Volume 509, January 2010
|Number of page(s)||7|
|Section||Interstellar and circumstellar matter|
|Published online||14 January 2010|
INTEGRAL probes the morphology of the Crab nebula in hard X-rays/soft -rays
ISDC Data Centre for Astrophysics, Geneva Observatory, University of Geneva, 16, ch. d'Ecogia, 1290 Versoix, Switzerland
2 INAF/IASF Milano, Via E. Bassini 15, 20133 Milano, Italy e-mail: firstname.lastname@example.org
3 IAAT, Abt. Astronomie, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
Accepted: 8 October 2009
Aims. We use the IBIS/ISGRI telescope on-board INTEGRAL to measure the position of the centroid of the 20–200 keV emission from the Crab region.
Methods. We find that the astrometry of the IBIS telescope is affected by the temperature of the IBIS mask during the observation. After correcting for this effect, we show that the systematic errors in the astrometry of the telescope are of the order of 0.5 arcsec. In the case of the Crab nebula and several other bright sources, the very large number of photons renders the level of statistical uncertainty in the centroid smaller or comparable to this value.
Results. We find that the centroid of the Crab nebula in hard X-rays (20–40 keV) is shifted by 8.0 arcsec with respect to the Crab pulsar in the direction of the X-ray centroid of the nebula. A similar shift is also found at higher energies (40–100 and 100–200 keV). We observe a trend of decreasing shift with energy, which can be explained by an increase in the pulsed fraction. To differentiate between the contribution of the pulsar and the nebula, we divide our data into an on-pulse and off-pulse sample. Surprisingly, the nebular emission (i.e., off-pulse) is located a significant distance away from the X-ray centroid of the nebula.
Conclusions. In all 3 energy bands (20–40, 40–100 and 100–200 keV), we find that the centroid of the nebula is offset significantly from the predicted position. We interpret this shift in terms of a cut-off in the electron spectrum in the outer regions of the nebula, which is probably the origin of the observed spectral break around 100 keV. From a simple spherically-symmetric model for the nebula, we estimate that the electrons in the external regions of the torus ( pc from the pulsar) reach a maximal energy slightly below 1014 eV.
Key words: pulsars: individual: Crab / ISM: supernova remnants / gamma rays: ISM
© ESO, 2010
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.