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News Article | November 17, 2016
Site: www.eurekalert.org

An international team of astrophysicists including Russian scientists from the Space Research Institute of the Russian Academy of Sciences (RAS), MIPT, and Pulkovo Observatory of RAS has detected an abrupt decrease of pulsar luminosity following giant outbursts. The phenomenon is associated with the so-called "propeller effect," which was predicted more than 40 years ago. However, this is the first study to reliably observe the transition of the two X-ray pulsars 4U 0115+63 and V 0332+53 to the "propeller regime." The results of the observations, the conclusions reached by the researchers, and the relevant calculations were published in Astronomy & Astrophysics. The two sources studied, 4U 0115+63 and V 0332+53, belong to a rather special class of transient X-ray pulsars. These stars alternately act as weak X-ray sources, undergo giant outbursts, and disappear from sight completely. The transitions of pulsars between different states provide valuable information about their magnetic field and the temperature of the surrounding matter. Such information is indispensable, as the immensely strong magnetic fields and extremely high temperatures make direct measurements impossible in a laboratory on Earth. The name of a pulsar is preceded by a letter designating the first observatory to discover it, which is followed by a numerical code containing the coordinates of the pulsar. The "V" refers to Vela 5B, a US military satellite that was launched to spy on the Soviets. As for the "4U" in the other name, it stands for the fourth Uhuru catalog, compiled by the first observatory in orbit dedicated specifically to X-ray astronomy. Following the discovery of the first pulsar, it was originally known as "LGM-1" (for "little green men"), because it was a source of regular radio pulses, leading scientists to believe that they might have received a signal from intelligent extraterrestrials. An X-ray pulsar is a rapidly spinning neutron star with a strong magnetic field. A neutron star can be part of a binary system. In a process that astrophysicists call accretion, the neutron star can channel gas from its normal star companion. The attracted gas spirals toward the neutron star forming an accretion disk, which is disrupted at the magnetosphere radius. During accretion the matter penetrates to a certain extent into the magnetosphere, "freezes into it," and flows along the lines of the magnetic field toward the magnetic poles of the neutron star. Falling toward the poles, the gas is heated to several hundred million degrees, which causes the emission of X-rays. If the magnetic axis of a neutron star is skewed relative to its rotational axis, the X-ray beams it emits rotate in a manner that resembles the way beacons work. For an "onshore" observer, the source appears to be sending signals at regular intervals ranging from fractions of a second to several minutes. A neutron star is one of the possible remnants left behind by a supernova. It can be formed at the end of stellar evolution, if the original star was massive enough to allow gravitation to compress the stellar matter enough to make electrons combine with protons yielding neutrons. The magnetic field of a neutron star can be more than ten orders of magnitude stronger that any magnetic field that could be achieved on Earth. A binary system where the normal star has filled its Roche lobe. In a binary system, an X-ray pulsar is observed when the neutron star is accreting matter from its normal star companion--often a giant or a supergiant characterized by a strong stellar wind (ejection of matter into space). Alternatively, it can be a smaller star like our own Sun that has filled its Roche lobe--the region beyond which it is unable to hold on to the matter attracted by the gravity of the neutron star companion. A NASA video showing the accretion of matter by a pulsar from its companion star. The 4U 0115+63 and V 0332+53 pulsars are irregular X-ray sources (transients), owing