1,721,061 research outputs found
Gamma-ray spectral analysis of three energetic millisecond pulsars
No full textPSRs J0218+4232, B1821−24 and B1937+21 are among the most energetic and fastest-spinning millisecond pulsars (MSPs). They have been studied in radio, X-rays and gamma-rays, and show aligned pulse profiles in all the bands. The Fermi LAT Pass 8 data was published in 2015 and has lots of advantages over the old Pass 7 data, such as increased effective area, wider energy range, and improved event reconstruction. Since the recent gamma-ray spectral anal- yses of the three MSPs are relatively old, I redo the gamma-ray spectral analy- ses of the three MSPs with four-year more Fermi LAT observational data and newly published Fermi LAT Pass 8 data. I obtain better fit results for the gamma-ray spectra of the three MSPs with smaller error bars and larger test statistic values. I also do numerical simulations to test the two-layer model using the new Fermi LAT data. By minimizing the differences between the predictions of the two-layer model and the observational data, I obtain the best-fit values of the three independent parameters of the two-layer model. It is discovered that the simplified two-layer model can predict the broadband spectra of the three MSPs which are very close to the observational data in gamma-rays (Fermi LAT) and X-rays (NuSTAR).published_or_final_versionPhysicsMasterMaster of Philosoph
X-ray observations of magnetar SGR 0501+4516 from outburst to quiescence
No full textMagnetars are neutron stars having extreme magnetic field strengths. Study of their emission properties in the quiescent state can help understand effects of a strong magnetic field on neutron stars. SGR 0501+4516 is a magnetar discovered in 2008 during an outburst and has recently returned to quiescence. I report its spectral and timing properties measured with new and archival observations from the Chandra X-Ray Observatory, XMM-Newton, and Suzaku. I found that the quiescent spectrum is best fit by a power-law plus two blackbody model, with temperatures of kT_low ∼ 0.25 keV and kT_high ∼ 0.61 keV and a photon index of Γ ∼ 3.2. I interpret these two blackbody components as emission from a hotspot and the entire surface. The hotspot radius shrinked from 1.5 km to 0.5 km since outburst and there is a significant correlation between its area and the X-ray luminosity, which well agrees with prediction by the twisted magnetosphere model.
I applied the two temperature spectral model to all magnetars in quiescence and found that it could be a common feature among the population. Moreover, the temperature of the cooler blackbody shows a general trend with the magnetar field strength, which supports the simple scenario of heating by magnetic field decaypublished_or_final_versionPhysicsMasterMaster of Philosoph
Globular clusters contributing to the nuclear star cluster and galaxy center gamma ray excess, moderated by galaxy assembly history
No full textTwo unresolved questions at galaxy centers, namely the formation of the nuclear star cluster (NSC) and the origin of the gamma-ray excess in the Milky Way (MW) and Andromeda (M31), are both related to the formation and evolution of globular clusters (GCs). They migrate towards the galaxy center due to dynamical friction, and get tidally disrupted to release the stellar mass content including millisecond pulsars (MSPs), which contribute to the NSC and gamma-ray excess.
In this study, we propose a semi-analytical model of GC formation and evolution that utilizes the Illustris cosmological simulation to accurately capture the formation epochs of GCs and simulate their subsequent evolution. Our analysis
confirms that our GC properties at z=0 are consistent with observations, and our model naturally explains the formation of a massive NSC in a galaxy similar to theMW and M31. We also find a remarkable similarity in our model prediction with
the gamma-ray excess signal in the MW. However, our predictions fall short by approximately an order of magnitude in M31, indicating distinct origins for the two gamma-ray excesses. Meanwhile, we utilize the catalog of Illustris halos to
investigate the influence of galaxy assembly history. We find that the earlier a galaxy is assembled, the heavier and spatially more concentrated its GC system behaves at z=0. This results in a larger NSC mass and brighter gamma-ray emission
from deposited MSPs.published_or_final_versionPhysicsDoctoralDoctor of Philosoph
Radio study of two pulsar wind nebulae powered by PSR B1706−44 and the Vela Pulsar
No full textPulsar wind nebulae (PWNe) are very energetic galactic objects. The pulsar wind carries away the majority of the pulsar spin down power and accelerates particles to relativistic speeds. To help understand related high energy physics in PWNe, I analyzed the PWN powered by PSR B1706−44 and that powered by the Vela pulsar with radio observations.
