On left, this HST image shows the inner part of the Crab Nebula with inside the red box the Crab pulsar and its near-by knot located 0.65” (1300 AU) south-east of the pulsar. On right, Zoom of the image. The arrows indicate at different period the polarisation angle in optical (HST in 2005 and GASP in 2012) and hard X-ray (Integral in the period 2003-2007 and 2012-2014). Also indicated are the directions of the proper motion (PM) and spin axis (SA) of the pulsar.
Jan 01 2016 Posted: 09:51 GMT

NUI Galway lead an international collaboration consisting of astronomers from the US and France to take optical and gamma-ray observations of the Crab Nebula


The Centre for Astronomy at the School of Physics in NUI Galway are the lead researchers and authors of a recent international study published today (01 January 2016) in one of the world’s leading primary research journals in astronomy and astrophysics, Monthly Notices of the Royal Astronomical Society (MNRAS).

A joint Irish-French-US set of observations have led to a better understanding of the unexpected flaring activity seen coming from the Crab supernova remnant. The project led by Irish astronomer Professor Andrew Shearer from the Centre of Astronomy at NUI Galway, involved using the NUI Galway developed, Galway Astronomical Stokes Polarimeter (GASP) polarimeter on the 200” Palomar telescope in California. Their work for the first time tied together changes in the optical polarisation with apparent changes in the gamma-ray (high energy x-ray) polarisation.

A supernova remnant occurs when a star explodes and spews its innards out across the sky, creating an expanding wave of gas and dust known as a supernova remnant. Arguably, the most famous of these remnants is the Crab Nebula, which exploded in 1054. The Crab Nebula has been studied extensively over the last fifty years and recently found to be the source of gamma-ray and X-ray flares.

It is not yet known where the flares are coming from and in an effort to understand their origin NUI Galway led the research programme of optical observations, which were carried out in association with gamma-ray observations using the European Space Agency’s (ESA) Integral gamma ray observatory. Uniquely both studies looked at the polarisation of both the light and the gamma rays in order to understand the origin of these flares.

For many years, the flux from the whole Crab Nebula was expected to be constant, in such a way that the Crab was always thought of as a ‘standard candle’ (known brightness). Some doubts were cast on this status from high energy gamma-ray and hard X-ray observations made by the Fermi and INTEGRAL satellites, both European Space Agency satellite missions used to detect energetic radiation that comes from space. Since 2007 strong high energy flaring activities have been detected by the Agile and Fermi gamma-ray telescopes at a rate of about 1 per year. Although, currently they have no clear origin, these high energy flares show the complex timing behavior of this source.

The NUI Galway team published observations of the polarisation of optical and hard X-ray photons from the Crab Nebula and pulsar system using the GASP, which was installed on the 200” Hale telescope at Mount Palomar in California, the Hubble Space Telescope/Advanced Camera for Surveys (HST/ACS) and the International Gamma-Ray Astrophysics Laboratory satellite, Integral. The NUI Galway study when compared to the Integral observations show that the polarisation of the optical light and gamma-ray seem to change in the same way, which was an unexpected result.

Professor Andrew Shearer from the School of Physics at the Centre of Astronomy in NUI Galway, said: “Our studies show how Galway’s GASP polarimeter will be important for future observations of these high energy astronomical sources. After the recent Government announcement that Ireland will join the European Southern Observatory (ESO) we hope to contribute to future world class telescope projects such as the European Extremely Large Telescope.”

Indeed, a change in the optical polarisation angle has been observed by this work, from 109.5° in 2005 to 85.3° in 2012. On the other hand, the gamma-ray polarisation angle changed from 115° to 80° during a similar period. Strong flaring activities at higher gamma-ray energies have been detected in the Crab nebula during this period and magnetic reconnection processes have been suggested to explain these observations.

The change in the polarised optical and gamma-ray emission of the Crab Nebula/pulsar system as observed, for the first time, by GASP and the Integral satellite may indicate that magnetic reconnection is possibly at work in the Crab Nebula. The study also reported for the first time, a non-zero measure of the optical circular polarisation from the Crab pulsar + knot system. These results outline the strong scientific potential of polarimetric studies in particular in systems like the Crab Nebula where magnetic fields play a key role.

The research was part-funded by a Ulysses grant for Irish-French collaboration.

To read the study published in MNRAS visit: http://arxiv.org/pdf/1511.07641v1.pdf

ENDS

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