to the fact that their stellar companions belong to the rather unusual Be star class. The axial rotation of a Be star is so rapid that it occasionally starts "bulging" at the equator, whereby a gas disk is formed around it, filling the Roche lobe. The neutron star starts rapidly accreting the gas from its "donor" companion, causing a sharp increase in X-ray emission called an X-ray outburst. At some point, after the matter in the equatorial bulge starts to deplete, the accretion disk becomes exhausted, and the gas can no longer fall onto the neutron star due to the influence of the magnetic field and the centrifugal force. This gives rise to a phenomenon known as the "propeller effect": the pulsar enters a state where accretion does not occur, and the X-ray source is no longer observed. Astronomers use the term "luminosity" to refer to the total amount of energy emitted by a celestial body per unit time. The red line in the diagram represents the threshold luminosity for the 4U 0115+63 pulsar. Observations of the other source (V 0332+53) produced similar results. The blue lines mark the moments in time, when the distance between the pulsar and the companion was at a minimum. This proximity of the companion star might cause the neutron star to go into overdrive and resume emission (see diagram), provided that sufficient amounts of matter are still available for accretion. The Russian scientists used the X-ray telescope (XRT) based on NASA's Swift space observatory to measure the threshold luminosity that marks the transition of a pulsar to the propeller regime. This parameter depends on the magnetic field and the rotational period of the pulsar. The rotational periods of the sources in this study are known based on the intervals between the pulses that we can register: 3.6 s in the case of 4U 0115+63 and 4.3 s for V 0332+53. Knowing both the threshold luminosity and the rotational period, one can calculate the strength of the magnetic field. The research findings are in agreement with the values obtained using other methods. However, the luminosity was only reduced by a factor of 200, as compared to the expected 400 times reduction. The researchers hypothesized that there could be two possible explanations for this discrepancy. Firstly, the neutron star surface could become an additional source of X-rays, as it cools down following an outburst. Secondly, the propeller effect could leave some room for matter transfer between the two stars, as opposed to sealing the neutron star off completely. In other words, an unaccounted for mechanism could be involved enabling accretion to continue to a certain extent. The transition of a pulsar into the propeller mode is challenging to observe, as the low luminosity state cannot be detected easily. For 4U 0115+63 and V 0332+53, this was attempted following the previous outbursts of these sources. However, the instruments available at the time were not sensitive enough to see the pulsars in the "off-mode." This study is the first to demonstrate reliably that these two sources do indeed "black out." Moreover, the researchers showed that knowledge of the luminosity that marks the transition of pulsars into the propeller regime can be used to learn more about the structure and intensity of the magnetic fields around neutron stars. Prof. Dr. Alexander Lutovinov of the Russian Academy of Sciences, Head of Laboratory at the Space Research Institute (IKI RAS) and a professor at MIPT, comments, "Knowledge of the structure of the magnetic fields of neutron stars is of paramount importance for our understanding of their formation and evolution. In this research, we determined the dipole magnetic field component, which is linked to the propeller effect, for two neutron stars. We demonstrate that this independently calculated value can be compared to the available results of magnetic field measurements based on the detection of cyclotron lines in the spectra of sources. By doing this, it is possible to estimate the contribution of the other, higher-order components in the field structure."


