116 research outputs found
On the Statistical Properties of Cospectra
In recent years, the cross-spectrum has received considerable attention as a means of characterizing the variability of astronomical sources as a function of wavelength. The cospectrum has only recently been understood as a means of mitigating instrumental effects dependent on temporal frequency in astronomical detectors, as well as a method of characterizing the coherent variability in two wavelength ranges on different timescales. In this paper, we lay out the statistical foundations of the cospectrum, starting with the simplest case of detecting a periodic signal in the presence of white noise, under the assumption that the same source is observed simultaneously in independent detectors in the same energy range. This case is especially relevant for detecting faint X-ray pulsars in detectors heavily affected by instrumental effects, including NuSTAR, Astrosat, and IXPE, which allow for even sampling and where the cospectrum can act as an effective way to mitigate dead time. We show that the statistical distributions of both single and averaged cospectra differ considerably from those for standard periodograms. While a single cospectrum follows a Laplace distribution exactly, averaged cospectra are approximated by a Gaussian distribution only for more than ~30 averaged segments, dependent on the number of trials. We provide an instructive example of a quasi-periodic oscillation in NuSTAR and show that applying standard periodogram statistics leads to underestimated tail probabilities for period detection. We also demonstrate the application of these distributions to a NuSTAR observation of the X-ray pulsar Hercules X-1
Quasi-Periodic Oscillations in Short Recurring Bursts of the magnetars SGR 1806-20 and SGR 1900+14 Observed With RXTE
Quasi-periodic oscillations (QPOs) observed in the giant flares of magnetars
are of particular interest due to their potential to open up a window into the
neutron star interior via neutron star asteroseismology. However, only three
giant flares have been observed. We therefore make use of the much larger data
set of shorter, less energetic recurrent bursts. Here, we report on a search
for QPOs in a large data set of bursts from the two most burst-active
magnetars, SGR 1806-20 and SGR 1900+14, observed with the Rossi X-ray Timing
Explorer (RXTE). We find a single detection in an averaged periodogram
comprising 30 bursts from SGR 1806-20, with a frequency of 57 Hz and a width of
5 Hz, remarkably similar to a giant flare QPO observed from SGR 1900+14. This
QPO fits naturally within the framework of global magneto-elastic torsional
oscillations employed to explain the giant flare QPOs. Additionally, we uncover
a limit on the applicability of Fourier analysis for light curves with low
background count rates and strong variability on short timescales. In this
regime, standard Fourier methodology and more sophisticated Fourier analyses
fail in equal parts by yielding an unacceptably large number of false positive
detections. This problem is not straightforward to solve in the Fourier domain.
Instead, we show how simulations of light curves can offer a viable solution
for QPO searches in these light curves.Comment: accepted for publication in ApJ; 12 pages, 7 figures; code +
instructions at https://github.com/dhuppenkothen/MagnetarQPOSearchPaper ;
associated data products at
http://figshare.com/articles/SGR_1900_14_RXTE_Data/1184101 (SGR 1900+14) and
http://figshare.com/articles/SGR_1806_20_Bursts_RXTE_data_set/1184427 (SGR
1806-20
Magnetar giant flare high-energy emission
High energy ( keV) emission has been detected persisting for several
tens of seconds after the initial spike of magnetar giant flares. It has been
conjectured that this emission might arise via inverse Compton scattering in a
highly extended corona generated by super-Eddington outflows high up in the
magnetosphere. In this paper we undertake a detailed examination of this model.
We investigate the properties of the required scatterers, and whether the
mechanism is consistent with the degree of pulsed emission observed in the tail
of the giant flare. We conclude that the mechanism is consistent with current
data, although the origin of the scattering population remains an open
question. We propose an alternative picture in which the emission is closer to
that star and is dominated by synchrotron radiation. The observations
of the December 2004 flare modestly favor this latter picture. We assess the
prospects for the Fermi Gamma-Ray Space Telescope to detect and characterize a
similar high energy component in a future giant flare. Such a detection should
help to resolve some of the outstanding issues.Comment: 20 pages, 14 figure
Rotational phase dependence of magnetar bursts
The trigger for the short bursts observed in -rays from many magnetar
sources remains unknown. One particular open question in this context is the
localization of burst emission to a singular active region or a larger area
across the neutron star. While several observational studies have attempted to
investigate this question by looking at the phase dependence of burst
properties, results have been mixed. At the same time, it is not obvious a
priori that bursts from a localized active region would actually give rise to a
detectable phase-dependence, taking into account issues such as geometry,
relativistic effects, and intrinsic burst properties such brightness and
duration. In this paper, we build a simple theoretical model to investigate the
circumstances under which the latter effects could affect detectability of a
dependence of burst emission on rotational phase. We find that even for
strongly phase-dependent emission, inferred burst properties may not show a
rotational phase dependence depending on the geometry of the system and the
observer. Furthermore, the observed properties of bursts with durations short
as 10-20% of the spin period can vary strongly depending on the rotational
phase at which the burst was emitted. We also show that detectability of a
rotational phase dependence depends strongly on the minimum number of bursts
observed, and find that existing burst samples may simply be too small to rule
out a phase dependence.Comment: 15 pages, 17 figure
Constraining the limiting brightness temperature and Doppler factors for the largest sample of radio bright blazars
Relativistic effects dominate the emission of blazar jets complicating our
understanding of their intrinsic properties. Although many methods have been
proposed to account for them, the variability Doppler factor method has been
shown to describe the blazar populations best. We use a Bayesian hierarchical
code called {\it Magnetron} to model the light curves of 1029 sources observed
by the Owens Valley Radio Observatory's 40-m telescope as a series of flares
with an exponential rise and decay, and estimate their variability brightness
temperature. Our analysis allows us to place the most stringent constraints on
the equipartition brightness temperature i.e., the maximum achieved intrinsic
brightness temperature in beamed sources which we found to be . Using our findings we estimated the
variability Doppler factor for the largest sample of blazars increasing the
number of available estimates in the literature by almost an order of
magnitude. Our results clearly show that -ray loud sources have faster
and higher amplitude flares than -ray quiet sources. As a consequence
they show higher variability brightness temperatures and thus are more
relativistically beamed, with all of the above suggesting a strong connection
between the radio flaring properties of the jet and -ray emission.Comment: 14 pages, 8 figures, accepted for publication in AP
Accurate X-ray Timing in the Presence of Systematic Biases With Simulation-Based Inference
Because many of our X-ray telescopes are optimized towards observing faint
sources, observations of bright sources like X-ray binaries in outburst are
often affected by instrumental biases. These effects include dead time and
photon pile-up, which can dramatically change the statistical inference of
physical parameters from these observations. While dead time is difficult to
take into account in a statistically consistent manner, simulating dead
time-affected data is often straightforward. This structure makes the issue of
inferring physical properties from dead time-affected observations fall into a
class of problems common across many scientific disciplines. There is a growing
number of methods to address them under the name of Simulation-Based Inference
(SBI), aided by new developments in density estimation and statistical machine
learning. In this paper, we introduce SBI as a principled way to infer
variability properties from dead time-affected light curves. We use Sequential
Neural Posterior Estimation to estimate the posterior probability for
variability properties. We show that this method can recover variability
parameters on simulated data even when dead time is variable, and present
results of an application of this approach to NuSTAR observations of the
galactic black hole X-ray binary GRS 1915+105
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