Manu Linares

Neutron stars are cool!
Not only fascinating astronomical objects, neutron stars are also unique laboratories for extreme physics. Just like our planet Earth, a neutron star has an atmosphere, a solid surface, a crust and a liquid core. Neutron stars are [4-7]x10⁵ times more massive than Earth, but they are densely and stably packed in a radius of about 8-15 km. As a consequence, their surface gravity is 10¹¹ times stronger than that on Earth. Escape velocities approach the speed of light and even photon trajectories are severely bent in their intense gravitational fields. Neutron stars can also have mountains, quakes, and a large scale magnetic field, 10⁸-10¹⁵ times stronger than that on Earth.

More than five decades after neutron stars were discovered, some of their properties remain largely concealed. Their inner cores reach densities above nuclear saturation, where nucleon interactions are ill-constrained. Learning how matter behaves at such densities is one of the main motivations of my past, present and future research. The study of neutron stars has also profound implications for numerous fields of astronomy, including supernovae and their remnants, gravitational waves from binary mergers, jets and accretion flows. For all these reasons, my research is aimed at accreting compact objects with a special focus on neutron stars.

Resuscitating neutron stars.
Neutron stars are often considered stellar corpses, formed after the death of a massive star in a supernova explosion. Left alone, they cool down and decrease their rotation rate until they cross the `death line' and cease to exist as radio pulsars. Accretion of mass in a binary system provides a way to resurrect them. Thanks to the in-falling gas stripped from the outermost layers of a nearby star, neutron stars in the so-called low-mass X-ray binaries release tremendous amounts of energy, mostly in the X-ray band. They also regain their nuclear energy, becoming thermonuclear reactors on the surface, and a handful of them are seen as accreting millisecond pulsars (showing accretion-powered millisecond X-ray pulsations). Moreover, the accretion of angular momentum from the innermost gas spins up or `recycles' those neutron stars, decreasing their spin periods to just a few milliseconds. This allows them, once accretion stops, to become active again as rotation-powered millisecond radio pulsars.

In the following I summarize my three main lines of research which I like to divide, according to the energy source, in three broad types of phenomena: those powered by the rotational energy of the neutron star; those powered by nuclear reactions in the neutron star envelope; and those powered by the gravitational energy released by the accretion of mass onto the neutron star.

ROTATION POWER: A lighthouse in the wind
The launch of the Fermi Gamma-ray Space Telescope triggered remarkable neutron star discoveries, which have uncovered a new population of millisecond pulsars in compact binaries. In these tight binaries, with orbital periods shorter than about a day, the intense relativistic pulsar wind (powered by the rotational energy reservoir of the neutron star) can strongly irradiate or even ablate the companion star. This growing nearby population of pulsars offers a new opportunity to measure the masses of recycled neutron stars. Furthermore, a handful of rapidly spinning, low magnetic field neutron stars are now seen to alternate between accretion-powered and rotation-powered states (the so-called "transitional millisecond pulsars"). These recent findings have confirmed the evolutionary link between millisecond radio pulsars (MSPs) and low-mass X-ray binaries, and have opened a new era where the metamorphosis of a neutron star from a rotation- to an accretion-powered state (and vice versa) can be directly observed!

THERMONUCLEAR POWER: The ticking bomb
The plasma accreted onto neutron stars piles up until nuclear reactions become thermally unstable and trigger a thermonuclear runaway, which burns the accumulated fuel. This produces recurrent, sudden, bright bursts of X-rays: thermonuclear bursts. One fundamental prediction of thermonuclear burst models clashed with observations for over thirty years: we expect more frequent bursts when the mass accretion rate on the neutron star increases, but little or no bursts were seen at high mass accretion rates, where helium burning should still be unstable. In 2010, we proposed a solution to this puzzle, thanks to a slowly spinning neutron star discovered in the globular cluster Terzan 5 (T5X2). Thermonuclear bursts from this particular system behaved like theory predicts! Based on the unusually slow spin of T5X2, we argued that rapid rotation has a drastic effect on the stability of thermonuclear burning in all other bursters.

ACCRETION POWER: Strong gravity rules
Accreting neutron stars shine more than anything else in the X-ray sky (our nearby Sun aside), thanks to the efficient conversion of gravitational potential energy into radiation that takes place in the accretion disc. Theories of gravity make falsifiable predictions in the strong field regime, such as the presence of an innermost stable circular orbit or the existence of relativistic nodal precession. Studies of rapid X-ray variability can test these predictions, probing the vicinity closest to neutron stars and black holes, where dynamical timescales are as short as milliseconds.

EWASS 2015: "Neutron stars at the crossroads: X-ray binaries and transitional millisecond pulsars". Program, talks and posters from our Symposium at the European Week of Astronomy and Space Science (Tenerife, Spain, June 25-26, 2015). Chairs: Manu Linares and Carlo Ferrigno.

Swift-XRT Globular Cluster Monitor. Swift-XRT monitoring of selected GCs in order to: i) find new low-luminosity transient LMXBs, ii) detect short and faint outbursts from known transient NS LMXBs and iii) monitor the X-ray activity of the known MSP population to search for more MSP-LMXB transition pulsars. PIs: Manu Linares and Jerome Chenevez.

• Observational Astrophysics (FY3215).
  Lectures, Planetarium, Astro-LAB sessions and Observing Project: Manu Linares
  First year M. Sc. course, Physics/Applied Physics & Math.
  Norwegian University of Science and Technology, 2022-.
  


• Quantum Physics (UPC-EF-Q3-2A-FISQ).
  Lectures: Manu Linares and Jordi José.
  Second year B. Sc. course, Eng. Physics.
  Polytechnic University of Catalonia, 2017-2021.
  
  DOWNLOAD (old-school) LECTURE NOTES, HYDROGEN ATOM:
  *Time-independent Schroedinger equation in 3D.
  *Orbital angular momentum in the one-electron atom.
  *Magnetic dipoles, spin and spin-orbit coupling.
  


• Physics I: Fundamentals of Mechanics (UPC-EEBE-Q1-F1FM).
  Lectures and Lab sessions.
  First year B. Sc. course, Engineering Degrees.
  Polytechnic University of Catalonia, 2018-2021.