News Article | November 18, 2016
Site: www.rdmag.com

An international team of astrophysicists including Russian scientists from the Space Research Institute of the Russian Academy of Sciences (RAS), MIPT, and Pulkovo Observatory of RAS has detected an abrupt decrease of pulsar luminosity following giant outbursts. The phenomenon is associated with the so-called "propeller effect," which was predicted more than 40 years ago. However, this is the first study to reliably observe the transition of the two X-ray pulsars 4U 0115+63 and V 0332+53 to the "propeller regime." The results of the observations, the conclusions reached by the researchers, and the relevant calculations were published in Astronomy & Astrophysics. The two sources studied, 4U 0115+63 and V 0332+53, belong to a rather special class of transient X-ray pulsars. These stars alternately act as weak X-ray sources, undergo giant outbursts, and disappear from sight completely. The transitions of pulsars between different states provide valuable information about their magnetic field and the temperature of the surrounding matter. Such information is indispensable, as the immensely strong magnetic fields and extremely high temperatures make direct measurements impossible in a laboratory on Earth. The name of a pulsar is preceded by a letter designating the first observatory to discover it, which is followed by a numerical code containing the coordinates of the pulsar. The "V" refers to Vela 5B, a US military satellite that was launched to spy on the Soviets. As for the "4U" in the other name, it stands for the fourth Uhuru catalog, compiled by the first observatory in orbit dedicated specifically to X-ray astronomy. Following the discovery of the first pulsar, it was originally known as "LGM-1" (for "little green men"), because it was a source of regular radio pulses, leading scientists to believe that they might have received a signal from intelligent extraterrestrials. An X-ray pulsar is a rapidly spinning neutron star with a strong magnetic field. A neutron star can be part of a binary system. In a process that astrophysicists call accretion, the neutron star can channel gas from its normal star companion. The attracted gas spirals toward the neutron star forming an accretion disk, which is disrupted at the magnetosphere radius. During accretion the matter penetrates to a certain extent into the magnetosphere, "freezes into it," and flows along the lines of the magnetic field toward the magnetic poles of the neutron star. Falling toward the poles, the gas is heated to several hundred million degrees, which causes the emission of X-rays. If the magnetic axis of a neutron star is skewed relative to its rotational axis, the X-ray beams it emits rotate in a manner that resembles the way beacons work. For an "onshore" observer, the source appears to be sending signals at regular intervals ranging from fractions of a second to several minutes. A neutron star is one of the possible remnants left behind by a supernova. It can be formed at the end of stellar evolution, if the original star was massive enough to allow gravitation to compress the stellar matter enough to make electrons combine with protons yielding neutrons. The magnetic field of a neutron star can be more than ten orders of magnitude stronger that any magnetic field that could be achieved on Earth. A binary system where the normal star has filled its Roche lobe. In a binary system, an X-ray pulsar is observed when the neutron star is accreting matter from its normal star companion--often a giant or a supergiant characterized by a strong stellar wind (ejection of matter into space). Alternatively, it can be a smaller star like our own Sun that has filled its Roche lobe--the region beyond which it is unable to hold on to the matter attracted by the gravity of the neutron star companion. A NASA video showing the accretion of matter by a pulsar from its companion star. The 4U 0115+63 and V 0332+53 pulsars are irregular X-ray sources (transients), owing to the fact that their stellar companions belong to the rather unusual Be star class. The axial rotation of a Be star is so rapid that it occasionally starts "bulging" at the equator, whereby a gas disk is formed around it, filling the Roche lobe. The neutron star starts rapidly accreting the gas from its "donor" companion, causing a sharp increase in X-ray emission called an X-ray outburst. At some point, after the matter in the equatorial bulge starts to deplete, the accretion disk becomes exhausted, and the gas can no longer fall onto the neutron star due to the influence of the magnetic field and the centrifugal force. This gives rise to a phenomenon known as the "propeller effect": the pulsar enters a state where accretion does not occur, and the X-ray source is no longer observed. Astronomers use the term "luminosity" to refer to the total amount of energy emitted by a celestial body per unit time. The red line in the diagram represents the threshold luminosity for the 4U 0115+63 pulsar. Observations of the other source (V 0332+53) produced similar results. The blue lines mark the moments in time, when the distance between the pulsar and the companion was at a minimum. This proximity of the companion star might cause the neutron star to go into overdrive and resume emission (see diagram), provided that sufficient amounts of matter are still available for accretion. The Russian scientists used the X-ray telescope (XRT) based on NASA's Swift space observatory to measure the threshold luminosity that marks the transition of a pulsar to the propeller regime. This parameter depends on the magnetic field and the rotational period of the pulsar. The rotational periods of the sources in this study are known based on the intervals between the pulses that we can register: 3.6 s in the case of 4U 0115+63 and 4.3 s for V 0332+53. Knowing both the threshold luminosity and the rotational period, one can calculate the strength of the magnetic field. The research findings are in agreement with the values obtained using other methods. However, the luminosity was only reduced by a factor of 200, as compared to the expected 400 times reduction. The researchers hypothesized that there could be two possible explanations for this discrepancy. Firstly, the neutron star surface could become an additional source of X-rays, as it cools down following an outburst. Secondly, the propeller effect could leave some room for matter transfer between the two stars, as opposed to sealing the neutron star off completely. In other words, an unaccounted for mechanism could be involved enabling accretion to continue to a certain extent. The transition of a pulsar into the propeller mode is challenging to observe, as the low luminosity state cannot be detected easily. For 4U 0115+63 and V 0332+53, this was attempted following the previous outbursts of these sources. However, the instruments available at the time were not sensitive enough to see the pulsars in the "off-mode." This study is the first to demonstrate reliably that these two sources do indeed "black out." Moreover, the researchers showed that knowledge of the luminosity that marks the transition of pulsars into the propeller regime can be used to learn more about the structure and intensity of the magnetic fields around neutron stars. Prof. Dr. Alexander Lutovinov of the Russian Academy of Sciences, Head of Laboratory at the Space Research Institute (IKI RAS) and a professor at MIPT, comments, "Knowledge of the structure of the magnetic fields of neutron stars is of paramount importance for our understanding of their formation and evolution. In this research, we determined the dipole magnetic field component, which is linked to the propeller effect, for two neutron stars. We demonstrate that this independently calculated value can be compared to the available results of magnetic field measurements based on the detection of cyclotron lines in the spectra of sources. By doing this, it is possible to estimate the contribution of the other, higher-order components in the field structure."


