Chameleon pulsar baffles astronomers

An international team of scientists - including scientists from National Centre for Radio Astrophysics (NCRA), TIFR India, has made an amazing discovery about the way pulsars radiate. Using data from both a satellite X-ray telescope and terrestrial radio telescopes, they found that the pulsar's X-ray and radio emission flips its state within a second, in a way that cannot be understood by existing theories. The Indian effort was led by Dr. Dipanjan Mitra of NCRA, Pune, and the full results reported in the 25 January issue of the journal Science.

This unique behaviour was found in a particular pulsar named PSR B0943+10 and revealed through a challenging project involving simultaneous observations with the European Space Agency (ESA)'s X-ray satellite XMM-Newton and two radio telescopes, the Giant Meterwave Radio Telescope (GMRT) in Khodad near Pune and the Low Frequency Array (LOFAR) in the Netherlands.

Prior to this project, team members Dr. Dipanjan Mitra of NCRA- TIFR, Pune, India and Prof. Joanna Rankin of the University of Vermont, USA, had been trying to characterize the daily routine of this pulsar in the radio wavelength using the GMRT. “We monitored the pulsar for the first time continuously from rise to set in 2009. We caught the pulsar in a bright organized state. Then it began to show subtle changes in its behaviour, and all of a sudden it transformed to a weaker and disorganized state and stayed there for hours. This behaviour confirmed earlier speculation and provided needed foundation for the the current project.”, said Dr. Dipanjan Mitra, scientist from NCRA – TIFR, who played a major role in this international endeavour, leading the experimental effort from GMRT.

Pulsars are small fast spinning neutron stars (a few seconds to a few hundred times a second) having diameters of about 20 km – more or less the size of a small city. They are more massive than the Sun and are so dense that a sugar cube of neutron star matter would weigh about 100 million tons. Pulsars harbour superstrong magnetic fields, which are a million times stronger than the strongest field that can be created in terrestrial laboratories, and are capable of emitting beams of radiation along their magnetic axes. As the pulsar spins, its radio beam sweeps repeatedly over Earth and we detect short pulses of radiation, a bit like lighthouse. Some pulsars show emission across the entire electromagnetic spectrum, including both at X-ray and radio wavelengths. The pulsar emission arises due to flow of relativistic charged particles, moving very close to the speed of light in their superstrong magnetic field. But the physics of pulsar emission is still a mystery even 45 years of their discovery. Thus, pulsars are space-based objects having extreme physical properties.

It has been known for some time that some radio pulsars flip their behavior between two (or even more) states, changing the pattern and intensity of their radio pulses. The moment of flip is both unpredictable and sudden (often within a single rotation period). It is also known from satellite-borne telescopes that a handful of radio pulsars can also be detected at X-ray frequencies. However, the X-ray signal is so weak that nothing is known of its variability - could it be that the X-rays also flip? The pulsar PSR B0943+10, changes its form and brightness every few hours, with the flip occurring within a second. It is as if the pulsar has two distinct personalities. As PSR B0943+10 is one of the few pulsars which is known to emit X-ray radiation, knowing how this behaved during the changes in the radio, could provide new insight into the nature of the emission process.

Since B0943+10 is a weak X-ray emitter; the team used the most sensitive X-ray telescope in existence, the ESA-funded XMM-Newton. To identify the exact moments of flip in the pulsar’s radio behaviour, the X-ray observations were tracked simultaneously with two of the world’s most powerful radio telescopes, GMRT and LOFAR for several days.

The results were totally surprising. The X-rays did indeed change their behaviorsynchronously with the radio emission. But in the state where the radio signal is strong and organized, the X-rays were weak. And when the radio emission switched to weak, the X-rays intensified. “To our surprise we found that when the brightness of the radio emission halved, the X-ray emission doubled!” said project leader Professor Wim Hermsen of SRON, the Netherlands Institute for Space Research.

Moreover, the intense X-rays have a very different character from those in the radio-bright state, since they seem to be thermal in origin and pulse with the neutron star’s rotation period. This suggest that a temporary “hotspot” appears close to the pulsar’s magnetic pole which switches on and off with the change of state.

At the radio frequency where GMRT observed this pulsar, an additional emission component (called precursor) is prominently seen when the pulsar is in the radio quiet (or X-ray-bright) state. “We are not yet sure if the radio precursor and the thermal hotspot in the X-ray has a connection. These results are completely beyond expectations. Earlier we modelled the radio observations from GMRT and predicted that the X-ray emission would only change marginally between the two states. But clearly we were wrong. Something fundamental is changing within a flip of a second at such extreme environment, and we don’t know what that fundamental thing is.” said Dr. Dipanjan Mitra of the NCRA-TIFR.

Team:
The research was led by Wim Hermsen (SRON Netherlands Institute for Space Research, UvA), Lucien Kuiper and Jelle de Plaa (SRON), Jason Hessels and Joeri van Leeuwen (ASTRON en UvA), Dipanjan Mitra (NCRA-TIFR, Pune, India), Joanna Rankin (University of Vermont, USA), Ben Stappers (University of Manchester, UK), Geoffrey Wright (University of Sussex, UK). Rahul Basu researcher from NCRA- TIFR, the Pulsar Working Group and the Builders Group of LOFAR-telescope, which was at the time still in the commissioning phase, gave support to these observations. The results of this research, entitled: Synchronous X-ray and Radio Mode Switches: a Rapid Transformation of the Pulsar Magnetosphere will appear in Science , dated 25th January, 2013.

Contact: Dr. Dipanjan Mitra - 09764707558, dmitra@ncra.tifr.res.in

Image captions for article, title:

1) The two states of pulsar PSR B0943+10 as observed with XMM-Newton and GMRT 

Caption: This illustration shows the two states of emission observed from pulsar PSR B0943+10, which is well known for switching between a 'bright' and 'quiet' mode at radio wavelengths. Observations of PSR B0943+10, performed simultaneously with ESA's XMM-Newton X-ray observatory and ground-based radio telescopes, revealed that this source exhibits variations in its X-ray emission that mimic in reverse the changes seen in radio waves. No current model is able to predict what could cause such sudden and drastic changes to the pulsar's entire magnetosphere and result in such a curious emission.

In the upper part of the illustration, the artist's impression on the left shows the pulsar with glowing cones of radiation stemming from its magnetic poles – a state referred to as 'radio-bright' mode. Radio emission from pulsars is known to arise from these cones, and we see it pulsate because the pulsar's rotation and magnetic axes are misaligned. The graphs on the right side show data from X-ray observations, performed with XMM-Newton (upper graph), and from radio observations, performed with the Giant Meterwave Radio Telescope  (GMRT ; lower graph). The upper graph shows that, in the 'radio-bright' mode, the pulsar does not shine brightly in X-rays. The lower graph shows a bright and pulsating emission at radio wavelengths.

In the lower part of the illustration, the artist's impression on the left shows the pulsar in a different state, with glowing 'hot-spots' that are located at its magnetic poles. In particular, the illustration shows the pulsar in a state characterised by bright X-ray emission, arising from the polar caps, and relatively low radio emission from the cones that stem from the pulsar's magnetic poles ('X-ray-bright/radio-quiet' mode). The graphs on the right side show how, in this mode, the pulsar exhibits a brighter and pulsating X-ray emission, whereas the radio emission is fainter but still pulsating.

Credit: ESA/ATG medialab; ESA/XMM-Newton; GMRT/NCRA