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? 

Medium-sized black holes? Probably not.

Ultraluminous X-ray sources (ULXs) are accreting black holes, i.e. black holes that are "eating" matter from a companion star. They are called ultraluminous because their luminosity is too high to be explained by "normal" accretion on stellar-mass black holes, i.e. black holes formed by the collapse of a single big star (from 8 up to ~100 masses of the Sun).
A step behind. During accretion, matter falls towards a central object. This matter heats up in the process, and this heat is freed in form of radiation. This radiation in turn "pushes" on the matter that keeps falling in, and a point is reached when the luminosity produced is comparable to the push by the infalling matter and so no higher luminosity can be achieved. This is called the Eddington luminosity and is a well known quantity in accretion studies. This luminosity scales with the mass of the central object, so that supermassive black holes (billions of times the mass of the Sun) will be able to radiate at much larger luminosities than stellar-mass black holes (mass several times the Sun)
Well, ULXs radiate at much more than the Eddington luminosity for stellar-mass black holes, so the first things that comes to mind is that they are bigger than stellar-mass black holes, and so they are members of the evasive class of Intermediate-mass black holes. Right?
Not so fast, my friend. People like these japanese researchers have done a great deal of simulations to show that it is possible to overcome the Eddington limit some extent.
Only problem: it was impossible to tell which of the two hypotheses was more right until 2012, since the few X-ray satellites capable of observing ULXs were sensitive only up to 10 keV, where the models started to really give incompatible predictions.
In this old post we talked about the launch of the NuSTAR satellite. Well, that was the turning point.
NuSTAR observed several ULXs and was able to finally strongly point in one direction. See the animated GIF above: the blue points are NuSTAR data, and they clearly follow better two models (the ones cutting off above 10 keV) than the others, while data from the XMM satellite weren't able to make a difference. This cutoff is considered a signature of super-Eddington accretion and permitted to estimate the mass of these ULXs to be in the high-end of the stellar-mass range.

Here is a press release of these studies. Enjoy!

NuSTAR on the spin of supermassive BHs

Some month ago, in this post, we reported about the launch of the Nuclear Spectroscopic Timing Array (NuSTAR).

Today NASA scheduled a media teleconference to announce one of the first scientific achievements of this mission: the first precise measurement of the spin of a supermassive black hole. 

The discovery will appear in this week Nature's issue.

It turns out that NGC 1365 hosts a rapidly spinning supermassive black hole at its center, and this black hole rotates very close to its theoretical limit imposed by Einstein's General Relativity.

Curiosity: 

one usually refers to these black holes as "extremal" because their spin is close to its maximum value. In "God-given" natural units, the angular momentum of an extremal black hole is J=M^2. 

Indeed, what is usually called the "Kerr limit" is J/M^2 = 1.  This is by no means an "extremal" value!! [For example, Earth also spins around its axis and it has J/M^2~ 10^9 in natural units!]

This doesn't mean that these black monsters in the sky don't spin fast, quite the opposite: because their mass is HUGE, the angular momentum of an extremal black hole of about 10^6 solar masses [roughly the mass of the black hole at the center of the NGC 1365 galaxy]  is as large as 10^13 times that of Earth... a pretty huge and fast spinning top!

PS:

The first author of the paper, Guido Risaliti is an Italian astrophysicists who works here at the Harvard-Smithsonian CfA and at Italian Institute for Astrophysics in Arcetri.

Welcome to space, NuStar!

The Nuclear Spectroscopic Timing Array, NuStar, is now in orbit. On June 13th, the satellite left Earh on a Pegasus XL rocket launched from a Lockeed L-101 close to the Marshall Islands. The trajectories of the three stages of the rocket were nominal, and the satellite has immediately deployed its solar panels and started communicating with the control rooms.
After some hitches with the attitude control system on June 14th, immediately solved, the satellite has been behaving as expected with the alignment operations using star trackers.
Today, it will deploy its 10m mast to take the focusing optics at the right focal distance, and in about 20 days from now it will be ready to do science.
Check the news on Fiona Harrison's blog.

EDIT: The mast was deployed successfully. Great news (midnight June 21st)!

What's so exciting about it?
As I wrote in a former post, NuStar is the first X-ray satellite that will focus hard X-rays (3 - 80 keV) and obtain images with ~25" resolution. It will be a natural complement of XMM-Newton, an ESA satellite with a slightly better resolution, but in the soft X-rays (0.3-12 keV).
This satellite has nice timing ( 0.1ms) and spectral (0.5keV@10keV, 1keV@60keV) resolution, and will be an extraordinary tool to study, among other things, accreting black holes and neutron stars.
The soft spectra of these objects have in fact delivered a lot of information on their structure and geometry, but often degeneracies arise when fitting them with concurring models. NuSTAR will solve most of these degeneracies: for example, it will be able to tell if some ultraluminous X-ray sources are intermediate mass black holes or unusually bright stellar-mass black holes; it will detect line emission from supernova remnants, to investigate their composition; it will be able to see accreting objects that, because of high absorption from the surrounding and interstellar medium, are too faint to be studied with with soft satellites, allowing us to map the presence of black holes in our galaxy and in others. 

Launch pins

I received them!
These are launch pins. According to a well-established ritual in space missions, the days before launch, and during launch, all members of the team must wear one of these pins. I specify MUST, because it's bad luck not doing so!

Those Russian non-believers, for example, did not wear launch pins, and that's the result. True story.

NuSTAR press conference at Nasa today.

Today, at 10am Pacific Time, 1pm Eastern time, or 7 pm Italian time, the official press conference about NuSTAR, the next NASA Small Explorer mission, will be held. See all information here.
The launch is scheduled for June 13. 
An older post on NuSTAR in this blog, instead, can be found here.