180 research outputs found
Blazar sequence - an artefact of Doppler boosting
The blazar sequence is a scenario in which the bolometric luminosity of the
blazar governs the appearance of its spectral energy distribution. The most
prominent result is the significant negative correlation between the
synchrotron peak frequencies and the synchrotron peak luminosities of the
blazar population. Observational studies of the blazar sequence have, in
general, neglected the effect of Doppler boosting. We study the dependence of
both the synchrotron peak frequency and luminosity with Doppler-corrected
quantities. We determine the spectral energy distributions of 135 radio-bright
AGN and find the best-fit parabolic function for the distribution to quantify
their synchrotron emission. The corresponding measurements of synchrotron peak
luminosities and frequencies are Doppler-corrected with a new set of Doppler
factors calculated from variability data. The relevant correlations for the
blazar sequence are determined for these intrinsic quantities. The Doppler
factor depends strongly on the synchrotron peak frequency, the lower energy
sources being more boosted. Applying the Doppler correction to the peak
frequencies and luminosities annuls the negative correlation between the two
quantities, which becomes positive. For BL Lacertae objects, the positive
correlation is particularly strong. The blazar sequence, when defined as the
anticorrelation between the peak frequency and luminosity of the synchrotron
component of the spectral energy distribution, disappears when the intrinsic,
Doppler-corrected values are used. It is an observational phenomenon created by
variable Doppler boosting across the synchrotron peak frequency range.Comment: 9 pages, 5 figures + 2 tables. The published version with minor
changes, the main conclusions are unchange
Locating the gamma-ray emission site in Fermi/LAT blazars from correlation analysis between 37 GHz radio and gamma-ray light curves
We address the highly debated issue of constraining the gamma-ray emission
region in blazars from cross-correlation analysis using discrete correlation
function between radio and gamma-ray light curves. The significance of the
correlations is evaluated using two different approaches: simulating light
curves and mixed source correlations. The cross-correlation analysis yielded 26
sources with significant correlations. In most of the sources, the gamma-ray
peaks lead the radio with time lags in the range +20 and +690 days, whereas in
sources 1633+382 and 3C 345 we find the radio emission to lead the gamma rays
by -15 and -40 days, respectively. Apart from the individual source study, we
stacked the correlations of all sources and also those based on sub-samples.
The time lag from the stacked correlation is +80 days for the whole sample and
the distance travelled by the emission region corresponds to 7 pc. We also
compared the start times of activity in radio and gamma rays of the correlated
flares using Bayesian block representation. This shows that most of the flares
at both wavebands start at almost the same time, implying a co-spatial origin
of the activity. The correlated sources show more flares and are brighter in
both bands than the uncorrelated ones.Comment: 15 pages, 8 figures and 4 tables. Published in MNRAS. Online-only
Figure 6 is available as ancillary file with this submissio
Backtesting Value-at-Risk Models
Value-at-Risk has become one of the most popular risk measurement techniques in finance. However, VaR models are useful only if they predict future risks accurately. In order to evaluate the quality of the VaR estimates, the models should always be backtested with appropriate methods. Backtesting is a statistical procedure where actual profits and losses are systematically compared to corresponding VaR estimates.
The main contribution of this thesis consists of empirical studies. The empirical part of the thesis is carried out in close cooperation with a Finnish institutional investor. The primary objective of the study is to examine the accuracy of a VaR model that is being used to calculate VaR figures in the companyâs investment management unit. As a secondary objective the empirical research tries to figure out which backtests are the most reliable, and which tests are suitable for forthcoming model validation processes in the company.
The performance of the VaR model is measured by applying several different tests of unconditional coverage and conditional coverage. Three different portfolios (equities, bonds and equity options) with daily VaR estimates for one year time period are used in the backtesting process.
The results of the backtests provide some indication of potential problems within the system. Severe underestimation of risk is discovered, especially for equities and equity options. However, the turbulent market environment causes problems in the evaluation of the backtesting outcomes since VaR models are known to be accurate only under normal market conditions
The radio structure of ultra-high-energy synchrotron peak BL Lacs
We present the results of EVN and MERLIN 5 GHz observations of nine
ultra-high-energy synchrotron peak BL Lacs (UHBLs) selected as all BL Lacs with
\textbf{log (} from Nieppola et al.. The radio
structure was investigated for these sources, in combination with the available
VLBA archive data. We found that the core-jet structure is detected in five
sources, while four sources only have a compact core on pc scale. The core of
all sources shows high brightness temperature (with mean and median values
\textbf{log (}, which implies that the beaming
effect likely present in all sources. When the multi-epoch VLBI data are
available, we found no significant variations either for core or total flux
density in two sources (2E 0414+0057 and EXO 0706.1+5913), and no evident
proper motion in 2E 0414+0057, while the superluminal motion is likely detected
in EXO 0706.1+5913. Our sources are found to be less compact than the typical
HBLs in Giroletti et al, by comparing the ratio of the VLBI total flux to the
core flux at arcsec scale. Combining all our results, we propose that the
beaming effect might be present in the jets of UHBLs, however, it is likely
weaker than that of typical HBLs. Moreover, we found that UHBLs could be less
Doppler beamed versions of HBLs with similar jet power, by comparing the
distribution of redshift, and radio luminosities. The results are in good
consistence with the expectations from our previous work.Comment: 14 pages, 15 figures and 5 tables. Accepted by MNRA
Synchrotron emission from blazar jets - energy distributions and radio variability
This thesis studies the synchrotron radiation of blazars from two perspectives: the spectral energy distributions, and radio emission and its variability. The first gives insight to the stable continuum emission originating in blazar jets, while the latter tells about the ephemeral events occuring in the jet flow, causing an outburst of radiation. The main goal is to establish the typical range of synchrotron emission properties for the fragmented blazar population, and use those data to test for any correlation between the wavelength of maximum synchrotron energy and source luminosity.
