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New Cosmological Standard Candles from Gamma Ray Burst Correlations

Final Report Summary - COSMOLOGICAL CANDLES (New Cosmological Standard Candles from Gamma Ray Burst Correlations)

The study of cosmic Gamma-Ray Bursts (GRBs), the most powerful and among the most distant phenomena ever observed so far, is of fundamental importance for modern astrophysics and cosmology. Due to their huge luminosity, GRBs are observed at distances (redshift) much farther than Type Ia Supernovae (SNe Ia), which are the most robust probes of the accelerating Universe. This discovery was awarded the Nobel Prize in 2011 and implies the existence of a “Dark Energy”. Several proposed models for Dark energy can be discriminated only with astrophysical probes with distances extending much beyond those of SNe-Ia, limited to redshift of 2. Thus, GRBs, which can be observed even beyond the epoch when the first stars start re-ionizing the intergalactic medium (i.e. up to redshift 10 or even more, that is a few hundred million years after the Big-Bang) are potentially a unique probe to test the evolution of Dark Energy. To this end, it is crucial to understand if they can be considered standard candles (astronomical objects whose luminosity is known or can be derived from other distance-independent observables) as SNe Ia and thus can be used as precise distance indicators. Indeed, GRBs seem to be far from standard candles, with their luminosities spanning over eight orders of magnitude. Thus, discovering universal relations linking observable GRB properties and their luminosity or radiated energy is a crucial step toward using them as precise distance indicators for cosmological applications. Moreover, investigating relations between important GRB characteristics will shed light not only on their use as possible standard candles, but can also provide new constraints for the physical models of the GRB explosion mechanism.
The project’s main aim is to extend in a tridimensional domain and in a multi-wavelength range, from 0.3 keV to 300 GeV, the luminosity-time correlation (Lx-Tx) for GRBs (known in the literature as the “Dainotti relation”). This correlation is between the luminosity at the end of the plateau phase, Lx and the end time of the plateau phase itself. The extension to 3D relation with the additional variable of the peak luminosity of the prompt emission, Lpeak, has brought to the identification a subclass of GRBs with well-defined properties and bias-free. Namely, this subclass is corrected by redshift evolution and selection effects due to instrumental threshold through refined statistical methods. This subclass will be the candidate for GRB standard candles. The bias-free Lx-Tx-Lpeak correlation might be the basis for a new independent and powerful cosmological tool. In addition, the improved characterization of this correlations will provide a fundamental step forward in shedding light on the still poorly understood physics of GRB “prompt” (i.e. the burst of gamma-rays itself) and “afterglow emission” (i.e. the multi-wavelength fading and weaker emission which follows the burst). Finally, the results and expertise from this project were used to further refine the science case and scientific requirements for the THESEUS (Transient High-Energy Sky and Early Universe Surveyor) space mission concept, a project developed by a large International collaboration led by L. Amati aimed at fully exploiting GRB for cosmology and providing a fundamental contribution to the newly born field of multi-messenger astrophysics (http://www.isdc.unige.ch/theseus/). THESEUS has been selected by ESA for a Phase 0/A study for next M5 space mission.
The main results achieved during the Fellowship have been featured in publications (refereed papers, conference proceedings, and additional papers in preparation), presented at several international conferences and meetings, advertised through press-releases and TV news channel broadcasting (e.g. national Italian television news, RAI, TG2), and soon, in June 2018 will be awarded with the International Princess Sichelgaita Prize, bestowed by the Presidency of the Italian Republic, are summarized in the points below.
1. Prompt-afterglow correlations
The discovery of a unique luminosity-time (hereafter LT) correlation which spans from prompt to afterglow is presented in [1]. Within this framework, the Researcher updated the Lx-Tx relation and the Lpeak-Tγ relation. With a sample, which is doubled compared to previous work it is possible to explain the presence of an offset between the two LT relations (those ones related to the prompt and the afterglow). The Researcher applied the Efron and Petrosian method and demonstrated that the Lpeak-Lx relation is not due to selection bias, but it is intrinsic to the physics of GRBs.
