arXiv subjects

Xavier Hernandez

Publications and source records attributed to Xavier Hernandez.

15 recordsLinked to original sources

arXiv2021

On the Existence of the Plateau Emission in High-Energy Gamma-Ray Burst LightCurves observed byFermi-LAT

The Large Area Telescope (LAT) on board the \Fermi Gamma-ray Space Telescope (\Fermi) shows long-lasting high-energy emission in many gamma-ray bursts (GRBs), similar to X-ray afterglows observed by the Neil Gehrels Swift Observatory \citep[\textit{Swift};][]{gehrels2004}. Some LAT light curves (LCs) show a late-time flattening reminiscent of X-ray plateaus. We explore the presence of plateaus in LAT temporally extended emission analyzing GRBs from the second \lat GRB Catalog \citep[2FLGC;][]{Ajello2019apj} from 2008 to May 2016 with known redshifts, and check whether they follow closure relations corresponding to 4 distinct astrophysical environments predicted by the external forward shock (ES) model. We find that three LCs can be fit by the same phenomenological model used to fit X-ray plateaus \citep{Willingale2007} and show tentative evidence for the existence of plateaus in their high-energy extended emission. The most favorable scenario is a slow cooling regime, whereas the preferred density profile for each GRBs varies from a constant density ISM to a $r^{-2}$ wind environment. We also compare the end time of the plateaus in $γ$-rays and X-rays using a statistical comparison with 222 \textit{Swift} GRBs with plateaus and known redshifts from January 2005 to August 2019. Within this comparison, the case of GRB 090510 shows an indication of chromaticity at the end time of the plateau. Finally, we update the 3-D fundamental plane relation among the rest frame end time of the plateau, its correspondent luminosity, and the peak prompt luminosity for 222 GRBs observed by \textit{Swift}. We find that these three LAT GRBs follow this relation.

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arXiv2020

On the investigation of the closure relations for Gamma-Ray Bursts observed by Swift in the post-plateau phase and the GRB fundamental plane

Gamma-Ray Bursts (GRBs) are the most explosive phenomena in the Universe after the Big Bang. A large fraction of GRB lightcurves (LCs) shows X-ray plateaus. We perform the most comprehensive analysis of all GRBs (with known and unknown redshifts) with plateau emission observed by The Neil Gehrels Swift Observatory from its launch until August 2019. We fit 455 LCs showing a plateau and explore whether these LCs follow closure relations, relations between the temporal and spectral indices of the afterglow, corresponding to 2 distinct astrophysical environments and cooling regimes within the external forward shock (ES) model, and find that the ES model works for the majority of cases. The most favored environments are a constant density interstellar or wind medium with slow cooling. We also confirm the existence of the fundamental plane relation between the rest-frame time and luminosity at the end of the plateau emission and the peak prompt luminosity for this enlarged sample, and test this relation on groups corresponding to the astrophysical environments of our known redshift sample. The plane becomes a crucial discriminant corresponding to these environments in terms of the best fitting parameters and dispersions. Most GRBs for which the closure relations are fulfilled with respect to astrophysical environments have an intrinsic scatter sigma compatible within 1 sigma of that of the Gold GRBs, a subset of long GRBs with relatively flat plateaus. We also find that GRBs satisfying closure relations indicating a fast cooling regime have a lower sigma than ever previously found in literature.

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arXiv2019

Gamma-ray Bursts as distance indicators through a machine learning approach

Gamma-ray bursts (GRBs) are spectacularly energetic events, with the potential to inform on the early universe and its evolution, once their redshifts are known. Unfortunately, determining redshifts is a painstaking procedure requiring detailed follow-up multi-wavelength observations often involving various astronomical facilities, which have to be rapidly pointed at these serendipitous events. Here we use Machine Learning algorithms to infer redshifts from a collection of observed temporal and spectral features of GRBs. We obtained a very high correlation coefficient ($0.96$) between the inferred and the observed redshifts, and a small dispersion (with a mean square error of $0.003$) in the test set. The addition of plateau afterglow parameters improves the predictions by $61.4\%$ compared to previous results. The GRB luminosity function and cumulative density rate evolutions, obtained from predicted and observed redshift are in excellent agreement indicating that GRBs are effective distance indicators and a reliable step for the cosmic distance ladder.