Korzinin E.Y.,Mendeleev Institute for Metrology | Ivanov V.G.,Pulkovo Observatory | Karshenboim S.G.,Max Planck Institute of Quantum Optics
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2013

Corrections to energy levels in light muonic atoms are investigated in order α2(Zα)4m. We pay attention to corrections which are specific for muonic atoms and include the electron vacuum polarization loop. In particular, we calculate relativistic and relativistic-recoil two-loop electron vacuum polarization contributions. The results are obtained for the levels with n=1, 2 and in particular for the Lamb shift (2p1/2-2s1/2) and fine-structure intervals (2p3/2-2p1/2) in muonic hydrogen, deuterium, and muonic helium ions. © 2013 American Physical Society.


Karshenboim S.G.,Max Planck Institute of Quantum Optics | Karshenboim S.G.,Pulkovo Observatory
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2014

To date the magnetic radius of the proton has been determined only by means of electron-proton scattering, which is not free of controversies. Any existing atomic determinations are irrelevant because they are strongly model dependent. We consider a so-called Zemach contribution to the hyperfine interval in ordinary and muonic hydrogen and derive a self-consistent model-independent value of the magnetic radius of the proton. More accurately, we constrain not a value of the magnetic radius by itself, but its certain combination with the electric-charge radius of the proton, namely, RE2+RM2. The result from the ordinary hydrogen is found to be RE2+RM2=1.35(12)fm2, while the derived muonic value is 1.49(18)fm2. That allows us to constrain the value of the magnetic radius of proton RM=0.78(8)fm at the 10% level. © 2014 American Physical Society.


Karshenboim S.G.,Max Planck Institute of Quantum Optics | Karshenboim S.G.,Pulkovo Observatory
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2014

Recently a high-precision measurement of the Lamb shift in muonic hydrogen has been performed. An accurate value of the proton charge radius can be extracted from this datum with a high accuracy. To do that a sufficient accuracy should be achieved also on the theoretical side, including an appropriate treatment of higher-order proton-structure effects. Here we consider a higher-order contribution of the finite size of the proton to the Lamb shift in muonic hydrogen. Only model-dependent results for this correction have been known up to date. Meanwhile, the involved models are not consistent either with the existing experimental data on the electron-proton scattering or with the value for the electric charge radius of the proton extracted from the Lamb shift in muonic hydrogen. We consider the higher-order contribution of the proton finite size in a model-independent way and eventually derive a self-consistent value of the electric radius of the proton. The reevaluated value of the proton charge radius is found to be RE=0.84022(56)fm. © 2014 American Physical Society.


