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| Objects like GRB 150309A tend to be located deep within galaxies. Photo Credit: Graham Holtshausen |
An international group of scientists has presented the results of a detailed spectral analysis of the instantaneous and residual X-ray emission (afterglow) from the intense two-episode dark gamma-ray burst GRB 150309A. The researchers' task was to determine the nature of the instantaneous emission and the composition of the jet ejected in the burst. In addition, based on optical and X-ray spectral analysis of the energy distribution, the researchers performed modeling of the parent galaxy of GRB 150309A to study the surrounding interstellar medium in which this outburst occurred. The results of the analysis are presented in a paper published in the journal Astronomy and Astrophysics.
A bright flash GRB 150309A lasting about 52 seconds was detected on March 9, 2015, by the Gamma-ray Burst Observatory of the Fermi Gamma-ray Space Telescope, a space observatory in low Earth orbit. The event consisted of two bursts: about 200 seconds after the first, more powerful burst, an episode of faint and quiet emission followed.
Despite the strong gamma-ray emission, optical observations with the BOOTES (Burst Observer and Optical Transient Exploration System) and GTC (Gran Telescopio Canarias) telescopes were inconclusive: only the parent galaxy of the outburst signal was detected at optical wavelengths. The X-ray afterglow of GRB 150309A was detected about 5.2 hours after the outburst by the CIRCE instrument installed on the GTC at the Spanish La Palma Observatory.
The optical inaccessibility under intense gamma-ray emission and the intense red X-ray afterglow detected in the near-infrared with CIRCE led scientists to suggest that GRB 150309A belongs to a subclass of dark bursts.
"Gamma-ray bursts are called dark bursts when, despite intense gamma-ray emission, their optical afterglow is minimal or absent. The absence or weakness of the optical afterglow contrasts with the typical behavior of GRBs, where a gamma-ray burst is followed by bright optical emission. The term "dark" underscores the difficulty of detecting and studying these bursts in the optical regime of electromagnetic waves; revealing the properties of such bursts requires multi-wavelength observations", explains Alberto J. Castro-Tirado, Research Leader, Professor at the Institute of Astrophysics of Andalucía (Spain).
Objects like GRB 150309A are usually found in the depths of galaxies characterized by a high concentration of cosmic dust, i.e., presumably in the fading stage. According to Maria Gritsevich, Associate Professor at the University of Helsinki, Senior Researcher at the Ural Federal University, research participant and co-author of the paper, the host galaxy from which GRB 150309A originated has unique characteristics.
"This galaxy is described as very dusty, with deeply embedded gamma-ray emission. The high dust content and active star formation processes probably account for the complex environment, which in turn is indicated by the high gas density in this galaxy and its special attenuation properties", describes Maria Gritsevich.
According to the scientists' conclusions, GRB 150309A is one of the most intense dark gamma-ray bursts known to date, and the most likely cause of its obscuration is the high concentration of dust in the parent galaxy. Spectral analysis of the gamma-ray emission showed that the jet ejected in the gamma-ray burst has a hybrid composition with a mixed dominance of matter and magnetic field. As Maria Gritsevich explains, the causes of this burst could be diverse, including the collapse of a star, a merger of cosmic objects, or a combination of both.
Detecting an object with a very red X-ray afterglow is fundamental.
"GRB 150309A provides valuable insights into the early Universe, in particular the environment of gamma-ray bursts and the conditions in young galaxies. The detection of the red afterglow of GRB 150309A provides a powerful tool for understanding the dusty environments associated with the early evolution of galaxies. In addition, our study provides a good starting point for future observations of similar but fainter afterglows of dark gamma-ray bursts with larger telescopes", emphasizes Alberto Castro-Tirado.
The research group included scientists from Spain, the United Kingdom, Germany, India, Ireland, Italy, China, Russia, the United States, Ukraine, Finland, the Czech Republic, South Korea, and Germany.
Reference: Gamma-ray bursts (GRBs) are the sudden and most powerful gamma-ray bursts in the Universe, occurring at great cosmological distances from the Solar System. A few seconds of such a burst releases as much energy as our Sun would release in 10 billion years of luminosity. Gamma-ray bursts can be divided into two families: long bursts (lasting more than two seconds) and short bursts (lasting less than two seconds). The initial burst is usually followed by a long-lived and gradually fading afterglow.
Short-duration GRBs are thought to result from the merger of compact objects such as neutron stars or black holes. The parent galaxies of short-duration GRBs are usually elliptical or lenticular galaxies that have exhausted most of their interstellar gas and dust, limiting the formation of new stars.
Long gamma-ray bursts are thought to occur in regions of star formation as a result of the collapse of the core of a massive star, when a black hole or neutron star forms, followed by the emission of a powerful gamma-ray burst.
Recently, however, with the development of observational tools, hybrid events have been observed, including long-duration merger gamma-ray bursts and short-duration collapsar gamma-ray bursts. These events, which combine characteristics of both traditional families, are providing new insights into the nature of gamma-ray bursts.
Scientists suggest that the optical dimming of gamma-ray burst afterglows may be caused by the deceleration of a bolide in a rarefied environment, or by dust in parent galaxies.
Published in journal: Astronomy and Astrophysics
Title: Revealing the characteristics of the dark GRB 150309A: Dust extinguished or high-z?
Authors: A. J. Castro-Tirado, R. Gupta, S. B. Pandey, A. Nicuesa Guelbenzu, S. Eikenberry, K. Ackley, A. Gerarts, A. F. Valeev, S. Jeong, I. H. Park, S. R. Oates, B.-B. Zhang, R. Sánchez-Ramírez, A. Martín-Carrillo, J. C. Tello, M. Jelínek, Y.-D. Hu, R. Cunniffe, V. V. Sokolov, S. Guziy, P. Ferrero, M. D. Caballero-García, A. K. Ror, A. Aryan, M. A. Castro Tirado, E. Fernández-García, M. Gritsevich, I. Olivares, I. Pérez-García1, J. M. Castro Cerón, and J. Cepa
Source/Credit: Ural Federal University | Alexander Zadorozhny
Reference Number: astr040824_01