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arXiv2018

Reply to the claim by van Dokkum et al. for a galaxy not containing dark matter

The following is a short comment on the recent claim made by van Dokkum and collaborators about the existence of a low surface brightness galaxy, NGC1052-DF2, not containing dark matter. A discovery used by the authors to both reject proposals of a failure of Newtonian dynamics in the low acceleration regime (e.g., MOND), and prove the dark matter hypothesis is correct. It is shown here that the claim in untenable.

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arXiv2017

A Study of the Gamma-Ray Burst Fundamental Plane

Long gamma-ray bursts (GRBs) with a plateau phase in their X-ray afterglows obeys a three-dimensional (3D) relation (Dainotti et al. 2016), between the rest-frame time at the end of the plateau, Ta, its corresponding X-ray luminosity, La, and the peak luminosity in the prompt emission, Lpeak, an extension of the two-dimensional Dainotti relation. This 3D relation identifies a GRB fundamental plane whose existence we here confirm. We extend the original analysis with X-ray data untill July 2016 gathering 183 Swift GRBs with afterglow plateaus and known redshifts. We compare several GRB categories, such as shorts with extended emission, SEE, X-ray Flashes, GRBs associated with SNe, a sample of only long-duration GRBs (132), selected from the total sample by excluding GRBs of the previous categories, and the gold sample, composed by GRBs with light curves with good data coverage and relatively flat plateaus. The relation planes for each of these categories are not statistically different from the gold fundamental plane, with the exception of the SEE, which are hence identified as a physically distinct class of objects. 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. Additionally, we tested this 3D relations by using GRBs observed at high energy, namely the peak luminosity values derived by the Fermi-Gamma Ray Burst Monitor (GBM). The 3D relation is also confirmed for GRBs observed by the GBM, thus showing its independence from the energy range. Furthermore, 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.

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arXiv2016

A fundamental plane for long gamma-ray bursts with X-ray plateaus

A class of long Gamma-Ray Bursts (GRBs) presenting light curves with an extended plateau phase in their X-ray afterglows obeys a correlation between the rest frame end time of the plateau, $T_a$, and its corresponding X-ray luminosity, $L_{a}$, Dainotti et al. (2008). In this work we perform an analysis of a total sample of 176 {\it Swift} GRBs with known redshifts, exhibiting afterglow plateaus. By adding a third parameter, that is the peak luminosity in the prompt emission, $L_{peak}$, we discover the existence of a new three parameter correlation. The scatter of data about this plane becomes smaller when a class-specific GRB sample is defined. This sample of 122 GRBs is selected from the total sample by excluding GRBs with associated Supernovae (SNe), X-ray flashes and short GRBs with extended emission. {\bf With this sample the three parameter correlation identifies a GRB `fundamental plane'}. Moreover, we further limit our analysis to GRBs with lightcurves having good data coverage and almost flat plateaus, 40 GRBs forming our `gold sample'. The intrinsic scatter, $σ_{int}=0.27 \pm 0.04$, for the three-parameter correlation for this last subclass is more than twice smaller than the value for the $L_{a}-T_a$ one, making this the tightest three parameter correlation involving the afterglow plateau phase. Finally, we also show that a slightly less tight correlation is present between $L_{peak}$ and a proxy for the total energy emitted during the plateau phase, $L_a T_a$, confirming the existence of an energy scaling between prompt and afterglow phases.

