1,997 research outputs found
No Time for Dead Time: Use the Fourier Amplitude Differences to Normalize Dead-time-affected Periodograms
Dead time affects many of the instruments used in X-ray astronomy, by
producing a strong distortion in power density spectra. This can make it
difficult to model the aperiodic variability of the source or look for
quasi-periodic oscillations. Whereas in some instruments a simple a priori
correction for dead-time-affected power spectra is possible, this is not the
case for others such as NuSTAR, where the dead time is non-constant and long
(~2.5 ms). Bachetti et al. 2015 suggested the cospectrum obtained from light
curves of independent detectors within the same instrument as a possible way
out, but this solution has always only been a partial one: the measured rms was
still affected by dead time, because the width of the power distribution of the
cospectrum was modulated by dead time in a frequency-dependent way.
In this Letter we suggest a new, powerful method to normalize cospectra and,
with some caveats, even power density spectra. Our approach uses the difference
of the Fourier amplitudes from two independent detectors to characterize and
filter out the effect of dead time. This method is crucially important for the
accurate modelling of periodograms derived from instruments affected by dead
time on board current missions like NuSTAR and ASTROSAT, but also future
missions such as IXPEComment: 8 pages, 5 figures, Published on ApJL on 2018 January 3
Searching data for periodic signals
We present two statistical tests for periodicities in the time series. We
apply the two tests to the data taken from Glasgow prototype interferometer in
March 1996. We find that the data contain several very narrow spectral
features. We investigate whether these features can be confused with
gravitational wave signals from pulsars.Comment: 7 pages, 2 figure
Cloud Atlas: Discovery of Rotational Spectral Modulations in a Low-mass, L-type Brown Dwarf Companion to a Star
Observations of rotational modulations of brown dwarfs and giant exoplanets
allow the characterization of condensate cloud properties. As of now rotational
spectral modulations have only been seen in three L-type brown dwarfs. We
report here the discovery of rotational spectral modulations in LP261-75B, an
L6-type intermediate surface gravity companion to an M4.5 star. As a part of
the Cloud Atlas Treasury program we acquired time-resolved Wide Field Camera 3
grism spectroscopy (1.1--1.69~m) of LP261-75B. We find gray spectral
variations with the relative amplitude displaying only a weak wavelength
dependence and no evidence for lower-amplitude modulations in the 1.4~m
water band than in the adjacent continuum. The likely rotational modulation
period is 4.780.95 h, although the rotational phase is not well sampled.
The minimum relative amplitude in the white light curve measured over the whole
wavelength range is 2.410.14\%. We report an unusual light curve with
seemingly three peaks approximately evenly distributed in rotational phase. The
spectral modulations suggests that the upper atmosphere cloud properties in
{LP261-75B} are similar to two other mid-L dwarfs of typical infrared colors,
but differ from that of the extremely red L-dwarf WISE0047
Periodogram and likelihood periodicity search in the SNO solar neutrino data
In this work a detailed spectral analysis for periodicity search of the time
series of the 8B solar neutrino flux released by the SNO Collaboration is
presented. The data have been publicly released with truncation of the event
times to the unit of day (1 day binning); they are thus suited to undergo the
traditional Lomb-Scargle analysis for periodicity investigation, as well as an
extension of such a method based on a likelihood approach. The results of the
analysis presented here confirm the absence of modulation signatures in the SNO
data. For completeness, a more refined "1 day binned" likelihood is also
illustrated, which approximates the unbinned likelihood methodology, based upon
the availability of the full time information, adopted by the SNO
collaboration. Finally, this work is completed with two different joint
analyses of the SNO and Super-Kamiokande data, respectively, over the common
and the entire data taking periods. While both analyses reinforce the case of
the constancy of the neutrino flux, the latter in addition provides evidence of
the detection at the 99.7% confidence level of the annual modulation spectral
line due to the Earth's orbit eccentricity around the SunComment: 27 pages, 29 figures. Joint periodicity analysis of the SNO and
Super-Kamiokande data added. Accepted for publication on Phys. Rev.
Four Years of Extreme Ultraviolet Observations of Markarian 421: II. Temporal Analysis
The Extreme Ultraviolet Explorer (EUVE) satellite accumulated ~one million
seconds of public data between 1994 and 1997 for the BL Lacertae object
Markarian 421. This is the second of two papers in which we present the results
of spectral and temporal analysis of this EUVE data set. We analyze in the
present paper the imaging data by means of power spectrum and structure
function techniques, while the spectral analysis is presented in a companion
paper. We find for MRK 421 a power spectrum with slope -2.14 +- 0.28 with a
break at ~3 days. This is the first time that a break in the power spectrum of
a BL Lacertae object has been found. We also find evidence of non-stationarity
for MRK 421 EUV emissionComment: Accepted for publication in the Astrophysical Journal. 16 pages, 14
Postscript figures, 3 Table
Cloud Atlas: High-Contrast Time-Resolved Observations of Planetary-Mass Companions
Directly-imaged planetary-mass companions offer unique opportunities in
atmospheric studies of exoplanets. They share characteristics of both brown
dwarfs and transiting exoplanets, therefore, are critical for connecting
atmospheric characterizations for these objects. Rotational phase mapping is a
powerful technique to constrain the condensate cloud properties in ultra-cool
atmospheres. Applying this technique to directly-imaged planetary-mass
companions will be extremely valuable for constraining cloud models in low mass
and surface gravity atmospheres and for determining the rotation rate and
angular momentum of substellar companions. Here, we present Hubble Space
Telescope Wide Field Camera 3 near-infrared time-resolved photometry for three
planetary-mass companions, AB Pic B, 2M0122B, and 2M1207b. Using two-roll
differential imaging and hybrid point spread function modeling, we achieve
sub-percent photometric precision for all three observations. We find tentative
modulations () for AB Pic B and 2M0122B but cannot reach
conclusive results on 2M1207b due to strong systematics. The relatively low
significance of the modulation measurements cannot rule out the hypothesis that
these planetary-mass companions have the same vertical cloud structures as
brown dwarfs. Our rotation rate measurements, combined with archival period
measurements of planetary-mass companions and brown dwarfs do not support a
universal mass-rotation relation. The high precision of our observations and
the high occurrence rates of variable low-surface gravity objects encourage
high-contrast time-resolved observations with the James Webb Space Telescope.Comment: Accepted for publication in AAS Journa
Optical Monitoring of the Seyfert Galaxy NGC 4151 and Possible Periodicities in the Historical Light Curve
We report B, V, and R band CCD photometry of the Seyfert galaxy NGC 4151
obtained with the 1.0-m telescope at Weihai Observatory of Shandong University
and the 1.56-m telescope at Shanghai Astronomical Observatory from 2005
December to 2013 February. Combining all available data from literature, we
have constructed a historical light curve from 1910 to 2013 to study the
periodicity of the source using three different methods (the Jurkevich method,
the Lomb-Scargle periodogram method and the Discrete Correlation Function
method). We find possible periods of P_1=4\pm0.1, P_2=7.5\pm0.3 and
P_3=15.9\pm0.3 yr.Comment: 8 pages, 5 figures, Accepted by Research in Astronomy and
Astrophysic
- …