Payaswini SaikiaAssociate Research Scientist, Yale University

Research

Accretion and jets from stellar-mass to supermassive black holes

I study how matter falls onto black holes, neutron stars and white dwarfs, and how part of that energy is launched back out as jets and winds. Combining radio, infrared, optical, ultraviolet and X-ray observations with modelling, I test whether the same physics holds across the full range of black hole mass.

Across the spectrum

The observatories on the ground and in space that my work draws on, from long radio waves to X-rays.

Me at the Westerbork Synthesis Radio Telescope, the Netherlands

Radio> 1 mm

ESO’s La Silla Observatory, Chile, where I observed with SOFI on the NTT in 2020

Infrared1 mm – 0.7 µm

The Palomar 200-inch Hale Telescope, California, during my 2026 observing run

Optical700 – 400 nm

Artist’s view of NASA’s Neil Gehrels Swift Observatory

Ultraviolet400 – 10 nm

Artist’s view of NASA’s Chandra X-ray Observatory

X-ray10 – 0.01 nm

Radio, infrared and optical photos are my own. Swift and Chandra images: NASA.

Research themes

From stellar-mass black holes in our own Galaxy to supermassive black holes in distant galaxies, my research follows how matter falls in, how jets and winds are launched, and how they shape their surroundings.

X-ray binaries and supermassive black holes on the Fundamental Plane
X-ray binaries (blue) and supermassive black holes (red) fall on one Fundamental Plane linking radio luminosity, accretion luminosity and black hole mass. Adapted from Saikia et al. 2015, MNRAS, Fig. 2.

Black Hole Mass Scaling

Black holes ranging from a few to billions of solar masses appear to feed and launch jets in much the same way. This apparent scale invariance allows each population to reveal what the other cannot. Millions of known supermassive black holes provide statistics that the few dozen known stellar-mass black holes cannot match. Stellar-mass black holes, in turn, let us follow entire cycles of activity over days to months—processes that could take millions of years in an active galaxy.

My research tests how far this shared behaviour extends, from accretion discs to jets. I discovered the optical Fundamental Plane, which links black hole mass to jet power, traced by radio emission, and accretion power, traced by the [O III] emission line, bringing X-ray binaries and active galaxies onto a common plane (Saikia et al. 2015). We have since found that accretion discs re-emit X-rays as optical light following a common relation across the black hole mass range (Cho et al. 2026). These similarities extend to jets: some stellar-mass black holes launch ejections with Lorentz factors of about 8, reaching speeds comparable to those in jets from supermassive black holes (Zhang et al. 2026).

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Colour–magnitude diagram of Swift J1910.2−0546
Swift J1910.2−0546 becomes bluer as it brightens, following a heated disc; the black line marks blackbody temperatures. Adapted from Saikia et al. 2023, ApJ.

Accretion Physics & Variability

Accretion—the flow of matter onto a compact object—powers some of the brightest sources in the Universe. Yet how accretion discs transport matter, become unstable and sustain activity at very low luminosities remains poorly understood. Black hole and neutron star X-ray binaries let us follow these processes through bright outbursts, repeated brightenings and long periods of quiescence.

My research follows how accretion changes across this full cycle, using nearly three decades of optical monitoring with SMARTS and the Las Cumbres Observatory (LCO) network, combined with X-ray and radio data. In Aquila X-1, we showed that the optical rise consistently precedes the X-rays across 29 outbursts over 27 years, helping us trace how changes in the disc travel inwards. This evolution does not always end with a steady decline: in Swift J1910.2−0546, we found seven reflares and a mini-outburst, consistent with heating and cooling fronts repeatedly travelling through the disc (Saikia et al. 2023). We also investigate what happens once these brightenings end. In A0620−00 and MAXI J1820+070, optical colours, rapid variability and long-term changes in brightness allow us to probe the faint accretion flow that persists in quiescence.

Featured papers, by stage of activity

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Schematic of jet and disc viewed at different inclinations
How viewing angle changes the infrared excess from a beamed compact jet compared with the accretion disc. Adapted from Saikia et al. 2019, ApJ.

