arXiv subjects

Michael Kramer

Publications and source records attributed to Michael Kramer.

194 records · Page 11Linked to original sources

arXiv2006

The Origin and Motion of PSR J0538+2817 in S147

We report on VLBA astrometry and CXO imaging of PSR J0538+2817 in the supernova remnant S147. We measure a parallax distance of $1.47^{+0.42}_{-0.27}$kpc along with a high-precision proper motion, giving a transverse velocity $V_\perp=400^{+114}_{-73}$km/s. A small extended wind nebula is detected around the pulsar; the symmetry axis of this structure suggests that the spin axis lies $12\arcdeg\pm4\arcdeg$ from the velocity vector (2-D), but the emission is too faint for robust model independent statements. The neutron star is hot, consistent with the young ~40kyr kinematic age. The pulsar progenitor is likely a runaway from a nearby cluster, with NGC 1960 (M36) a leading candidate.

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arXiv2006

A Multi-wavelength study of the Pulsar PSR B1929+10 and its X-ray trail

We report on the emission properties of PSR B1929+10 and its putative X-ray trail from a multi-wavelength study performed with XMM-Newton, the ESO NTT, the HST, the Effelsberg 100m Radio Telescope and the Jodrell Bank Radio Observatory. The XMM-Newton observations confirm the existence of the diffuse emission with a trail morphology lying in a direction opposite to the transverse motion of the pulsar. The trail has a length of ~15 arcmin. Its spectrum is non-thermal and produced by electron-synchrotron emission in the shock between the pulsar wind and the surrounding medium. Assuming that the electron lifetime against synchrotron cooling is comparable to the source transit time over the X-ray trail length, the magnetic field strength in the trail emitting region is inferred to be ~5 uG. Inspecting data from the Effelsberg 11cm radio continuum survey of the Galactic plane we discovered an elongated feature apparently coincident with the X-ray trail. The emission properties observed from PSR 1929+10 are found to finally challenge the picture of the emission properties of old non-recycled rotation powered pulsars. Both, the temporal and spectral X-ray emission properties of PSR 1929+10 are in excellent agreement with a non-thermal and, thus, magnetospheric radiation dominated emission scenario. A flux contribution from the thermal emission of heated polar caps of ~7% is inferred from a best fitting composite Planckian and power law spectral model. The X-ray pulse profile is found to be markedly different from the broad sinusoidal pulse profile seen in the low statistic ROSAT data. Simulations in the framework of an outer-gap emission model are able to reproduce the observed X-ray pulse profile and its phase relation relative to the radio pulse.

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arXiv2005

A 2.1 Solar Mass Pulsar Measured by Relativistic Orbital Decay

PSR J0751+1807 is a millisecond pulsar in a circular 6 hr binary system with a helium white dwarf secondary. Through high precision pulse timing measurements with the Arecibo and Effelsberg radio telescopes, we have detected the decay of its orbit due to emission of gravitational radiation. This is the first detection of the relativistic orbital decay of a low-mass, circular binary pulsar system. The measured rate of change in orbital period, corrected for acceleration biases, is dP_b/dt=(-6.4+-0.9)x10^-14. Interpreted in the context of general relativity, and combined with measurement of Shapiro delay, it implies a pulsar mass of 2.1+-0.2 solar masses, the most massive pulsar measured. This adds to the emerging trend toward relatively high neutron star masses in neutron star--white dwarf binaries. Additionally, there is some evidence for an inverse correlation between pulsar mass and orbital period in these systems. We consider alternatives to the general relativistic analysis of the data, and we use the pulsar timing data to place limits on violations of the strong equivalence principle.

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arXiv2005

A multiwavlength study of PSR B0628-28: The first overluminous rotation-powered pulsar?