We determined the synchrotron peak frequencies and luminosities for two samples of blazars using a parabolic fit to archival data. The first was a very large sample of BL Lacertae objects (BLOs) and the second a complete sample of northern 1 Jy active galactic nuclei (AGN), for which we studied especially the Doppler-corrected properties. The range of synchrotron peak frequencies varied from the infrared to X-ray domain, and their distribution was smooth. Neither sample exhibited anti-correlation between the peak frequency and peak luminosity, contrary to the so-called blazar sequence scenario. In fact, when the Doppler-correction is properly applied, the two quantities have a positive correlation. This result was unexpected, but helps solve some inconsistencies in previous blazar research. It will also shed light on the relationship between the spectral energy distributions (SED) and the physics of the nucleus, as well as allows us to link the SED shape to other fundamental jet parameters, such as the jet speed and viewing angle.
To find the limits of the typical BLO radio behaviour, we observed a sample of almost 400 BL Lacertae objects in MetsĂ€hovi Radio Observatory for almost 4 years. Typically, BLOs are faint at 37 GHz, as only a third of the sample was detected at S â N > 4. However, there are also very bright and variable objects, which were studied in two separate works. Their radio flux curves were examined at several frequencies and their flaring behaviour determined, both individually and with respect to other subgroups of AGN. When the variable radio emission of blazars is considered, the range of typical behaviour is very large. The bright sources have intense flares of up to 50 Jy. The average flare duration is 2.5 years at 22 and 37 GHz
Simultaneous spectra and radio properties of BL Lac's
We present the results of nine years of the blazar observing programme at the
RATAN-600 radio telescope (2005-2014). The data were obtained at six frequency
bands (1.1, 2.3, 4.8, 7.7, 11.2, 21.7 GHz) for 290 blazars, mostly BL Lacs. In
addition, we used data at 37 GHz obtained quasi-simultaneously with the
Metsahovi radio observatory for some sources. The sample includes blazars of
three types: high-synchrotron peaked (HSP), low-synchrotron peaked (LSP), and
intermediate-synchrotron peaked (ISP). We present several epochs of flux
density measurements, simultaneous radio spectra, spectral indices and
properties of their variability. The analysis of the radio properties of
different classes of blazars showed that LSP and HSP BL Lac blazars are quite
different objects on average. LSPs have higher flux densities, flatter spectra
and their variability increases as higher frequencies are considered. On the
other hand, HSPs are very faint in radio domain, tend to have steep low
frequency spectra, and they are less variable than LSPs at all frequencies.
Another result is spectral flattening above 7.7 GHz detected in HSPs, while an
average LSP spectrum typically remains flat at both the low and high frequency
ranges we considered.Comment: 14 pages, 6 figures. Accepted for publication in Astronomische
Nachrichte
Continuance Behavior of Smart Wearable Devices : by Finnish university students
Smart wearable devices (SWDs), like smart watches and smart rings, are well accepted and used among the general public. Although the attributes that contribute to SWD adoption are comprehensively researched, there is still a gap in knowledge, what factors promote continuous SWD usage.
This research uses the unified model of IT continuance to study continuance behavior of SWDs by Finnish university students. The study was conducted via questionnaire, which was distributed to Finnish university students who are users of SWDs. In total, 100 usable responses were collected, and the results were analyzed using PLS-SEM method in SmartPLS4 to find the linkages between different continuous use contributors.
The study found that disconfirmation, satisfaction, continuance intention, and habit promote continuous SWD usage. Perceived usefulness and subjective norm were not found to have a significant contribution towards continuous use. Age, gender, student status, educational level, and gross income did not alter usersâ continuance behavior.
For future research, it is suggested to study how culture and geographical location affect the significance of certain contributors. A study done by using a more comprehensive continuance behavior model and greater sample size is also recommended for results with higher overall validity
Long-term variability of radio-bright BL Lacertae objects
Radio-bright BL Lacertae objects (BLOs) are typically variable and exhibit
prominent flaring. We use a sample of 24 BLOs to get a clear idea of their
flaring behavior and to find possible commonalities in their variability
patterns. Our goal was to compare the results given by computational time
scales and the observed variability parameters determined directly from the
flux curves. Also, we wanted to find out if the BLO flares adhere to the
generalized shock model. We use long-term monitoring data from 4.8, 8, 14.5,
22, 37, 90 and 230 GHz. The structure function, discrete correlation function
and Lomb-Scargle periodogram time scales, calculated in a previous study, are
analyzed in more detail. We determine flare durations, rise and decay times,
absolute and relative peak fluxes from the monitoring data. We find that BLOs
demonstrate a wide range of variability behavior. BLOs include sources with
fast and strong variability, such as OJ 287, PKS 1749+096 and BL Lac, but also
sources with more rolling fluctuations like PKS 0735+178. The most extreme
flares can last for up to 13 years or have peak fluxes of approximately 12 Jy
in the observer's frame. When the Doppler boosting effect is taken into
account, the peak flux of a flare does not depend on the duration of the flare.
A rough analysis of the time lags and peak flux evolution indicates that BLO
flares in the mm - cm wavelengths are high-peaking, i.e., are in the adiabatic
stage. Thus, the results concur with the generalized shock model.Comment: Published in the Astronomical Journa
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