What Happened
- Astrophysicists from the Indian Institute of Technology Kanpur (IIT Kanpur) and the National Centre for Radio Astrophysics (NCRA), Pune, have developed a novel method to estimate distances to pulsars with greater precision.
- The new technique combines two independent radio-wave effects experienced by pulsar signals as they travel through interstellar space: Dispersion Measure (DM) and scatter broadening — eliminating dependence on existing galactic electron density models that are known to carry significant uncertainties.
- The study was published in the Monthly Notices of the Royal Astronomical Society (MNRAS) — one of the world's oldest and most prestigious astronomy journals.
- The method was demonstrated using pulsars toward the Gum Nebula, a large supernova remnant in the southern Milky Way.
- The technique is applicable to hundreds of known pulsars and can also be extended to Fast Radio Bursts (FRBs), improving distance and environment characterisation for these enigmatic extragalactic events.
Static Topic Bridges
Pulsars — Nature, Properties, and Scientific Significance
Pulsars are rapidly rotating neutron stars — the ultra-dense, collapsed remnant cores of massive stars that have undergone supernova explosions. They emit narrow beams of electromagnetic radiation (typically radio waves) that sweep across Earth like a lighthouse beam, producing precisely timed pulses detectable on Earth. Millisecond pulsars spin hundreds of times per second and maintain such extraordinary rotational stability that they rival atomic clocks in precision. This makes pulsars uniquely powerful natural instruments for probing fundamental physics and the interstellar medium.
- A neutron star packs approximately 1.4 solar masses into a sphere ~20 km in diameter — density exceeds nuclear density (~10^17 kg/m³).
- First pulsar discovered: 1967, by Jocelyn Bell Burnell and Antony Hewish at Cambridge (Hewish received Nobel Prize in Physics, 1974).
- Millisecond pulsars are so stable they are used as cosmic clocks in Pulsar Timing Arrays (PTAs) — including the Indian Pulsar Timing Array (InPTA) — to detect nanohertz gravitational waves.
- Pulsar distances: Previously estimated using models of galactic electron density (YMW16, NE2001 models), which carry 20–30% uncertainties in many directions.
- GMRT (Giant Metrewave Radio Telescope) near Pune, operated by NCRA-TIFR, is India's premier radio telescope for pulsar research. Its upgraded version (uGMRT) offers simultaneous multi-band coverage from 120 MHz to 1,460 MHz.
Connection to this news: The new distance measurement method directly improves upon the known weaknesses of current pulsar distance models, enabling more accurate astrophysical inferences from pulsar observations globally.
Interstellar Medium and Radio Wave Propagation Effects
As radio waves from a pulsar travel through the interstellar medium (ISM) — the diffuse gas and plasma filling the space between stars — they experience several propagation effects that encode information about the ISM structure. These effects are observable and scientifically exploitable:
- Dispersion Measure (DM): Lower-frequency radio waves travel slower than higher-frequency waves through ionised plasma. The delay between arrival times at different frequencies (dispersion) is proportional to the integrated free electron column density along the line of sight — the DM.
- Scatter Broadening: Radio waves scatter off density irregularities in the ISM, causing pulse broadening. The degree of broadening scales steeply with distance and is independent of DM in a complementary way.
The new IIT Kanpur-NCRA method exploits the different distance-scaling laws of DM and scatter broadening simultaneously — providing a geometric distance constraint that does not rely on assumed galactic electron density models.
- DM is measured in units of parsecs per cm³ (pc/cm³).
- Scatter broadening scales approximately as the 4th power of wavelength — making low-frequency observations particularly sensitive.
- The uGMRT's wideband receivers (300–750 MHz) are ideally suited for simultaneous DM and scattering measurements.
- The Gum Nebula (used in the study) is a known complicating structure for distance estimates of background pulsars — the new method helps untangle the local ISM contribution.
Connection to this news: The dual-observable technique is specifically designed to work where conventional methods fail — in the direction of complex ISM structures like the Gum Nebula — making it a significant methodological advance.
Indian Radio Astronomy — GMRT and InPTA
India hosts the Giant Metrewave Radio Telescope (GMRT), located near Narayangaon (Pune district), operated by the National Centre for Radio Astrophysics (NCRA), a branch of the Tata Institute of Fundamental Research (TIFR). It is one of the world's largest and most sensitive low-frequency radio telescope arrays. The upgraded GMRT (uGMRT, operational since 2017) significantly enhanced sensitivity and frequency coverage. India's contribution to global pulsar timing science includes the Indian Pulsar Timing Array (InPTA) — a collaborative network involving NCRA, TIFR Mumbai, IIT Roorkee, IISER Bhopal, IIT Hyderabad, IMSc Chennai, Raman Research Institute Bengaluru, and Kumamoto University (Japan).
- GMRT: 30 dish antennas, each 45 m in diameter, spread over ~25 km baseline in Pune, Maharashtra.
- uGMRT upgrade (completed 2017): Seamless frequency coverage 120 MHz–1,460 MHz; ~3× better sensitivity than original GMRT.
- InPTA's major achievement (2023): First Indian contribution to global pulsar timing array evidence for the nanohertz gravitational wave background — a historic detection.
- NCRA-TIFR Pune: Primary institutional home for Indian pulsar research.
- Fast Radio Bursts (FRBs): Millisecond-duration extragalactic radio transients of unknown origin — the new distance method may help constrain FRB host distances.
Connection to this news: The new pulsar distance method is a product of India's world-class radio astronomy infrastructure and the collaborative InPTA research network, adding to India's growing contribution to fundamental astrophysics.
Key Facts & Data
- Institutions: IIT Kanpur (Dept. of Physics, SPASE) and NCRA, Pune (TIFR).
- Key researchers: Dr. Ashish Kumar (NCRA Pune), Prof. Avinash A. Deshpande (formerly RRI Bengaluru), Prof. Pankaj Jain (IIT Kanpur).
- Journal: Monthly Notices of the Royal Astronomical Society (MNRAS).
- Technique: Joint analysis of Dispersion Measure (DM) + scatter broadening for model-independent pulsar distance estimates.
- Study object: Pulsars toward the Gum Nebula (large supernova remnant, southern Milky Way).
- Applicability: Hundreds of known pulsars; extendable to Fast Radio Bursts (FRBs).
- Instrument: uGMRT, Narayangaon, Pune (NCRA-TIFR); 30 dishes × 45 m diameter.
- uGMRT frequency range: 120 MHz–1,460 MHz.
- InPTA gravitational wave detection: 2023 (nanohertz GW background).
- GMRT location: ~80 km north of Pune, Maharashtra.