Consider these cosmic phenomena: 1. Black holes emit X-rays directly from their event horizon 2. Pulsars are rapidly rotating neutron stars. Which of the above is/are correct?

Correct — B. Statement 1 is false. Nothing, not even light, escapes from inside a black hole's event horizon. The X-rays astronomers detect come from gas in the accretion disc outside the horizon, heated by friction as it spirals inward.

Statement 2 is true. A pulsar is a highly magnetised, rotating neutron star whose beams from its magnetic poles sweep past Earth like a lighthouse, so it is seen as regular pulses.

Why the others are wrong

• (a)
  Statement 1 only — Statement 1 is false: no radiation leaves the event horizon itself, and the X-rays come from hot matter just outside it. This option also drops statement 2, which is true.
• (c)
  Both 1 and 2 — Both fails on statement 1. The event horizon is the boundary from inside which nothing escapes, so it cannot be the X-ray source.
• (d)
  Neither 1 nor 2 — Statement 2 is true: pulsars are rotating, highly magnetised neutron stars, first found by Jocelyn Bell and Antony Hewish in 1967. So 'neither' cannot be right.

A black hole is a region whose gravity is so strong that nothing crossing its event horizon can come back out, so it is found indirectly. Gas falling towards it forms an accretion disc, and friction in that fast-orbiting gas heats it until it glows in X-rays.

A neutron star is the collapsed, extremely dense core left by a supernova. When it spins and beams radiation from its magnetic poles, it is seen as a pulsar.

Key facts

• Nothing, not even light, escapes from inside a black hole's event horizon.
• Black-hole X-rays come from matter heated in the accretion disc outside the horizon.
• A pulsar is a highly magnetised, rotating neutron star that emits beams from its magnetic poles.
• The first pulsar was discovered in 1967 by Jocelyn Bell, working with Antony Hewish, who shared the 1974 Nobel Prize in Physics with Martin Ryle.

Common traps

• Accepting statement 1 because black holes are known X-ray sources, when the X-rays come from outside the horizon.
• Confusing pulsars with quasars: a quasar is powered by accretion onto a supermassive black hole, a pulsar is a spinning neutron star.

No directly related past PYQ was found.