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Mode Change of a Gamma-Ray Pulsar, PSR J2021+4026
Zhao, J.,Ng, C. W.,Lin, L. C. C.,Takata, J.,Cai, Y.,Hu, C.-P.,Yen, D. C. C.,Tam, P. H. T.,Hui, C. Y.,Kong, A. K. H.,Cheng, K. S. American Astronomical Society 2017 The Astrophysical journal Vol.842 No.1
<P>A glitch of a pulsar is known as a sudden increase in the spin frequency and spin-down rate (frequency time derivative), and it can be caused by a sudden release of the stress built up in the solid crust of the star or pinned vortices in the superfluid interior. PSR J2021+4026 is the first pulsar that shows a significant change in the gamma-ray flux and pulse profile at the glitch that occurred around 2011 October 16. We report the results of timing and spectral analysis of PSR J2021+4026 using similar to 8 yr Fermi. Large Area Telescope data. We find that the pulsar stayed at a high spin-down rate (similar to 4% higher than the pre-glitch value) and a low gamma-ray state (similar to 18% lower) for about 3 yr. after the glitch. Around 2014 December, the spin-down rate and gamma-ray flux gradually returned to pre-glitch values within a timescale of a. few months. The phase-resolved spectra and pulse profiles after the relaxation are also consistent with those before the glitch. The observed long-term evolution of the spin-down rate and the gamma-ray flux indicates that the glitch triggered a mode change in the global magnetosphere. We speculate that the glitch changed. the local magnetic field structure around the polar cap and/or the inclination angle of the. dipole axis, leading to. a change in the electric current circulating in the. magnetosphere.</P>
Magnetic Properties and Magnetocaloric Effect in Ni0.5Mn0.5.xSbx Alloys
N. H. Duc,T. D. Thanh,N. H. Yen,P. T. Thanh,N. H. Dan,판더롱 한국물리학회 2012 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.60 No.3
Ni0.5Mn0.5.xSbx (x = 0.1, 0.2, 0.3, and 0.4) ingots fabricated by arc-melting high-pure metals have been studied to determine their structure and magnetic properties and to identify the magnetocaloric effect. X-ray diffraction analyses reveal that the sample with x = 0.2 is close to a single phase in a cubic structure while the others have the secondary phases. This influences strongly the magnetic properties of Ni0.5Mn0.5.xSbx. With increasing Sb content, the Curie temperature (TC) increases from 210 (for x = 0.1) to 435 K (for x = 0.4). Among the studied alloys, two samples, x = 0.2 and 0.3, have the greatest saturation magnetization values, which were recorded at 300 K. Under an applied field of 12.0 kOe, the maximum magnetic entropy changes are about 1.0 and 0.5 J.kg.1·K.1 for x = 0.2 and 0.3, respectively. Detailed analyses related to isothermal M-H curves in the vicinity of TC by using the modified Arrott method reveal that these samples undergo a second-order phase transition with critical exponents of = 0.40 ± 0.01 and = 1.27± 0.08 for x = 0.2 and of = 0.69 ± 0.09 and = 0.85 ± 0.10 for x = 0.3. The differences in the critical parameters are likely related to the presence of Ni-related secondary phases.
DISCOVERY OF X-RAY PULSATIONS FROM THE NEXT GEMINGA, PSR J1836+5925
Lin, L. C. C.,Hui, C. Y.,Li, K. T.,Takata, J.,Hu, C. P.,Kong, A. K. H.,Yen, D. C. C.,Chou, Y. IOP Publishing 2014 ASTROPHYSICAL JOURNAL LETTERS - Vol.793 No.1
<P>We report recent XMM-Newton investigations of PSR J1836+5925, the 'next Geminga,' which determined an X-ray pulsation of similar to 173.3 ms. Its X-ray periodicity is consistent with the gamma-ray ephemeris at the same epoch. The X-ray folded light curve has a sinusoidal structure that is different from the double-peaked gamma-ray pulse profile. We have also analyzed the X-ray phase-averaged spectra which shows that the X-ray emission from PSR J1836+5925 is thermal dominant. This suggests that the X-ray pulsation mainly originates from the modulated hot spot on the stellar surface.</P>
Influences of Ru-doping on the Magnetic Properties of Ca0.85Pr0.15Mn1−xRuxO3
The Long Phan,Y.D. Zhang,유성초,P. Q. Thanh,P. D. H. Yen 한국물리학회 2012 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.61 No.10
CaMnO<SUB>3</SUB> is an antiferromagnet, where the super-exchange interaction taking place between Mn<SUP>4+</SUP> ions plays an important role. The doping of a small amount of 15% Pr into the Ca site, Ca<SUB>0.85</SUB>Pr<SUB>0.15</SUB>MnO<SUB>3</SUB>, leads the appearance of Mn<SUP>3+</SUP> ions, and introduces the ferromagnetic double-exchange interaction between Mn<SUP>3+</SUP> and Mn<SUP>4+</SUP> ions dominant in a narrow temperature range of 90~115 K. The ferromagnetic interaction becomes strong as Ca<SUB>0.85</SUB>Pr<SUB>0.15</SUB>MnO<SUB>3</SUB> doped with 4 and 8 % Ru into the Mn site (<I>i.e.</I>, Ca<SUB>0.85</SUB>Pr<SUB>0.15</SUB>Mn<SUB>1-x</SUB>Ru<SUB>x</SUB>O<SUB>3</SUB> with <I>x</I> = 0.04 and 0.08). The Curie temperature obtained for <I>x</I> = 0.04 and 0.08 are about 135 and 180 K, respectively. While the FM interaction in the former is dominant due to Mn<SUP>3+</SUP>-Mn<SUP>4+</SUP> exchange pairs, the latter has the contribution of Ru ions. This results in the remarkable difference in the feature of their FM-paramagnetic phase transitions, and coercive field <I>H</I><SUB>c</SUB>.