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Posts: 131433
Date:
Be Star SS 2883
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Title: Modelling the Wind of the Be Star SS 2883
Authors: A. I. Bogomazov

Observations of eclipses of the radio pulsar B1259-63 by the disk of its Be-star companion SS 2883 provide an excellent opportunity to study the winds of stars of this type. The eclipses lead to variations in the radio flux (due to variations in the free-free absorption), dispersion measure, rotation measure, and linear polarisation of the pulsar. We have carried out numerical modelling of the parameters of the Be-star wind and compared the results with observations.

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Posts: 131433
Date:
PSR B1259-63/SS2883
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Title: Gamma-rays from binary system with energetic pulsar and Be star with aspherical wind: PSR B1259-63/SS2883
Authors: Agnieszka Sierpowska-Bartosik, Wlodek Bednarek

At least one massive binary system containing an energetic pulsar, PSR B1259-63/SS2883, has been recently detected in the TeV gamma-rays by the HESS telescopes. These gamma-rays are likely produced by particles accelerated in the vicinity of the pulsar and/or at the pulsar wind shock, in comptonisation of soft radiation from the massive star. However, the process of gamma-ray production in such systems can be quite complicated due to the anisotropy of the radiation field, complex structure of the pulsar wind termination shock and possible absorption of produced gamma-rays which might initiate leptonic cascades. In this paper we consider in detail all these effects. We calculate the gamma-ray light curves and spectra for different geometries of the binary system PSR B1259-63/SS2883 and compare them with the TeV gamma-ray observations. We conclude that the leptonic IC model, which takes into account the complex structure of the pulsar wind shock due to the aspherical wind of the massive star, can explain the details of the observed gamma-ray light curve.

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Posts: 131433
Date:
PSRB1259-63
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Astronomers have witnessed a never-seen-before event in observations by ESA's XMM-Newton spacecraft - a collision between a pulsar and a ring of gas around a neighbouring star.

The rare passage, which took the pulsar plunging into and through this ring, illuminated the sky in gamma- and X-rays.
It has revealed a remarkable new insight into the origin and content of 'pulsar winds', which has been a long-standing mystery. The scientists described the event as a natural but 'scaled-up' version of the well-known Deep Impact satellite collision with Comet Tempel 1.
Their final analysis is based on a new observation from XMM-Newton and a multitude of archived data which will lead to a better understanding of what drives well-known 'pulsar nebulae', such as the colourful Crab and Vela pulsars.

"Despite countless observations, the physics of pulsar winds have remained an enigma. Here we had the rare opportunity to see pulsar wind clashing with stellar wind. It is analogous to smashing something open to see what's inside" - Masha Chernyakova, lead author, Integral Science Data Centre, Versoix, Switzerland.

A pulsar is a fast-spinning core of a collapsed star that was once about 10 to 25 times more massive than our Sun. The dense core contains about a solar mass compacted in a sphere about 20 kilometres across.
The pulsar in this observation, called PSR B1259-63, is a radio pulsar, which means most of the time it emits only radio waves. The binary system lies in the general direction of the Southern Cross about 5000 light-years away.
Pulsar wind comprises material flung away from the pulsar. There is ongoing debate about how energetic the winds are and whether these winds consist of protons or electrons. What Chernyakova's team has found, although surprising, ties in neatly with other recent observations.



The team observed PSR B1259-63 orbiting a 'Be' star named SS 2883, which is bright and visible to amateur astronomers. 'Be' stars, so named because of certain spectral characteristics, tend to be a few times more massive than our Sun and rotate at astonishing speeds.

They rotate so fast that their equatorial region bulges and they become flattened spheres. Gas is consistently flung off such a star and settles into an equatorial ring around the star, with an appearance somewhat similar to the planet Saturn and its rings.
The pulsar plunges into the Be star's ring twice during its 3.4-year elliptical orbit; but the plunges are only a few months apart, just before and after 'periastron', the point when the two objects in orbit are closest to each other. It is during the plunges that X-rays and gamma rays are emitted, and XMM-Newton detects the X-rays.

"For most of the 3.4-year orbit, both sources are relatively dim in X-rays and it is not possible to identify characteristics in the pulsar wind. As the two objects draw closer together, sparks begin to fly" - Andrii Neronov, co-author.

