work we report the results of a nineteen-month Fast Radio Burst
observational campaign carried out with the North-South arm of the Medicina
Northern Cross radio telescope at 408~MHz in which we monitored four repeating
sources: FRB20180916B, FRB20181030A, FRB20200120E and FRB20201124A. We present
the current state of the instrument and the detection and characterisation of
three bursts from FRB20180916B. Given our observing time, our detections are
consistent with the event number we expect from the known burst rate ( above our 10, 38~Jy~ms detection threshold) in the 5.2 day active
window of the source, further confirming the source periodicity. We detect no
bursts from the other sources. We turn this result into a 95\% confidence level
lower limit on the slope of the differential fluence distribution to
be and for FRB20181030A and FRB20200120E
respectively. Given the known rate for FRB20201124A, we expect
bursts from our campaign, consistent with our non-detection.Comment: MNRAS Accepted, 10 pages, 6 figure
Milliarcsecond Localization of the Repeating FRB 20201124A
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 sigma uncertainty of 2.7 mas. We use the relatively large burst sample to investigate astrometric accuracy and find that for greater than or similar to 20 baselines (greater than or similar to 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 similar to 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 sigma 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\u27s position to nearby knots of star formation
Simultaneous and panchromatic observations of the fast radio burst FRB 20180916B
Aims. Fast radio bursts are bright radio transients whose origins are not yet understood. The search for a multi-wavelength counterpart of those events can set a tight constraint on the emission mechanism and the progenitor source.Methods. We conducted a multi-wavelength observational campaign on FRB 20180916B between October 2020 and August 2021 over eight activity cycles of the source. Observations were carried out in the radio band by the SRT both at 336 and 1547 MHz and the uGMRT at 400 MHz. Simultaneous observations were conducted by the optical telescopes Asiago (Galileo and Copernico), CMO SAI MSU, CAHA 2.2 m, RTT-150 and TNG, and X/?-ray detectors on board the AGILE, Insight-HXMT, INTEGRAL, and Swift satellites.Results. We present the detection of 14 new radio bursts detected with the SRT at 336 MHz and seven new bursts with the uGMRT from this source. We provide the deepest prompt upper limits in the optical band for FRB 20180916B to date. In fact, the TNG/SiFAP2 observation simultaneous to a burst detection by uGMRT gives an upper limit E-optical/E-radio < 1.3 x 10(2). Another burst detected by the SRT at 336 MHz was also co-observed by Insight-HXMT. The non-detection in the X-rays yields an upper limit (1 - 30 keV band) of EX - ray/E-radio in the range of (0.9 - 1.3) x 10(7), depending on the model that is considered for the X-ray emission
Simultaneous and panchromatic observations of the Fast Radio Burst FRB 20180916B
Aims. Fast Radio Bursts are bright radio transients whose origin has not yet
explained. The search for a multi-wavelength counterpart of those events can
put a tight constrain on the emission mechanism and the progenitor source.
Methods. We conducted a multi-wavelength observational campaign on FRB
20180916B between October 2020 and August 2021 during eight activity cycles of
the source. Observations were led in the radio band by the SRT both at 336 MHz
and 1547 MHz and the uGMRT at 400 MHz. Simultaneous observations have been
conducted by the optical telescopes Asiago (Galileo and Copernico), CMO SAI
MSU, CAHA 2.2m, RTT-150 and TNG, and X/Gamma-ray detectors on board the AGILE,
Insight-HXMT, INTEGRAL and Swift satellites. Results. We present the detection
of 14 new bursts detected with the SRT at 336 MHz and seven new bursts with the
uGMRT from this source. We provide the deepest prompt upper limits in the
optical band fro FRB 20180916B to date. In fact, the TNG/SiFAP2 observation
simultaneous to a burst detection by uGMRT gives an upper limit E_optical /
E_radio < 1.3 x 10^2. Another burst detected by the SRT at 336 MHz was also
co-observed by Insight-HMXT. The non-detection in the X-rays yields an upper
limit (1-30 keV band) of E_X-ray / E_radio in the range of (0.9-1.3) x 10^7,
depending on which model is considered for the X-ray emission.Comment: A&A accepte
Accurate multiple time step in biased molecular simulations
Many recently introduced enhanced sampling techniques are based on biasing coarse descriptors (collective variables) of a molecular system on the fly. Sometimes the calculation of such collective variables is expensive and becomes a bottleneck in molecular dynamics simulations. An algorithm to treat smooth biasing forces within a multiple time step framework is here discussed. The implementation is simple and allows a speed up when expensive collective variables are employed. The gain can be substantial when using massively parallel or GPU-based molecular dynamics software. Moreover, a theoretical framework to assess the sampling accuracy is introduced, which can be used to assess the choice of the integration time step in both single and multiple time step biased simulations
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