A Glimpse of a New Transient Source in the Milky Way

A new X-ray source in a Milky Way star cluster has puzzled astronomers. What’s the most likely explanation for this source’s unusual properties?

Things That Go Bump in the Night Sky

plot of X-ray flux for two time periods as well as model fits and residuals

NuSTAR X-ray spectra of MAXI J1848-015 during the outburst (blue) and as the outburst was fading (gold). Model fits are shown as dashed lines. [Pike et al. 2022]

In December 2020, the Monitor of All-sky X-ray Image (MAXI) spotted a new X-ray source from its vantage point on the International Space Station. Though the source — MAXI J1848-015 — was too close to the Sun for many telescopes to observe it, the Nuclear Spectroscopic Telescope Array (NuSTAR) managed to make follow-up observations a few days after the source was discovered, and again a week later.

The observations revealed that the source suddenly flared into view before beginning to fade just five days later. What kind of astrophysical object was responsible for this brief outburst? A team led by Sean Pike (California Institute of Technology) has performed an in-depth analysis of the object’s X-ray spectra to find out.

An X-ray Exploration

Pike and collaborators found that during the outburst, the source’s emission was dominated by soft, low-energy X-rays, peaking around 5 kiloelectronvolts. A week later, as the outburst was fading, harder, higher-energy X-rays surged, and though the source was fainter overall, its emission peaked between 20 and 30 kiloelectronvolts. This transition from soft and bright emission to hard and faint emission is typical for X-ray binaries — systems containing a compact object like a neutron star or black hole that is accreting material from a stellar companion.

illstration of a black hole with an accretion disk and a jet

This artist’s impression shows an outflowing jet of energetic particles and a bright corona, which appears just above an accreting black hole. [NASA/JPL-Caltech]

The authors modeled the spectra obtained during the soft and hard emission states and found further evidence for the X-ray binary scenario. Specifically, the team found that models containing a close-in accretion disk that reflects X-rays from a hot corona near the source fit the data well. Intriguingly, the models also showed evidence that the accretion disk changed as a result of the outburst — the innermost edge of the disk appears to have moved away from the accreting object in the later observations.


Neutron Star or Black Hole?

infrared image of a star cluster

The Spitzer Space Telescope discovered GLIMPSE-C01, the star cluster where MAXI J1848-015 resides, in 2004. [NASA/JPL-Caltech/H. Kobulnicky (Univ. of Wyoming)]

The observations are consistent with a binary system containing a star partnered with either a neutron star or a black hole. But which is it? Pike and collaborators considered several pieces of evidence:

  • Spin: The source spins almost as fast as is physically possible for an object of its size and angular momentum — far faster than even the most quick-whirling neutron stars.
  • Luminosity: Archival X-ray observations of the star cluster where the source is located show that when the object is in a quiescent state, its luminosity is roughly 1,000 times fainter than a typical neutron star.
  • Outburst length: The outburst is short compared to those from most — but not all — other accreting black holes.

Based on these findings, the authors rule that MAXI J1848-015 is most likely an accreting black hole, though some pieces of the puzzle aren’t a perfect fit. One lingering mystery is how an accreting black hole with a close-in accretion disk can have such a low luminosity. Catching another outburst in the act should help illuminate the nature of this intriguing source!


“MAXI and NuSTAR Observations of the Faint X-Ray Transient MAXI J1848-015 in the GLIMPSE-C01 Cluster,” Sean N. Pike et al 2022 ApJ 927 190. doi:10.3847/1538-4357/ac5258