Explain what X-ray binary pulsars are. How does studying secondary emission help astronomers probe structures that cannot be resolved spatially?

An X-ray binary pulsar is a highly magnetised neutron star (a pulsar) and a companion star held together by gravity [1]. In October 2026 the Raman Research Institute (RRI) studied Centaurus X-3. Its work shows that secondary emission can reveal structures that no telescope can image directly [1].

X-ray binary pulsars: nature and working

• Neutron star: extremely dense, roughly the Sun's mass packed into a ball the size of Bengaluru [1].
• Accretion: the pulsar's gravity pulls in matter thrown off by the companion, and this infalling matter gives off X-rays [1].
• Pulsation: primary emission leaves from the magnetic poles and sweeps past the detector with every rotation, like a lighthouse beam [1].
• HMXBs: in high-mass X-ray binaries the companion has more than 10 solar masses, and the pulsar usually captures its stellar wind directly. Cen X-3, about 24,000 light years away, is one of the first X-ray pulsars discovered [1]. RRI has studied another HMXB, LMC X-4, using AstroSat's LAXPC instrument [2].

Companion star --(wind / disk)--> Accretion stream --> Magnetic poles --> Pulsed X-rays

Fig: How matter flows in an X-ray binary pulsar

How secondary emission reveals unresolved structures

• The limit: at these distances the whole system looks like a single point of light, so a disk or stream cannot be photographed [1].
• Re-emitted light: nearby structures absorb primary X-rays and give them off again. Possible emitters are the neutron star's surface, the companion's surface, the stellar wind and the accretion flow [1].
• Timing test: RRI analysed four emission lines. Their strength rises and falls with the pulsar's spin, so whatever emits them must co-rotate with the pulsar. The companion and its wind do not, so they are ruled out [1].
• Blocking signature: when the rotating accretion stream passes in front of the pulsar, primary emission falls and secondary emission rises [1].
• Inference: an accretion stream can exist only if there is an accretion disk, so this is evidence of disk-fed accretion, which is rare in HMXBs [1].
• Wider use: the method also helps estimate the system's orientation and magnetic field strength, and Cen X-3's emission models must now include the disk [1].
• Independent check: RRI also built POLIX on XPoSat, which measures X-ray polarisation at 8–30 keV and can test the same geometry another way [3].

Secondary emission lets astronomers work out the structure of systems they cannot see, using timing and spectral lines. Combining it with AstroSat timing and XPoSat polarimetry would make the results stronger. It would also add to India's own capability in frontier space science, as GS-III's theme of indigenisation of technology envisages.