Karshenboim S.G.,Max Planck Institute of Quantum Optics | Karshenboim S.G.,Pulkovo Observatory
Annalen der Physik | Year: 2013

A brief overview on determination of the values of fundamental constants by means of atomic physics is given. Recommended values of CODATA-2010 least square adjustment are discussed as well as the related input data. © 2013 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


Volobuev D.M.,Pulkovo Observatory
Climate Dynamics | Year: 2014

Antarctic "Vostok" station works most closely to the center of the ice cap among permanent year-around stations. Climate conditions are exclusively stable: low precipitation level, cloudiness and wind velocity. These conditions can be considered as an ideal model laboratory to study the surface temperature response on solar irradiance variability during 11-year cycle of solar activity. Here we solve an inverse heat conductivity problem: calculate the boundary heat flux density (HFD) from known evolution of temperature. Using meteorological temperature record during (1958-2011) we calculated the HFD variation about 0.2-0.3 W/m2 in phase with solar activity cycle. This HFD variation is derived from 0.5 to 1 °C temperature variation and shows relatively high climate sensitivity per 0.1 % of solar radiation change. This effect can be due to the polar amplification phenomenon, which predicts a similar response 0.3-0.8 °C/0.1 % (Gal-Chen and Schneider in Tellus 28:108-121, 1975). The solar forcing (TSI) is disturbed by volcanic forcing (VF), so that their linear combination TSI + 0.5VF empirically provides higher correlation with HFD (r = 0.63 ± 0.22) than TSI (r = 0.50 ± 0.24) and VF (r = 0.41 ± 0.25) separately. TSI shows higher wavelet coherence and phase agreement with HFD than VF. © 2013 Springer-Verlag Berlin Heidelberg.


Malkin Z.,Pulkovo Observatory
Journal of Geodesy | Year: 2011

Very Long Baseline Interferometry (VLBI) Intensive sessions are scheduled to provide operational Universal Time (UT1) determinations with low latency. UT1 estimates obtained from these observations heavily depend on the model of the celestial pole motion used during data processing. However, even the most accurate precession-nutation model, IAU 2000/2006, is not accurate enough to realize the full potential of VLBI observations. To achieve the highest possible accuracy in UT1 estimates, a celestial pole offset (CPO), which is the difference between the actual and modelled precession-nutation angles, should be applied. Three CPO models are currently available for users. In this paper, these models have been tested and the differences between UT1 estimates obtained with those models are investigated. It has been shown that neglecting CPO modelling during VLBI UT1 Intensive processing causes systematic errors in UT1 series of up to 20 μas. It has been also found that using different CPO models causes the differences in UT1 estimates reaching 10 μas. Obtained results are applicable to the satellite data processing as well. © 2011 Springer-Verlag.


Malkin Z.,Pulkovo Observatory
International Association of Geodesy Symposia | Year: 2013

Allan Variance (AVAR) was introduced more than 40 years ago as an estimator of the stability of frequency standards. Now it is also used for investigations of time series in astronomy and geodesy. However, there are several issues with this method that need special consideration. First, unlike frequency measurements, astronomical and geodetic time series usually consist of data points with unequal uncertainties. Thus one needs to apply data weighting during statistical analysis. Second, some sets of scalar time series naturally form multidimensional vector series. For example, Cartesian station coordinates form the 3D station position vector. The original AVAR definition does not allow one to process unevenly weighted and/or multidimensional data. To overcome these deficiencies, AVAR modifications were proposed in Malkin (2008. On the accuracy assessment of celestial reference frame realizations. J Geodesy 82: 325-329). In this paper, we give some examples of processing geodetic and astrometric time series using the classical and the modified AVAR approaches, and compare the results. © Springer-Verlag Berlin Heidelberg 2013.


Yudin R.V.,Pulkovo Observatory
Monthly Notices of the Royal Astronomical Society | Year: 2014

We present the analysis of early published by Yudin & Evans polarimetric data for TeV γ -ray binary HESS J0632+057 (HD 259440, MWC 148). It was found out that some fraction of the observed polarization (about 2 per cent) may have an intrinsic (stellar/circumstellar) origin. We supposed that a global net polarization of a few per cent can be produced by the circumstellar disc which may be oriented at the angle of either 165° or 75° in the plane of the sky. Investigation of long-term polarimetric changes (at different epochs) shows that the object exhibits significant variability (~0.7 per cent) in the blue spectral region (photometric band U). Short-term polarimetric variability (hours) may also occur but more precise measurements are needed to make definite conclusions. © 2014 The Authors.

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