I performed a radio study of the PWN powered by PSR B1706−44 at 3, 6, 13, and 21cm using observations taken with the Australia Telescope Compact Array (ATCA). The nebula shows an overall arc-like morphology at 3 and 6cm, and the “arc” resembles a double-lobe at 6cm. The inner PWN is faint in radio but bright in X-rays, and the radio emission only brightens beyond that. The radio PWN morphology is interpreted using a thick torus model with Doppler boosting effect in this study. The PWN morphological differences between 3 and 6 cm images are explained by introducing a spectral gradient, and the model suggests a bulk flow speed of ∼0.2c in the radio PWN, which is much lower than that (∼0.7c) in the X-ray PWN. The radio PWN shows a highly ordered toroidal B-field, with an equipartition strength of ∼10μG. 13 and 21cm radio images show that the surrounding supernova remnant (SNR)G343.1−2.3 has a semi-circular rim and an east-west ridge. The ridge has a radio spectrum, which is comparable to that of the PWN but significantly flatter than that of the SNR rim. In addition, our polarization maps reveal that the intrinsic B-field in the ridge generally aligns with the pulsar motion direction and the ridge never extends beyond the SNR rim. All these suggest that the ridge could be a tail of the PWN.
I also studied the radio Vela PWN with ATCA observations at 3, 6, and 21 cm. The overall PWN wraps around the Vela pulsar from the northwest, with two lobes northeast (NE) and southwest (SW) of the pulsar and fainter emission northwest of it. An X-ray radio anti-correlation is also found in this PWN: the radio emission is dim in the X-ray PWN and only becomes bright beyond the X-ray structures. The thick torus model with the Doppler boosting effect, which has a ≤0.2c bulk flow speed, can better explain the features of Vela PWN. Compared with 0.44c in the X-ray tori, it implies a deceleration as the flow goes out. The polarization images also show a toroidal B-field with an equipartition B-field strength is 39 μG. The radio spectra of the overall PWN, the NE lobe, and the SW lobe have similar spectral indices of −0.3.published_or_final_versionPhysicsDoctoralDoctor of Philosoph
Multi-wavelength observations of bow-shock pulsar wind nebulae
No full textA bow-shock pulsar wind nebula (BSPWN) is a synchrotron bubble, formed when the relativistic outflow of a fast-moving pulsar is confined by the ambient environment. BSPWNe are powerful particle acceleration sites nearby in the Universe, thus providing an ideal testing ground for the particle acceleration theories in the relativistic regime. In particular, unlike PWNe in early stage, BSPWNe formation does not involve their parent supernova remnants, so their structures are steady. This hence allows simple modeling. I studied spectral energy distributions of four BWPWNe, namely, PWN B0355+54 (the Mushroom Nebula), PWN J0357+3205, PWN J1740+1000, and the Mouse Nebula (G359.23−0.82). For the first three PWNe, deep Very Large Array (VLA) radio searches using the D configuration at 3GHz were carried out. However, no extended emission was detected. I reported a 3σ flux density limit and use them as constraints in multi-wavelength modeling to probe the injected particle distribution from shocks. The limits are 0.24, 0.041 and 0.093mJy for PWN B0355+54, PWN J0357+3205, and PWN J1740+1000, respectively. The Mouse Nebula (G359.23-0.82) is a prototypical BSPWN and are bright in both radio and X-ray. Its multi-wavelength properties are beneficial to spectral modeling. I constructed a multi-wavelength spectrum of the Mouse Nebula, using the radio, infrared, X-ray, and gamma-ray Fermi data. I developed a one- zone static model to infer and compare three injected particle distributions predicted from the theories, namely, a single power law, a broken power law, and a Maxwellian distribution with a high-energy power-law tail, based on the multi-wavelength spectrum of the Mouse Nebula. I considered the combined effects of synchrotron radiation, synchrotron cooling, as well as inverse-Compton scattering in the model.
