2,159 research outputs found
GRB Flares: UV/Optical Flaring (Paper I)
We present a new algorithm for the detection of flares in gamma-ray burst
(GRB) light curves and use this algorithm to detect flares in the UV/optical.
The algorithm makes use of the Bayesian Information Criterion (BIC) to analyze
the residuals of the fitted light curve, removing all major features, and to
determine the statistically best fit to the data by iteratively adding
additional `breaks' to the light curve. These additional breaks represent the
individual components of the detected flares: T_start, T_stop, and T_peak. We
present the detection of 119 unique flaring periods detected by applying this
algorithm to light curves taken from the Second Swift Ultraviolet/Optical
Telescope (UVOT) GRB Afterglow Catalog. We analyzed 201 UVOT GRB light curves
and found episodes of flaring in 68 of the light curves. For those light curves
with flares, we find an average number of ~2 flares per GRB. Flaring is
generally restricted to the first 1000 seconds of the afterglow, but can be
observed and detected beyond 10^5 seconds. More than 80% of the flares detected
are short in duration with Delta t/t of < 0.5. Flares were observed with flux
ratios relative to the underlying light curve of between 0.04 to 55.42. Many of
the strongest flares were also seen at greater than 1000 seconds after the
burst.Comment: Submitted to ApJ. 20 pages (including 8 figures and 1 table
Frequency evaluation of the doubly forbidden transition in bosonic Yb
We report an uncertainty evaluation of an optical lattice clock based on the
transition in the bosonic isotope Yb by use
of magnetically induced spectroscopy. The absolute frequency of the
transition has been determined through comparisons
with optical and microwave standards at NIST. The weighted mean of the
evaluations is (Yb)=518 294 025 309 217.8(0.9) Hz. The uncertainty
due to systematic effects has been reduced to less than 0.8 Hz, which
represents in fractional frequency.Comment: 4 pages, 3 figure -Submitted to PRA Rapid Communication
High accuracy measure of atomic polarizability in an optical lattice clock
Despite being a canonical example of quantum mechanical perturbation theory,
as well as one of the earliest observed spectroscopic shifts, the Stark effect
contributes the largest source of uncertainty in a modern optical atomic clock
through blackbody radiation. By employing an ultracold, trapped atomic ensemble
and high stability optical clock, we characterize the quadratic Stark effect
with unprecedented precision. We report the ytterbium optical clock's
sensitivity to electric fields (such as blackbody radiation) as the
differential static polarizability of the ground and excited clock levels:
36.2612(7) kHz (kV/cm)^{-2}. The clock's fractional uncertainty due to room
temperature blackbody radiation is reduced an order of magnitude to 3 \times
10^{-17}.Comment: 5 pages, 3 figures, 2 table
Quenched Narrow-Line Laser Cooling of 40Ca to Near the Photon Recoil Limit
We present a cooling method that should be generally applicable to atoms with
narrow optical transitions. This technique uses velocity-selective pulses to
drive atoms towards a zero-velocity dark state and then quenches the excited
state to increase the cooling rate. We demonstrate this technique of quenched
narrow-line cooling by reducing the 1-D temperature of a sample of neutral 40Ca
atoms. We velocity select and cool with the 1S0(4s2) to 3P1(4s4p) 657 nm
intercombination line and quench with the 3P1(4s4p) to 1S0(4s5s)
intercombination line at 553 nm, which increases the cooling rate eight-fold.
Limited only by available quenching laser power, we have transferred 18 % of
the atoms from our initial 2 mK velocity distribution and achieved temperatures
as low as 4 microK, corresponding to a vrms of 2.8 cm/s or 2 recoils at 657 nm.
This cooling technique, which is closely related to Raman cooling, can be
extended to three dimensions.Comment: 5 pages, 4 figures; Submitted to PRA Rapid Communication
Measurement of excited-state transitions in cold calcium atoms by direct femtosecond frequency-comb spectroscopy
We apply direct frequency-comb spectroscopy, in combination with precision cw
spectroscopy, to measure the transition
frequency in cold calcium atoms. A 657 nm ultrastable cw laser was used to
excite atoms on the narrow ( Hz) clock transition, and the direct output of the frequency comb was
used to excite those atoms from the state to the state. The resonance of this second stage was detected by observing a
decrease in population of the ground state as a result of atoms being optically
pumped to the metastable states. The transition frequency is measured to be kHz; which is an improvement by almost four orders of magnitude over
the previously measured value. In addition, we demonstrate spectroscopy on
magnetically trapped atoms in the state.Comment: 4 pages 5 figure
Kilohertz-resolution spectroscopy of cold atoms with an optical frequency comb
We have performed sub-Doppler spectroscopy on the narrow intercombination
line of cold calcium atoms using the amplified output of a femtosecond laser
frequency comb. Injection locking of a 657-nm diode laser with a femtosecond
comb allows for two regimes of amplification, one in which many lines of the
comb are amplified, and one where a single line is predominantly amplified. The
output of the laser in both regimes was used to perform kilohertz-level
spectroscopy. This experiment demonstrates the potential for high-resolution
absolute-frequency spectroscopy over the entire spectrum of the frequency comb
output using a single high-finesse optical reference cavity.Comment: 4 pages, 4 Figure
Magnetic field-induced spectroscopy of forbidden optical transitions with application to lattice-based optical atomic clocks
We develop a method of spectroscopy that uses a weak static magnetic field to
enable direct optical excitation of forbidden electric-dipole transitions that
are otherwise prohibitively weak. The power of this scheme is demonstrated
using the important application of optical atomic clocks based on neutral atoms
confined to an optical lattice. The simple experimental implementation of this
method -- a single clock laser combined with a DC magnetic field-- relaxes
stringent requirements in current lattice-based clocks (e.g., magnetic field
shielding and light polarization), and could therefore expedite the realization
of the extraordinary performance level predicted for these clocks. We estimate
that a clock using alkaline earth-like atoms such as Yb could achieve a
fractional frequency uncertainty of well below 10^-17 for the metrologically
preferred even isotopes
Sub-femtosecond absolute timing precision with a 10 GHz hybrid photonic-microwave oscillator
We present an optical-electronic approach to generating microwave signals
with high spectral purity. By circumventing shot noise and operating near
fundamental thermal limits, we demonstrate 10 GHz signals with an absolute
timing jitter for a single hybrid oscillator of 420 attoseconds (1Hz - 5 GHz)
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