The Influence of Selection Effects on the Properties of BL Lacs

Author(s):  
M. J. M. Marchã ◽  
I. W. A. Browne
Keyword(s):  
1994 ◽  
Vol 159 ◽  
pp. 515-515
Author(s):  
M.J.M. Marchã ◽  
I.W.A. Browne

The fact that the recognition of BL Lacs always requires optical confirmation, regardless of whether the objects were first selected in the radio, or X-ray frequencies means that deep surveys will miss some objects simply because the optical emission from the host galaxy outshines that of the BL Lac. In particular, the deeper the survey, the more difficult it will become to recognize low luminosity BL Lacs in the nuclei of luminous galaxies. This recognition effect will modify the intrinsic distribution of objects, and influence their statistical properties in general.


1985 ◽  
Vol 24 (03) ◽  
pp. 120-130 ◽  
Author(s):  
E. Brunner ◽  
N. Neumann

SummaryThe mathematical basis of Zelen’s suggestion [4] of pre randomizing patients in a clinical trial and then asking them for their consent is investigated. The first problem is to estimate the therapy and selection effects. In the simple prerandomized design (PRD) this is possible without any problems. Similar observations have been made by Anbar [1] and McHugh [3]. However, for the double PRD additional assumptions are needed in order to render therapy and selection effects estimable. The second problem is to determine the distribution of the statistics. It has to be taken into consideration that the sample sizes are random variables in the PRDs. This is why the distribution of the statistics can only be determined asymptotically, even under the assumption of normal distribution. The behaviour of the statistics for small samples is investigated by means of simulations, where the statistics considered in the present paper are compared with the statistics suggested by Ihm [2]. It turns out that the statistics suggested in [2] may lead to anticonservative decisions, whereas the “canonical statistics” suggested by Zelen [4] and considered in the present paper keep the level quite well or may lead to slightly conservative decisions, if there are considerable selection effects.


Author(s):  
Wei‐Ping Zhang ◽  
Sai‐Nan Gao ◽  
Zhao‐Xin Li ◽  
Hua‐Sen Xu ◽  
Hao Yang ◽  
...  

2021 ◽  
pp. 019791832199478
Author(s):  
Wanli Nie ◽  
Pau Baizan

This article investigates the impact of international migration to the United States on the level and timing of Chinese migrants’ fertility. We compare Chinese women who did not leave the country (non-migrants) and were subject to restrictive family policies from 1974 to 2015 to those who moved to the United States (migrants) and were, thus, “emancipated” from these policies. We theoretically develop and empirically test the emancipation hypothesis that migrants should have a higher fertility than non-migrants, as well as an earlier timing of childbearing. This emancipation effect is hypothesized to decline across birth cohorts. We use data from the 2000 US census, the 2005 American Community Survey, the 2000 Chinese census, and the 2005 Chinese 1 percent Population Survey and discrete-time event history models to analyze first, second, and third births, and migration as joint processes, to account for selection effects. The results show that Chinese migrants to the United States had substantially higher childbearing probabilities after migration, compared with non-migrants in China, especially for second and third births. Moreover, our analyses indicate that the migration process is selective of migrants with lower fertility. Overall, the results show how international migration from China to the United States can lead to an increase in migrant women’s fertility, accounting for disruption, adaptation, and selection effects. The rapidly increased fertility after migration from China to the United States might have implications on other migration contexts where fertility in the origin country is dropping rapidly while that in the destination country is relatively stable.


2004 ◽  
Vol 217 ◽  
pp. 252-257
Author(s):  
M. T. Murphy ◽  
S. J. Curran ◽  
J. K. Webb

The chemical abundances in damped Lyman-α systems (DLAs) show more than 2 orders of magnitude variation at a given epoch, possibly because DLAs arise in a wide variety of host galaxies. This could significantly bias estimates of chemical evolution. We explore the possibility that DLAs in which H2 absorption is detected may trace cosmological chemical evolution more reliably since they may comprise a narrower set of physical conditions. The 9 known H2 absorption systems support this hypothesis: metallicity exhibits a faster, more well-defined evolution with redshift than in the general DLA population. The dust-depletion factor and, particularly, H2 molecular fraction also show rapid increases with decreasing redshift. We comment on possible observational selection effects which may bias this evolution. Larger samples of H2-bearing DLAs are clearly required and may constrain evolution of the UV background and DLA galaxy host type with redshift.