2. The discovery of the fundamental plane.
This discovery opened the way to a new 3 parameter correlation, namely the Lx-Ta-Lpeak correlation [3] which is also intrinsic to the physics of GRBs because both Lx-T*a and Lpeak-Lx are intrinsic observables. The LX-Ta-Lpeak correlation has an intrinsic dispersion 54% smaller than the (LX, Ta) one, and therefore will result in an improved correlation that can allow the use of GRBs as standard candles. This result has been chosen as highlight from the American Astronomical Society and as a press conference at AAS [4] and they have been mentioned in Scientific American, as highlight research of Stanford KIPAC 2016. The update of this correlation with more data has shown that plane on which short GRBs with extended emission (SEE) lie is statistically different from the gold fundamental plane. The gold fundamental plane has an intrinsic scatter smaller than any plane derived from the other sample categories. Thus, the distance of any particular GRB category from this plane becomes a key parameter. We computed the several category planes with T a as a dependent parameter obtaining for each category smaller intrinsic scatters (reaching a reduction of 24% for the long GRBs). The fundamental plane is independent from several prompt and afterglow parameters. Results of this analysis have been published in ApJ [9] and have been featured by INAF press release, by Stanford KIPAC blog, by Nature Index and broadcast via the National TV news channel, see in the dissemination section for details.
3. The magnetar model and the Galactic magnetar population
Within the context of the magnetar model, a millisecond spinning neutron star with high magnetic field, [2] debated on the reliability of this model as the correct interpretation for the X-ray plateaus. The GRB-magnetar model in its present form is safe only if GRB magnetar progenitor should be different from Galactic magnetars and should be considered super-magnetars. The Researcher further investigated the explanation of X-ray plateaus within the magnetar model. Findings are reported in a paper in preparation for ApJ [10]. The discovery is that short GRBs with extended emission follow a different magnetic field, spin period plane, thus the distinction among the planes obtained with the fundamental plane is physical indeed within this theoretical model.
4. The study of the Lx-Ta relation and GRBs associated with SNe.
To look for a standard candle in [8] the Researcher revealed that the subsample of long GRBs associated with SNe (LONG-SNe) presents a very high correlation coefficient for the Lx-Tx correlation and a slope of roughly -2. This possibly suggests that this category might not require a standard energy reservoir in the plateau phase unlike the long GRBs for which no SNe has been observed (LONG-NO-SNe). Therefore, this analysis may open new perspectives in future theoretical investigations of the GRBs with plateau emission and associated with SNe. Namely, a different theoretical scenario from the magnetar and the accretion for which the energy reservoir of the plateau is not constant can be found. The Researcher also discusses how much this difference can be due to the jet opening angle effect.
5. Review on GRB prompt-afterglow correlations
The mechanism responsible for the afterglow emission of GRBs and its connection to the prompt is still a debated issue. Thus, the Researcher presents an overview of the afterglow and prompt-afterglow two parameter correlations, their physical interpretations, and their use as redshift estimators, as possible cosmological tools and on how they are corrected for selection biases [10].
6. The Lx-Ta relation and the decay index in the afterglow emission
In the context of reducing the scatter of the Lx-Tx correlation, [5] investigated the indices α after the plateau phase for a sample of 176 GRBs detected by Swift. They concluded that the LT correlation for the low and high luminous GRBs seems to depend differently on the α parameter, thus implying a diverse density medium dominating the afterglow emission.
7. The Fermi-LAT analysis
The Fermi-Large Area Telescope (LAT, 20 MeV-30 GeV) shows long lasting high energy emission in many GRBs similar to the X-ray afterglows observed by Swift. Some of the lightcurves (LCs) show a late time flattening reminiscent of the X-ray plateaus. The Researcher investigates how many GRBs seen by LAT show a plateau similar to the observed X-ray and if the mechanism that powers the gamma- and X-ray plateaus is the same. The Researcher has pioneered a thorough analysis of LAT LCs present in all Fermi LAT sources observed so far using the new event data algorithm PASS 8; has found that among the 120 GRBs observed by the LAT in 4 cases it is possible to identify a secure plateau emission; has also investigated if the LAT GRBs are generated via synchrotron emission in the external shock (ES) and verified the so called closure relations α=(3*β-1)/2 between the temporal decay index α of the LCs and the energy spectral index β above 100 MeV. This relation serves as a rough indication that the observed radiation is being produced in the ES. The paper is in preparation in ApJ.
