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NEW MACRONOVA'S MODEL


Image: The sequence illustrates the macronova model for the formation of a short-duration gamma-ray burst. 1. A pair of neutron stars in a binary system spiral together. 2. In the final milliseconds, as the two objects merge, they kick out highly radioactive material. This material heats up and expands, emitting a burst of light called a macronova. 3. The fading fireball blocks visible light but radiates in infrared light. 4. A remnant disk of debris surrounds the merged object, which may have collapsed to form a black hole Credit: NASA, ESA, and A. Feild (STScI)

A macronova (also called a 'kilonova' or an 'r-process supernova') occurs when two neutron stars or a neutron star and a black hole merge. It is a near-infrared/optical transient powered by the radioactive decay of heavy elements synthesized in the ejecta (~10-4-10-1 Msun with velocities ~ 0.1-0.3c) of a compact binary merger. Strong electromagnetic radiation is emitted due to the decay of heavy r-process ions that are produced and ejected fairly isotropically during the merger process-similar to a faint, short-lived supernova.


In the final milliseconds before the explosion, the two stars merge into a death spiral that kicks out highly radioactive material. This material heats up and expands, emitting a burst of light. The resulting "kilonova" is about 1,000 times brighter than a regular nova, which is caused by the eruption of a white dwarf.

The inspiral and merging of these compact objects are thought to be a strong source of gravitational waves (GWs). Macronova is also thought to be the progenitor of short gamma-ray bursts and the predominant source of stable r-process elements in the Universe.

These images taken by NASA's Hubble Space Telescope reveal a new type of stellar explosion produced from the merger of two compact objects. Hubble spotted the outburst while looking at the aftermath of a short-duration gamma-ray burst, a mysterious flash of intense high-energy radiation that appears from random directions in space. Short-duration blasts last at most a few seconds. They sometimes, however, produce faint afterglows in visible and near-infrared light that continue for several hours or days and help astronomers pinpoint the exact location of the burst.
In the image at left, the galaxy in the center produced the gamma-ray burst, designated GRB 130603B. The galaxy, cataloged as SDS J112848.22+170418.5, resides almost 4 billion light-years away. A probe of the galaxy with Hubble's Wide Field Camera 3 on June 13, 2013, revealed a glow in near-infrared light at the source of the gamma-ray burst, shown in the image at top, right. When Hubble observed the same location on July 3, the source had faded, shown in the image at below, right. The fading glow provided key evidence that it was the decaying fireball of a new type of stellar blast called a kilonova. Credit: NASA, ESA, N. Tanvir (University of Leicester), A. Fruchter (STScI), and A. Levan (University of Warwick)

The r-process is a nucleosynthesis process that occurs in supernovae and is responsible for the creation of approximately half of the neutron-rich atomic nuclei heavier than iron. The process entails a succession of rapid neutron captures (hence the name r-process) by heavy seed nuclei, typically 56Fe or other more neutron-rich heavy isotopes.

In a recent paper (Kisaka, Ioka & Nakar 2016, ApJ) the authors analyze the macronova observed as a infrared excess  several days after short gamma-ray burst GRB 130603B. They propose a new emission model in which the X-ray excess gives rise to the simultaneously observed infrared excess via thermal re-emission.

Schematic picture of the X-ray-powered model.


The X-ray-powered model explains the macronova with smaller ejecta mass. The new model is not based on the r-processes and can explain the X-ray and infrared excesses with a single energy source by the central engine like a black hole.


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