The ROSAT source RX J0630.8-2834 was suggested by positional coincidence to be the X-ray counterpart of the old field pulsar PSR B0628-28. This association, however, was regarded to be unlikely based on the computed energetics of the putative X-ray counterpart. In this paper we report on multiwavelength observations of PSR B0628-28 made with the ESO/NTT observatory in La Silla, the Jodrell Bank radio observatory and XMM-Newton. Although the optical observations do not detect any counterpart of RX J0630.8-2834 down to a limiting magnitude of V=26.1 mag and B=26.3 mag, XMM-Newton observations finally confirmed it to be the pulsar's X-ray counterpart by detecting X-ray pulses with the radio pulsar's spin-period. The X-ray pulse profile is characterized by a single broad peak with a second smaller peak leading the main pulse component by ~144 degree. The fraction of pulsed photons is (38 +- 7)% with no strong energy dependence in the XMM-Newton bandpass. The pulsar's X-ray spectrum is well described by a single component power law with photon index 2.63^{+0.23}_{-0.15}, indicating that the pulsar's X radiation is dominated by non-thermal emission processes. A low level contribution of thermal emission from residual cooling or from heated polar caps, cannot be excluded. The pulsar's spin-down to X-ray energy conversion efficiency is obtained to be ~16% for the radio dispersion measure inferred pulsar distance. If confirmed, PSR B0628-28 would be the first X-ray overluminous rotation-powered pulsar identified among all ~1400 radio pulsars known today.

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arXiv2004

Next-to-leading order QCD calculations with parton showers I: collinear singularities

Programs that calculate observables in quantum chromodynamics at next-to-leading order typically suffer from the problem that, when considered as event generators, the events generated consist of partons rather than hadrons and just a few partons at that. Thus the events generated are completely unrealistic. These programs would be much more useful if the few partons were turned into parton showers, in the style of standard Monte Carlo event generators. Then the parton showers could be given to one of the Monte Carlo event generators to produce hadron showers. We show how to generate parton showers related to the final state collinear singularities of the next-to-leading order perturbative calculation for the example of e+ + e- --> 3 jets . The soft singularities are left for a separate publication.

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arXiv2003

Observational manifestations of young neutron stars: Spin-powered pulsars

The largest number of known young neutron stars are observed as spin-powered pulsars. While the majority of those are detected at radio frequencies, an increasing number can be studied at other parts of the electromagnetic spectrum as well. The Crab pulsar is the prototype of a young pulsar which can be observed from radio to gamma-ray frequencies, providing a red thread of discussion during a tour through the pulsar properties observed across the electromagnetic spectrum. The basic observational features of pulsar emission are presented, preparing the ground for more detailed reviews given in these proceedings. Here, particular attention will be paid to those emission features which may provide a link between the radio and high-energy emission processes.

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arXiv2003

Astrophysical constraints on hypothetical variability of fundamental constants

(Abridged) Many-multiplet (MM) method applied to three inhomogeneous samples of Keck/HIRES quasar absorption spectra gives a shift in the value of the fine-structure constant of Delta alpha/alpha = (-5.4+/-1.2) 10^{-6} in the redshift range 0.2 < z < 3.7 (Murphy et al. 2003).The 1sigma error is, however, much too small and cannot be maintained by current observations of quasars. We present a modified MM method to set an upper limit on Delta alpha/alpha from a homogeneous sample of FeII lines identified in the up-to-date best quality VLT/UVES spectrum of HE 0515-4414. Our result is Delta alpha/alpha = (1.1+/-1.1) 10^{-5} at z = 1.149. Theoretical models of the fundamental physical interactions predict that the proton-to-electron mass ratio (mu = m_p/m_e) may relate to the shift in Delta alpha/alpha as Delta mu/mu = R Delta alpha/alpha. We use VLT/UVES high-resolution observations of molecular hydrogen H2 ultraviolet absorption lines at z = 3.025 toward Q 0347-3818 to bound the value of R. The obtained constraints on Delta ln mu = (2.1+/-3.6) 10^{-5} and on Delta ln alpha = (1.1+/-1.1) 10^{-5} rule out vary large values of |R| > 6.

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arXiv2003

Pulsars with the SKA

As for other areas in modern astronomy, the SKA will revolutionize the field of pulsar astrophysics. Not only will new science be possible by the shear number of pulsars discovered, but also by the unique timing precision achievable with the SKA. The combination of both will not simply mean a continuation of the successes already achieved by using pulsars as fundamental tools of physics but the SKA will provide a new quality of science.

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arXiv2003

The effect of HII regions on rotation measure of pulsars

We have obtained new rotation measure for 11 pulsars observed with the Effelsberg 100-m radio telescope, in the direction of the Perseus arm. Using a combination of 34 published and the 11 newly measured pulsar rotation measures we study the magnetic field structure towards the Perseus arm. We find that two pulsars towards l$\sim$ 149$^{\circ}$ (Region 1) and four pulsars towards l$\sim113^{\circ}$ (Region 2) lie behind HII regions which seriously affects the pulsar rotation measures. The rotation measure of PSR J2337+6151 seem to be affected by its passage through the supernova remnant G114.3+0.3. For Region 1, we are able to constrain the random component of the magnetic field to $5.7μ$G. For the large-scale component of the Galactic magnetic field we determine a field strength of $1.7\pm1.0μ$G. This average field is constant on Galactic scales lying within the Galactic longitude range of $85^{\circ} <$ l $ < 240^{\circ}$ and we find no evidence for large scale field reversal upto 5-6 kpc. We highlight the great importance to include the effects of foreground emission in any systematic study.

