Friday, October 17, 2014

The brightest pulsar ever

Last week the astrophysics community was hit by an exciting news. A very luminous X-ray source located in the "Cigar" galaxy M82 was discovered to be a spinning neutron star (a pulsar) rather than a black hole, as all models have assumed so far. This research was lead by astrophysicist Matteo Bachetti, who also happens to be a writer of this blog. Thus, we take the opportunity to ask Matteo a couple of questions whose answers you will not find in the excellent press (and radio) coverage that has followed the publication of this discovery.

Q: Matteo, first of all congratulations for what sounds like a great scientific achievement! How would you explain this discovery to my grandma? 

A: If you go to the stadium during a night game, you see that, in order to light it, you need very big, powerful lamps. If you see many stadiums, you'll see those big lamps over and over. Now imagine you go to the stadium and it's perfectly lit. If you don't see the lamps you'll think they are the usual big, fat lamps. But then you look closer and realize that, instead, what is producing that incredible light is a tiny led bulb... Well, a pulsar is at least 50 times smaller than the black holes that were thought to power that incredible X-ray lamp we were observing (according to some actually very nice papers, this alleged "black hole" was actually more than 10000 times bigger than a neutron star!). And such a luminosity in such a small object was theoretically "impossible".

Q: Where were you exactly at the time you realized what was going on? My grandma is still curious, could you explain her the steps that brought to this discovery?

A: I was at my desk in Toulouse and I asked my officemate Natalie Webb: "Do you know about any pulsars in M82?"... That's how it started.
In January a very nearby (by astronomical standards... it's still 12 million light-years) supernova has exploded in M82. A nearby supernova was among the "Priority A" targets of NuSTAR, the mission I'm involved with since 2011, and the satellite pointed at M82 for about 20 days. But in M82 there were these two ultraluminous X-ray sources, that we thought to be black holes. My group was allowed to see the data to analyze these sources. I was doing the timing analysis, that consists of looking for periodic signals. And I found a periodicity that was far too stable to be produced by a black hole! Of course, the explanation was that the source was not a black hole but a pulsar, that is a neutron star.
It took a while to test with the help of other colleagues that this pulsar was really that ULX, and when we were satisfied, with Fiona Harrison (the Principal Investigator of NuSTAR and coordinator of the ULX group) we put the material together in what became the Nature paper.

Q: What is NuSTAR and how much did it cost? Which kind of observations are expected in the next years?

A: NuSTAR is a satellite that carries two identical telescopes, the first capable of producing images of the sky in the so-called hard X-rays, that is, the X-rays used for medical diagnostics. It's a Small Explorer (SMEX) mission by NASA, built as a proof-of-concept of this new technology, and cost about 170 M$. The primary mission has been accomplished and has been a great success (3 Nature papers, besides the completion of all Primary science goals). But now, the most interesting part starts, called the Guest Investigator program. Every year, scientists from all around the world will be able to propose for observations that will be assigned on the base of peer-reviewed proposals.

Q: This result is puzzling from a theoretical viewpoint, because it challenges various models of accretion onto compact objects. Which are the most interesting consequences and the most urgent followups of this work? 

A: The rate at which black holes grow is an important parameter to keep in mind when one studies how the universe evolves. We have shown at least one system that accretes 100 times the Eddington limit, and this might mean that black holes can do the same. In order to understand what is the precise mechanism that produces this very high luminosity, and so if black holes could use it too or it's just a neutron star thing, we will be monitoring the source at multiple wavelengths, and analyzing the archival data from various missions. For example, at radio and optical frequencies there are multiple features that can be produced by strong outflows of matter, and this can give a measure of the mechanical output of the source, besides its luminosity.
I also want to mention that there have been other discoveries in the past few months, that together with ours have definitively confirmed that ultraluminous X-ray sources are not a single class of object: they range from neutron stars (our work), to super-Eddington stellar-mass black holes, to intermediate mass black holes. Very exciting times to be working on ULXs!

Q: At the time of this work, you were based in Toulouse and now you are working at the Sardinia Radio Telescope. Is your present job different or will you continue to investigate this pulsar?

A: I'll have time to dedicate to my own research, and of course most of it will be devoted to this source. Also, SRT is a fantastic radio telescope and I hope to be able to use it for some of the radio followup. My main job here is to analyze archival data of double neutron star binaries and participate to the astronomical validation of SRT. That's another story, I'll maybe write something about it.

Q: My grandma knows you also play guitar and has one final question. If this work were a song, which song would it be?

A: "This little light of mine I'm gonna let it shine"
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