1998 ◽  
Vol 11 (1) ◽  
pp. 570-570
Author(s):  
Johan Holmberg ◽  
Lennart Lindegren ◽  
Chris Flynn

We use the Hipparcos survey to derive an improved model of the local galactic structure. The availability of parallaxes for all the stars permits direct determination of stellar distributions, eliminating the basic indeterminacy of classical methods based on star counts. Hipparcos gives for the first time a truly three-dimensional view of the solar vicinity, and a complete, homogeneous and highly accurate set of magnitudes and colours. This means that new techniques can be applied in the treatment of the data which place strong constraints on a model that tries to describe the local Galactic structure. Here we investigate how well a static model of low complexitycan describe the Hipparcos observations. The interpretation of the Hipparcos data is complicated by various observational errors and selection effects that are hard to treat correctly. We do not try to correct the data, but instead use a model and subject this model to the same observational errors and selection effects. A model catalogue is created that can be compared with the observed catalogue directly in the observational domain, thereby eliminating the effects from various biases. Many features in the HR diagram are for the first time seen in field stars thanks to Hipparcos, such as the slanted red giant clump, previously seen in rich old open clusters such as Berkeley 18. This and other features ofthe observed HR diagram are well reproduced by the model thanks to the rather detailed modelling of the joint Mv/B — V distribution. Actually, separate distributions were derived for the three different components, disk, thick disk and halo, using the kinematic characteristics of the components to discriminate between them.


2021 ◽  
Vol 502 (4) ◽  
pp. 4669-4679
Author(s):  
Sofia Z Sheikh ◽  
Mariah G MacDonald

ABSTRACT Approximately 8 per cent of the ∼2800 known pulsars exhibit ‘nulling,’ a temporary broad-band cessation of normal pulsar emission. Nulling behaviour can be coarsely quantified by the nulling fraction, which describes the percentage of time a given pulsar will be found in a null state. In this paper, we perform the most thorough statistical analysis thus far of the properties of 141 known nulling pulsars. We find weak, non-linear correlations between nulling fraction and pulse width, as well as nulling fraction and spin period which could be attributed to selection effects. We also further investigate a recently hypothesized gap at 40 per cent nulling fraction. While a local minimum does exist in the distribution, we cannot confirm a consistent and unique break in the distribution when we investigate with univariate and multivariate clustering methods, nor can we prove the existence of two statistically distinct populations about this minimum. Using the same methods, we find that nulling pulsars are a statistically different population from normal, radio, non-nulling pulsars, which has never been quantitatively verified. In addition, we summarize the findings of the prior nulling pulsar statistics literature, which are notoriously contradictory. This study, in context, furthers the idea that nulling fraction alone does not contain enough information to describe the behaviour of a nulling pulsar and that other parameters such as null lengths and null randomness, in addition to a better understanding of selection effects, are required to fully understand this phenomenon.


1996 ◽  
Vol 175 ◽  
pp. 47-48
Author(s):  
A. Sillanpää ◽  
L. Takalo ◽  
K. Nilsson ◽  
T. Pursimo ◽  
P. Teerikorpi ◽  
...  

A widely accepted model for BL Lac objects is that they are radio galaxies with a relativistic jet pointing almost directly towards us. But we need a clear trigger mechanism for these jets. One possibility is the close interaction between the BL Lac host and the closeby galaxies (e.g. Heckman et al. 1986). This interaction has been seen many times in the case of quasars (Hutchings et al. 1989) but not so much is known about the close surroundings of the BL Lac objects although there has been some pioneer work like Stickel et al. (1993). The problem has usually been that the images are not deep enough and that the seeing has not been so good. To clarify the situation we have started an observing program to get very deep images in the sub-arcsecond seeing conditions from the whole 1 Jy sample (Stickel et al. 1991) of BL Lac objects. The aims of this study are: 1. to search for very close companions to the BL Lacs, 2. to study the large scale galaxy clustering around the BL Lacs and 3. to study the BL Lac hosts themselves.


1996 ◽  
Vol 160 ◽  
pp. 435-436
Author(s):  
H.-J. Wiebicke ◽  
U. Geppert

AbstractWe present a scenario of magnetic field (MF) evolution of newly-born neutron stars (NSs). Numerical calculations show that in the hot phase of young NSs the MF can be amplified by thermoelectric effects, starting from a moderately strong seed-field. Therefore, there is no need to assume a 1012G dipole field immediately after the gravitational collapse of the supernova (SN) event. The widely accepted scenario for such a field to be produced by flux conservation during the collapse is critically discussed. Instead, it can be generated by amplification and selection effects in the first 104yrs, and by the subsequent fast ohmic decay of higher multipole components, when the NS cools down.


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