The new XMM-Newton data was collected nearly simultaneously with a HESS observation. HESS, the High Energy Stereoscopic System, is a new ground-based gamma-ray telescope in Namibia.
Announced last year, the HESS observation was puzzling in that the gamma-ray emission fell to a minimum at periastron and had two maximums, just before and after the periastron, the opposite of what scientists were expecting.
The XMM-Newton observation supports the HESS observation by showing how the maximums were generated by the double plunging into the Be star's ring. By combining these two observations with radio observations from the last periastron event, the scientists now have a complete picture of this system.
Tracing the rise and fall of X-rays and gamma rays day after day as the pulsar dug through the Be star's disk, the scientists could conclude that the wind of electrons at an energy level of 10-100 MeV is responsible for the observed X-ray light. (1 MeV represents one million electron volts.)
Although 10-100 MeV is energetic, this is about 1000 times less than the expected energy level of 100 TeV. Even more puzzling is the multi-TeV gamma-ray emission, which, although surely emanating from the 10-100 TeV wind electrons, seems to be produced differently to how it was thought before.

"The only fact that is crystal clear at the moment is that this is the pulsar system to watch if we want to understand pulsar winds. Never have we seen pulsar wind in such detail. We are continuing with theoretical models now. We have some good explanation of the radio-to-TeV-gamma-ray behaviour of this funny system, but it is still 'under construction." - Masha Chernyakova.

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Posts: 131433
Date:
PSR B1259-63 & SS 2883
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(old news - April 2005)

Observations of the 47-ms pulsar PSR B1259-63 (PSR J1302-6350) and optical observations of its binary companion Be star, SS 2883 have shown that the binary system of the blue giant and pulsar operate as a natural particle accelerator.



Binary pair PSR B-1259-63 / SS 2883 is located some 5,000 light-years distant in the constellation Crux (the Southern Cross).
The two are in a highly eccentric orbit of period 3.4 yr.
The pulsar orbits the more massive primary that brings it within 100 million kilometres at closest approach and ten times that distance at their furthest separation.
During closest approach, signals from the pulsar drop off significantly as it is eclipsed by the massive blue giant.

The pulsar thus has a characteristic age of 0.33 Million years and a surface magnetic field strength of 3x10^11 G. Optical photometry of SS 2883 shows that the star is of spectral type of about B2e, indicating its mass to be 10 solar masses and its radius 6 Rsun.
It is likely that the inclination of the plane of the orbit and the inclination of the circumstellar disc are similar at about 35 deg.

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Assuming that the pulsar accelerates electrons in the strong electric field generated its the rotating magnetic moment, PSR B1259-63/SS 2883 was predicted as a variable source of TeV gamma rays.
The flow of highly relativistic electrons - the pulsar wind - ends in a termination shock, where the pressure of the wind is balanced by the ambient medium; in this shock, additional particles can be accelerated.
In particular during crossings of the stellar disk, the outflow from the Be star strongly confines the pulsar wind, resulting in a contact discontinuity with the shocked pulsar wind on one side and the shocked outflow on other.
Unlike in isolated pulsars, with a spherical or circular wind termination shock, the directed radiation pressure and ambient flow should result in a bow-shock or "cometary" shock geometry.

The gamma rays are generated when the high-energy electrons Inverse-Compton scatter photons of the Be star of such target photons varies with the inverse square of the distance between the Be star and the pulsar.
As another possibility for gamma-ray production, interactions between shock-accelerated protons and the outflow have been discussed.
In the enhanced ambient magnetic fields, the electrons also create synchrotron radiation, which is visible e.g. in X-rays.

Throughout the orbit, but in particular near periastron, different energy loss mechanisms - synchrotron radiation, Inverse Compton scattering and adiabatic losses caused by the expansion of pulsar wind against the external pressure - compete with each other and provide a fascinating laboratory to study pulsar winds and pulsar wind interactions.

H.E.S.S, observed PSR B1259-63 about two weeks before periastron in March 2004, and followed the object until June 2004, with observations interrupted by full moon periods, where the telescopes cannot be operated. A clear TeV signal was quickly detected from the direction of PSR B1259-63, with a second - yet unidentified - TeV source visible in the field of view.

The signal from PSR 1259-63 was found to vary significantly, making it the first variable galactic TeV source. The relatively high pre-periastron flux decreased towards periastron; the periastron passage itself coincided with full moon and could not be observed. After periastron, the initially very small flux increased and then faded out over the next months.

The TeV flux varied along the orbit. The flux reached its maxima near the times of the crossing of the disk of the Be star. The uncertainty in the disk orientation and disk width do not allow to decide if the maxima coincide exactly with the disk crossings, or if they are displaced, as seems the case for the nominal disk geometry assumed.

Radio and X-ray flux observed during the 2004 periastron passage show a similar time dependence, with a less pronounced minimum around periastron. The fact that both synchrotron and Inverse-Compton (TeV) fluxes are low during periastron excludes models where radiative energy losses of accelerated electrons dominate, and show that there must be other mechanism, through which electrons lose their energy faster then by radiation - presumably adiabatic losses.
The double-peaked TeV light curve, on the other hand, is qualitatively - although not quantitatively - described by a model where accelerated protons interact with the outflow.
To gain further insights, PSR B1259-63 will certainly be a target of future H.E.S.S. observations.

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