By Maria G. Dainotti
Gamma-ray bursts (GRBs) are some of the most energetic events known in astrophysics. In just a few seconds, a typical burst can release as much energy as our sun will emit over its entire 10 billion-year lifetime, so it is not surprising that GRBs have been detected billions of light years away. If the intrinsic brightness of GRBs were known, a comparison with their detected brightness would yield their effective distance, and given their observed recession velocity or redshift, GRBs could then be used as accurate distance estimators for cosmology. This would enable researchers to arrive at solid estimates for the distances of all manner of extremely faint, old objects, such as very early galaxies.
Indeed, GRBs are the only possible distance estimators known over such a great redshift interval (approximately 0.0085-9.4), which is critical to our understanding of the details of cosmological evolution over time - e.g., the characteristics of dark energy from the epoch of reionization to the present.
Unfortunately, as the SWIFT satellite has revealed, GRBs do not present uniform features: all their critical parameters vary widely over orders of magnitude. As the saying goes: if you've seen one GRB, you've seen one GRB. This applies not only to the prompt emission (the main event in the gamma rays), but also to the extended X-ray afterglow phase (the counterpart, which follows the prompt emission and can occur in several wavelengths). To complicate matters further, no single clear explanation as to their physical nature exists. Possible origins range from the collapse of massive stars, to magnetars in the process of spinning down, to the collapse of supernovae, to binary mergers.
Over the past decade, efforts have been made to find correlations between some characteristic parameters. Previous efforts looked for relationships among two parameters in the afterglow, such as the rest frame end time of the plateau phase, called Ta , and its corresponding luminosity, La.
Above: Swift satellite data for 101 GRBs with the different types of morphologies (red are for a category called ‘short extended emission’). The grouping shows some degree of scatter, which needs to be reduced if we want a set of more standard GRBs. At least this relation is a good start for identifying GRB subsets (from Determination of the Intrinsic Luminosity Time Correlation in the X-ray Afterglows of GRBs (2013)).
Our idea was to search for and identify a tighter correlation by introducing a third parameter, the peak prompt luminosity, Lpeak, in order to further reduce the scatter of this correlation. This new 3D correlation La-T a-Lpeak forms a plane in parameter space (and ideally, we would like to find another plane that contains all the bursts).
Existing correlations are very noisy, with GRBs showing a large dispersion related to the best fit of the correlations, very likely due to the fact that GRBs do not come from the same type of objects. For example, in sick people, a cough might signal a variety of ills, from a chest infection to a mild throat irritation to an allergy. Bunching together all patients with a cough and looking for correlations between cough frequency and some other health parameter (fever, stuffy nose) will yield correlations with wide scatter. Conversely, tighter correlations can be seen in objects which have more in common. While in the aviary at the zoo we can look for different species of birds that might be related to each other by examining different characteristics—plumage, wing span, and feet—to confirm a match. Similarly, isolating a single GRB class (as much as possible) gives hope of identifying a type-specific sample where correlations will be much clearer and hence provide better cosmological information and constraints on GRB emission mechanism scenarios.
Above: Adding in a third parameter (the peak luminosity of the prompt phase), and testing a particular GRB subclass. Choosing only the class of long GRBs unassociated with supernovae (points shown in gray) dramatically reduces the scatter, as shown in the plot of 122 long GRBs (all dots together).
In this paper, we have taken all long GRBs (i.e., having a duration > 2 s) from Swift with measured redshifts, and removed all of those classified as X-ray flashes (with the ratio between X-ray and gamma-ray flux in their spectra greater than 1), having associated supernovae, or showing a steep decline in their X-ray afterglows. The resulting high-quality data sample shows a correlation between Lpeak,Ta, and La. This correlation is more than twice as tight as the corresponding one for the full sample. The authors also controlled through Monte Carlo simulations that the reduced scatter of this correlation is not randomly produced.
Building on previous work (Dainotti et al. 2013a and 2015b) done under the supervision of Vahe Petrosian (which is related to even earlier work on selection bias (Efron & Petrosian 1992)), it has also been directly established that the parameters used in this study and their correlations (Lpeak-La and La-Ta) are intrinsic to the physical systems studied, and not the result of selection or observational effects. Thus, it naturally follows that the 3D correlation is intrinsic to GRBs and it is not the result of selection bias.
To be more explicit in what this tighter correlation allows us to do: from the location of the GRB point in the plane in the above figure, we can derive the luminosity distance as a function of three quantities, the flux at the end of the plateau, the peak flux in the prompt emission, and the rest frame end time of the plateau emission itself. The tighter the correlation is between these three, the less scatter among them, and the more precise the estimation of the luminosity distance ultimately will be.
To sum up - through careful study, what we find is that identifying and isolating class-specific GRB samples raises the possibility of significantly reducing the scatter in GRB correlations, which together with more solid and testable physical modeling for understanding these enigmatic and powerful events, opens the door wide for using GRBs as reliable and powerful cosmological tools.
------------- Further reading:
NASA Press Release: Swift Gamma-Ray Bursts: A 3D Step Toward Standard Candles (Presented at American Astronomical Society June 2016 meeting)