8. The Epeak-Eiso relation
Regarding the Epeak-Eiso or Epeak-Lpeak intrinsic correlation the Researcher has investigated so far the data which present a Band function and a cut-off power law in the spectrum of GRBs. The Researcher has preliminary approximated the Epeak distribution with a Gaussian. The intrinsic Epeak distribution peaks at higher energies than the observed one in the Swift data. The Researcher has computed the minimum and maximum Epeak for this distribution, written a code for the determination of the intrinsic distribution and make use of the R code publicly available in order to compare her code and the one available to the public in R.
9. GRBs as redshift estimators through a machine learning approach
The Researcher adopted several cutting edge methods of supervised machine learning to use different observables to build a GRB redshift estimator. Methods applied are the so called random trees, random forests and the generalized additive models, GAM. The first two methods are completely non-parametric thus allowing to identify the best predictors (the observed variables) of redshift as well as to determine groups of similar GRBs in terms of the values of the predictors and the redshift. GAM models builds a non-parametric relationship between the response variable (in our case the redshift) and the predictors in a relatively simple form and allows for an efficient extrapolation. The results of this analysis have shown that it is possible to find a redshift estimator for GRBs with a prediction of 70%. Results are in preparations in an ApJ paper.
10. Two Reviews on the prompt emission and on selection effects of the prompt emission
The Researcher finalized an exhaustive review on the prompt emission correlations and on selection biases possibly affecting the prompt correlations, with particular emphasis on the correlation between the spectral peak energy of GRB prompt emission, Ep, and the “Intensity” of the bursts (Eiso, Lp) and their application to cosmology. The first paper is published in Advances in Astronomy [11] and the latter is published on the Publications of the Astronomical Society of the Pacific [12].
11. Contribution to the THESEUS mission concept
The Researcher leveraged part of the results and expertise acquired within this project, for contributing actively to the further refinement and development of the science case, requirements and expected performances of the THESEUS mission concept resulted in 2 papers [13, 14].
12. Book on GRB correlations, selection biases and their application as cosmological tools.
The Researcher combined the material of the three reviews adding an extensive introduction and she wrote a book on the topic of GRB relations. The outline of the book has been already approved by IOP and the final version of the book has been sent to the Editorial board of IOP in April 2018. The Researcher is the only author of the book.
13. The interpretation of the plateau emission within the magnetar model. The researcher investigated the evidence of the discrimination among long GRBs and short GRBs with extended emission (SEE) in the Lx-Ta plane within the framework of the magnetar scenario. It was found that LGRBs and SEE are statistically consistent with two different populations in terms of magnetar magnetic fields B and initial spin periods P.
14. The investigation of the evolution of GRBs short with extended emission. Use Short GRBs and GRBs with extended emission as probes of Star formation rate (SFR) and check whether there are differences among results obtained employing Long GRBs. This is a more challenging task because of the smaller number of Short GRBs compared to Long GRBs, but with the more powerful non-parametric methods, the Efron & Petrosian method we have set meaningful constraints on distributions and evolution of Short GRBs as well. The analysis shows an evolution of (1+z)^3.09 of the short GRBs observed by GBM, Konus Wind and Swift all together. The density evolution has been calculated and it is similar to the LGRBs. Additional analyses are ongoing.
The wider societal implications relies on a broader and deeper understanding of the geometry and expansion rate of our Universe, including the nature of newly discovered and still mysterious components like “Dark Energy” and/or modifications to the general relativity theory, the fundament of modern cosmology and of the “Big-bang” standard cosmological model. This progress in the Universe understanding will also provide an increasing awareness of the perspective offered by the investigation of very far away astrophysical phenomena like GRBs and their use to map the evolution and properties of the Cosmos, thus constituting a unique bridge between astrophysics, cosmology and fundamental physics.
The high-level, success and strong scientific impact of this project are also demonstrated by the press releases by, e.g. the American Astronomical Society, the Scientific American, by the Le Scienze, by being issued among the most popular KIPAC blog and the Italian National Institute for Astrophysics, as well as the intense diffusion through media and news. These include broadcasting of an interview of the Researcher on the Italian national television (RAI).
(http://gallery.media.inaf.it/main.php/v/video/inafintv/Rai2/20180308-tg2-dainotti.mp4.html)