The All-Sky Automated Survey for SuperNovae (ASAS-SN, pronounced “assassin”) regularly scans the sky hunting for transient sources. optical luminosity diagram, shown with orange and yellow markers as it evolves through its outburst, place it within the region dominated by black hole LMXBs (blue markers) throughout the outburst. But a sneaky telescope has now caught one source in the process of waking up.ĪSASSN-18ey’s position on an X-ray vs. In particular, most observations only capture transient LMXBs after they’ve transitioned into an outbursting state. Where do theses outbursts begin? What causes the sudden eruption? What else can we learn about these odd sources? Though theorists have built detailed models of transient LMXBs, we need observations that can confirm our understanding. One type of LMXB, known as a transient/outbursting LMXB, has a peculiar quirk: though they often go undetected in their dim, quiescent accretion state, these sources exhibit sudden outbursts in which the brightness of the system increases by several orders of magnitude in less than a month. In a low-mass X-ray binary (LMXB), the donor star typically weighs less than one solar mass. X-ray binaries come in two main types, depending on the size of the donor star: low-mass and high-mass. The accreting material emits at X-ray wavelengths, providing these systems with their signature emission. In such a system, mass is siphoned off the donor star, forming an accretion disk around the feeding compact object. Some of the most easily discoverable stellar-mass black holes in our own galaxy are those in X-ray binaries: binary star systems that consist of a star in orbit with a compact object like a black hole or a neutron star. According to models, instabilities in this accretion disk can lead to the binary going into outburst. This representation of an X-ray binary shows the accretion disk that surrounds the black hole.

Black hole accretion disk xray