The results favor a broken power law and a Maxwellian distribution with a high-energy power-law tail injected spectra than a single power law, implying that simple predictions from conventional Fermi acceleration theory cannot explain the observations of the Mouse Nebula. In addition, it needs observations using high-frequency radio telescopes, e.g. ALMA, and infra-red telescopes to provide complete coverage for better modeling and further statistical tests to identify the best-fit inferred injected particle distribution, and therefore the most favorable particle acceleration theory.published_or_final_versionPhysicsMasterMaster of Philosoph
Radio study of the lighthouse pulsar wind nebula
No full textPulsar wind nebulae (PWNe) are created by magnetized relativistic outflows of pulsars that are shocked upon interaction with the ambient medium. A bow-shock PWN is formed when a pulsar moves supersonically in a medium. Previous radio polarization studies of bow-shock PWNe showed various mag- netic field configurations. It was claimed that the B-field preferentially aligns with the PWN elongation for systems traveling at a high speed of ∼ 1000 km/s.
In this thesis, I report radio observations of the Lighthouse PWN using the Australia Telescope Compact Array in 3cm and 6cm bands. This bow-shock PWN moving at a speed > 1000 km/s and has a ∼ 10 kpc-long tail trailing the pulsar motion. My new radio image shows that the PWN consists of a faint cube-like structure near the pulsar, a cone-like head with two bright cores inside, and a tail. These features resemble two other bow-shock systems, G319.9−0.7
and the Guitar Nebula. The morphology of the head and the two-core structure suggest that there could be flow instabilities driving bubbles into the interstellar medium (ISM) or the PWN just encountered several ISM density discontinuities. I developed a simple model with a bow-shock front and two expanding shocks to fit the PWN structure.
Radio polarimetric measurements reveal a helical B-field in the head of the Lighthouse and a possible switch of the field direction at the second core. This is the first time that such a field configuration is found in a fast-moving system. A direct comparison with the Frying Pan, G319.9−0.7, and the Mouse PWNe suggests that the alignment angle between the pulsar velocity and its spin axis direction could not decide the magnetic field configuration. The speed and axis should be both considered. Future polarimetric studies of bow-shock PWNe and magnetohydrodynamics simulations are needed to further confirm and explain this idea.published_or_final_versionPhysicsMasterMaster of Philosoph
High-resolution radio study of the pulsar wind nebula MSH 15-52
No full textPulsar winds are charged particles accelerated by the extremely strong magnetic field of rotation-powered pulsars. When these particles interact with the ambient circumstellar or interstellar material (ISM) around the pulsar, they emit broadband synchrotron radiation. The interacting region formed a nebula which is collectively known as a pulsar wind nebula (PWN). A PWN could be formed by a young pulsar within a supernova remnant (SNR) or by a fast-moving pulsar which gives a bow shock nebula. Synchrotron radiation is the major emission mechanism of PWNe, which is in the range from radio to X-rays. In gamma-rays, the emission mechanism is inverse-Compton scattering of low-energy ambient photons. Young PWNe inside SNRs are often complex and features-rich astronomical objects, their morphology based on the evolution stage of the SNR.
We present a high-resolution radio imaging study of the PWN MSH 15-52 with new Australia Telescope Compact Array observations at 6 cm and 3 cm. The system is powered by a young and energetic radio pulsar B1509-58 with a high-spin down luminosity of E ~ 2 x 10^37 erg/s. Previous X-ray studies found a complex morphology for the PWN. The overall shape resembles a hand, extending over 10 pc with features like a bright collimated jet, double arcs, filaments and enhanced emission knots in the associated HII region RCW 89.
Unlike the X-ray emission which shows the most recent conditions of PWNe, radio emission has a much longer synchrotron loss timescale. This reflects the integrated history of the injected particles to the system. The new radio images discovered the long-sought radio PWN and show different morphology than the X-ray counterpart. No radio emission is detected at the X-ray jet position, instead an enhanced emission in a shape of sheath surrounds the jet which consists of a bright arc and polarised filaments. Polarisation measurements show that the intrinsic orientation of the magnetic field aligns with the X-ray jet elongation. Small-scale features include a narrow polarised filament across the pulsar, network of filaments and knots in RCW 89. Counterparts of X-ray features like the inner arc, the ``thumb'' are also discovered. The 6km correlation data were used to measure the position of B1509. The proper motion result could imply the pulsar is moving in the X-ray jet outflow direction.published_or_final_versionPhysicsDoctoralDoctor of Philosoph
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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