Jet Physics & Disc–Jet Coupling

Relativistic jets carry a large share of the energy released by accretion back into their surroundings. Yet how they are launched, how fast they move and what switches them on and off remain open questions. Their speeds are especially difficult to measure because relativistic beaming changes how bright they appear depending on our viewing angle. Black hole X-ray binaries let us follow their evolution over days, as changes in the accretion flow cause jets to disappear and return.

I have developed new ways to measure jet speeds. Using the optical Fundamental Plane to estimate the intrinsic radio brightness of blazars, I showed that their Lorentz factors follow a power-law distribution (Saikia et al. 2016). In X-ray binaries, I showed that the infrared excess from the jet depends on our viewing angle, and used this to constrain compact-jet Lorentz factors, finding values between 1.3 and 3.5 (Saikia et al. 2019). We also track how jets respond to changes in the disc. In 4U 1543−47, we found that a compact jet briefly reappeared during the soft-intermediate state (Russell et al. 2020). Changes can also become visible just before a jet disappears: in MAXI J1807+132, we observed an optical polarization rotation of nearly 90 degrees shortly before the jet switched off (Rout et al. 2025).

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15-GHz radio luminosity function of nearby LLAGN
The 15-GHz radio luminosity function of nearby low-luminosity AGN, with a break in slope towards the faintest nuclei. Adapted from Saikia et al. 2018, A&A.

Supermassive Black Holes & Galaxy Evolution

Most supermassive black holes in nearby galaxies, including our own, are only weakly active. These faint nuclei offer a way to study how black holes feed between their brightest phases and how their activity relates to the evolution of their host galaxies. Finding them can be difficult, because their light is easily hidden by dust or overwhelmed by stars. High-resolution radio imaging lets us see through the dust and isolate compact emission from the central black hole.

I use this approach to uncover black hole activity that other observations miss. With deep VLA observations at 15 GHz, I detected compact radio cores in about 60% of previously undetected nearby low-luminosity active galaxies, and found a break in their radio luminosity function at the faintest end (Saikia et al. 2018). Extending this census to the 280 nearby galaxies in the LeMMINGs survey, we find that up to 30% host a radio-active nucleus (Williams-Baldwin et al. 2026). I also investigate how this activity appears in more distant galaxies. In the JWST North Ecliptic Pole field, our VLBA imaging revealed 12 compact, black-hole-powered cores among 106 radio sources, uncovering clear signs of accretion even in radio-quiet galaxies (Saikia et al. 2025). Through the PEARLS programme, we use JWST to identify the galaxies hosting faint radio sources and examine how black hole activity relates to their transitions between star formation and quiescence (Willner et al. 2026; Bowling et al. 2026).

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MeerKAT radio image of Circinus X-1 inside its natal supernova remnant, with shocks and bubbles labelled
MeerKAT 2.6 GHz image of the neutron star X-ray binary Circinus X-1 inside the supernova remnant in which it was born. Shocks driven by its outflows and bubbles inflated by its jets are labelled. Adapted from Cowie et al. 2026, MNRAS, Fig. 1.

Outflows, Shocks & Feedback

Jets and winds do not just carry energy away from black holes and stars; they push into the surrounding gas, heat it and drive shocks. This feedback can shape star formation and the growth of galaxies, yet how much energy outflows actually return to their environment is hard to measure. Shocks offer a way in, because the radio emission from shocked gas records the energy and particles that outflows deposit.

With MeerKAT, we made the first radio detection of the bow shock around the X-ray binary Vela X-1, and with ASKAP we expanded the sample of radio-detected bow shocks around runaway massive stars (van den Eijnden et al. 2022a, 2022b). In Circinus X-1, we traced precessing jets breaking out of the neutron star’s natal supernova remnant (Cowie et al. 2025, 2026; Gasealahwe et al. 2025). This interaction extends to galactic scales: in the dwarf galaxy NGC 4395, we found a weak radio jet aligned with shocked gas within about 10 parsecs of the black hole (Nandi et al. 2023).

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