(click on the images for details)
(click on the images for details)
Neutron stars are cool!
Not only fascinating astronomical objects, neutron stars are also unique laboratories of 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!
[See Duncan Lorimer's updated list of Galactic MSPs.]
[See Paulo Freire's list of pulsars in globular clusters.]
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.
Here is the talk I gave at the Institute for Astrophysics of the Canaries back in May 2013
LINKS TO RESEARCH PROJECTS/WEBS:
- Professor. NTNU, Department of Physics (Trondheim, Norway). From September 2021
- Associate Professor. UPC, Department of Physics (Barcelona, Spain). 2018-present
- Marie Curie Fellow. UPC, Group of Astronomy and Astrophysics (Barcelona, Spain). 2017-2018
- IAC Fellow. Instituto de Astrofísica de Canarias (Tenerife, Spain). 2012-2017
- Rubicon Fellow. Massachusetts Institute of Technology (Cambridge, US). 2009-2012
- PhD in Astronomy. Universiteit van Amsterdam. (Amsterdam, The Netherlands). 2004-2009
- Physics Degree. Universitat de Barcelona (Barcelona, Spain). 1998-2004
I have authored 62 refereed articles in high-impact journals (Nature, ApJ, JCAP, MNRAS, A&A), 14 of them as first and lead author. My publications have received more than 2600 citations and form an h-index of 29 (as of January 2021).
Because these numbers evolve rapidly, next you can find links to updated lists of my publications.Updated full publication lists:
NASA-ADS G-Scholar ORCID
ADS (refereed) ADS (1st. author) ADS (by citations)
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-.
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-.
We have currently no specific job openings. Check again later.
Current/former group members and students:
- Eda Vurgun (Ph.D. Candidate, 2020-2023)
- Carlos Celma (B.Sc. Thesis, 2019. Now at: ING, La Palma)
- Marta Rubio (B.Sc. research project, 2018. Now at: DTU, Copenhagen)
- Laura Portos, Andrea Valenzuela (B.Sc. research project, 2017. Now at: UU, Utrecht; AI, UPC)
We always welcome motivated hard-working students and highly qualified researchers to work in our group. See the following (incomplete list of) funding opportunities and research projects, and contact us if you are interested in joining the GAA!
- Ramon y Cajal (Senior tenure-track 5-yr fellowships, Spanish funding; link in ES)
- Beatriz Galindo (Senior/junior tenure-track 4-yr fellowships, Spanish funding; link in ES)
- Marie Curie (Highly competitive 2-yr fellowships, European funding)
- Beatriu de Pinos (Postdoctoral 2-yr fellowships, Catalan funding)
- La Caixa junior leader (Postdoctoral 3-yr fellowships, private funding)
- Juan de la Cierva (Postdoctoral 2-yr fellowships, Spanish funding; link in ES)
- PHAROS STSM (COST Action funds for short visits, 5-90 days)
- ChETEC STSM (COST Action funds for short visits, 5-90 days)
- Ph.D. Thesis grants. FI (Catalan funding) - FPU (Spanish funding) - La Caixa (PhD fellowships)
- M.Sc./B.Sc. Thesis projects in Engineering Physics (TFG/TFM)
- M.Sc./B.Sc. Thesis projects at EEBE (TFG/TFM)
Credit: Jordi José
AND THIS VIDEO
with an overview of some of our research lines:
UPC-EEBE, Av. Eduard Maristany 16 (Office C2.22)
08019 Barcelona, SPAIN.
Sant Ramon de Penyafort 261-269
08930 St. Adria de Besos, SPAIN.