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arXiv2002

Next-to-leading order numerical calculations in Coulomb gauge

Calculations of observables in quantum chromodynamics can be performed using a method in which all of the integrations, including integrations over virtual loop momenta, are performed numerically. We use the flexibility inherent in this method in order to perform next-to-leading order calculations for event shape variables in electron-positron annihilation in Coulomb gauge. The use of Coulomb gauge provides the potential to go beyond a purely order alpha_s^2 calculation by including, for instance, renormalon or parton showering effects. We expect that the approximations needed to include such effects at all orders in alpha_s will be simplest in a gauge in which unphysically polarized gluons do not propagate over long distances.

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arXiv2001

Electroweak radiative corrections to W-boson production at hadron colliders

The complete set of electroweak O(alpha) corrections to the Drell--Yan-like production of W bosons is calculated and compared to an approximation provided by the leading term of an expansion about the W-resonance pole. All relevant formulae are listed explicitly, and particular attention is paid to issues of gauge invariance and the instability of the W bosons. A detailed discussion of numerical results underlines the phenomenological importance of the electroweak corrections to W-boson production at the Tevatron and at the LHC. While the pole expansion yields a good description of resonance observables, it is not sufficient for the high-energy tail of transverse-momentum distributions, relevant for new-physics searches.

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arXiv2001

High time-resolution observations of the Vela pulsar

We present high time resolution observations of single pulses from the Vela pulsar (PSR B0833-45) made with a baseband recording system at observing frequencies of 660 and 1413 MHz. We have discovered two startling features in the 1413 MHz single pulse data. The first is the presence of giant micro-pulses which are confined to the leading edge of the pulse profile. One of these pulses has a peak flux density in excess of 2500 Jy, more than 40 times the integrated pulse peak. The second new result is the presence of a large amplitude gaussian component on the trailing edge of the pulse profile. This component can exceed the main pulse in intensity but is switched on only relatively rarely. Fluctutation spectra reveal a possible periodicity in this feature of 140 pulse periods. Unlike the rest of the profile, this component has low net polarization and emits predominantly in the orthogonal mode. This feature appears to be unique to the Vela pulsar. We have also detected microstructure in the Vela pulsar for the first time. These same features are present in the 660 MHz data. We suggest that the full width of the Vela pulse profile might be as large as 10 ms but that the conal edges emit only rarely.

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arXiv2000

Pulsar spectra of radio emission

We have collected pulsar flux density observations and compiled spectra of 281 objects. The database of Lorimer et al. (\cite{lorimer}) has been extended to frequencies higher than 1.4 GHz and lower than 300 MHz. Our results show that above 100 MHz the spectra of the majority of pulsars can be described by a simple power law with average value of spectral index $<α> = -1.8\pm0.2$. A rigorous analysis of spectral fitting revealed only about 5% of spectra which can be modelled by the two power law. Thus, it seems that single power law is a rule and the two power law spectrum is a rather rare exception, of an unknown origin, to this rule. We have recognized a small number of pulsars with almost flat spectrum ($α\geq -1.0$) in the wide frequency range (from 300 MHz to 20 GHz) as well as few pulsars with a turn-over at unusually high frequency ($\sim$~1 GHz).

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arXiv1998

Determination of the geometry of the PSR B1913+16 system by geodetic precession

New observations of the binary pulsar B1913+16 are presented. Since 1978 the leading component of the pulse profile has weakend dramatically by about 40%. For the first time, a decrease in component separation is observed, consistent with expectations of geodetic precession. Assuming the correctness of general relativity and a circular hollow-cone like beam, a fully consistent model for the system geometry is developed. The misalignment angle between pulsar spin and orbital momentum is determined giving direct evidence for an asymmetric kick during the second supernova explosion. It is argued that the orbital inclination angle is $132\fdg8$ (rather than $47\fdg2$). A prediction of this model is that PSR B1913+16 will not be observable anymore after the year 2025.

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