From AAS NOVA: “How a Black Widow Consumes Its Companion”

AASNOVA

American Astronomical Society

1 September 2017
Susanna Kohler

1
Artist’s impression of the optical and X-ray emission surrounding the original “Black Widow” pulsar B1957+20. [NASA/CXC/M.Weiss]

Hanging out in a binary system with a hot millisecond pulsar can be hazardous to your health! A new study has examined how these perilous objects can heat and evaporate away their companions.

2
Panel (a) shows the intrabinary shock and the companion star (the pulsar would lie to the right). Panel (b) shows the companion star and the magnetic field lines funneling into its front pole. [Adapted from Sanchez & Romani 2017]

Predatory Stars

Millisecond pulsars — highly magnetized neutron stars that we detect through their beamed pulses of radiation — lose energy rapidly as they spin slower and slower. When such an object is locked in a binary system with a star or a planetary-mass object, the energy lost by the pulsar can blast its companion, causing it to evaporate.

Such systems, termed “black widows” in an acknowledgement of how the pulsar effectively consumes its partner, show optical emission revealing their strong heating. We hope that by studying these systems, we can learn more about the properties of the energetic winds emitted by pulsars, and by measuring the companion dynamics we can determine the masses of the pulsars and companions in these systems.

3
Different geometries for the companion’s magnetic field lines can alter the resulting light curve for the system. [Sanchez & Romani 2017]

How Heating Happens

Past models of black widows — necessary to interpret the observations — have generally assumed that the companion’s evaporation was due only to direct heating by the energetic gamma-ray photons emitted by the pulsar. This scenario, however, doesn’t successfully reproduce some of the quirks we’ve observed for these systems, such as very large temperatures and asymmetric light curves.

This picture also ignores the fact that much of the pulsar’s spin-down energy — the energy lost as it gradually spins slower and slower — is carried away by not just the gamma-ray photons, but also a magnetized wind of electrons and positrons. Two scientists at Stanford University, Nicolas Sanchez and Roger Romani, asked the following: how could the particles in the pulsar wind contribute to the heating of a black widow’s companion?

4
The authors’ heating model, as fit to PSR J1301+0833. [Sanchez & Romani 2017]

A Shock Assists

Sanchez and Romani’s alternative model relies on the fact that somewhere between the pulsar and its companion lies an intrabinary shock — the collision point between the pulsar’s relativistic wind and the companion’s ordinary, baryonic wind. The shock is anchored to the companion via magnetic fields, which provides an entry point for shock particles to be funneled along the magnetic field lines onto the companion’s surface. These energetic particles, in addition to the direct irradiation by the pulsar’s photons, cause the heating of the companion that results in its evaporation.

Sanchez and Romani show via simulations that this model can reproduce the observed light curves of several known black widow systems — including the strange features that the direct-heating model didn’t account for. They then use their model to make estimates for the masses of the pulsars and companions in these systems.

The authors caution that this model is still incomplete, but it illustrates that other sources of heating are important to consider in addition to heating by photons. Applying this and similar models to more black-widow systems will surely help us to better understand how these predatory compact stars cause their companions’ ultimate demise.

Citation

Nicolas Sanchez and Roger W. Romani 2017 ApJ 845 42. doi:10.3847/1538-4357/aa7a02

See the full article here .

Related Journal Articles
See the full article for further references comlete with links.

Please help promote STEM in your local schools.

STEM Icon

Stem Education Coalition

1

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009