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arXiv2014

Non-parametric study of the evolution of the cosmological equation of state with SNeIa, BAO and high redshift GRBs

We study the dark energy equation of state as a function of redshift in a non-parametric way, without imposing any {\it a priori} $w(z)$ (ratio of pressure over energy density) functional form. As a check of the method, we test our scheme through the use of synthetic data sets produced from different input cosmological models which have the same relative errors and redshift distribution as the real data. Using the luminosity-time $L_{X}-T_{a}$ correlation for GRB X-ray afterglows (the Dainotti et al. correlation), we are able to utilize GRB sample from the {\it Swift} satellite as probes of the expansion history of the Universe out to $z \approx 10$. Within the assumption of a flat FLRW universe and combining SNeIa data with BAO constraints, the resulting maximum likelihood solutions are close to a constant $w=-1$. If one imposes the restriction of a constant $w$, we obtain $w=-0.99 \pm 0.06$ (consistent with a cosmological constant) with the present day Hubble constant as $H_{0}=70.0 \pm 0.6$ ${\rm km} \, {\rm s}^{-1} {\rm Mpc}^{-1}$ and density parameter as $Ω_{Λ0}=0.723 \pm 0.025$, while non-parametric $w(z)$ solutions give us a probability map which is centred at $H_{0}=70.04 \pm 1$ ${\rm km} \, {\rm s}^{-1} {\rm Mpc}^{-1}$ and $Ω_{Λ0}=0.724 \pm 0.03$. Our chosen GRB data sample with full correlation matrix allows us to estimate the amount, as well as quality (errors) of data, needed to constrain $w(z)$ in the redshift range extending an order of magnitude in beyond the farthest SNeIa measured.

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arXiv2004

On the Galactic Spiral Patterns: Stellar and Gaseous

The gas response to a proposed spiral stellar pattern for our Galaxy is presented here as calculated via 2D hydrodynamic calculations utilizing the ZEUS code in the disk plane. The locus is that found by Drimmel (2000) from emission profiles in the K band and at 240 $μm$. The self-consistency of the stellar spiral pattern was studied in previous work (see Martos et al. 2004). It is a sensitive function of the pattern rotation speed, $Ω_p$, among other parameters which include the mass in the spiral and its pitch angle. Here we further discuss the complex gaseous response found there for plausible values of $Ω_p$ in our Galaxy, and argue that its value must be close to $20 km s^{-1} kpc^{-1}$ from the strong self-consistency criterion and other recent, independent studies which depend on such parameter. However, other values of $Ω_p$ that have been used in the literature are explored to study the gas response to the stellar (K band) 2-armed pattern. For our best fit values, the gaseous response to the 2-armed pattern displayed in the K band is a four-armed pattern with complex features in the interarm regions. This response resembles the optical arms observed in the Milky Way and other galaxies with the smooth underlying two-armed pattern of the old stellar disk populations in our interpretation.

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arXiv2003

Dynamical consequences of CDM merger trees

Within the context of the standard structure formation scenario, massive present day elliptical galaxies are sometimes thought of as the result of a major merger of spiral systems. Through extensive SPH simulations of merging spirals, we have explored these processes with the aim of quantifying their relaxation times. This is important, as it sets a minimum time interval between the onset of a merger, and the appearance of an elliptical galaxy. We then compare this constraint with predictions of the hierarchical scenario, computed through Press-Schechter merger trees. We find evidence for elliptical systems which appear not to have been formed by a major merger of spirals.

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arXiv2003

On the newtonian limit and cut--off scales of isothermal dark matter halos with cosmological constant

We examine isothermal dark matter halos in hydrostatic equilibrium with a cosmological constant Lambda =Omega_Λrho_{crit}c^2, where Omega_Λ=0.7, and rho_{crit} is the present value of the critical density with h=0.65. The Newtonian limit of General Relativity yields equilibrium equations that are different from those arising by merely coupling an ``isothermal sphere'' to the Lambda-field within a Newtonian framework. The conditions for the existence and stability of circular geodesic orbits show the existence of (I) an ``isothermal region'' (0 r_1) dominated by the Lambda-field, where the Newtonian potential oscillates and circular orbits exist in disconnected patches of the domain of r; (III) a ``transition region'' (r_2 0.008 M_\odot {pc}^3, in agreement with rotation curve studies of dwarf galaxies. Since r_2 marks the largest radius of a stable circular orbit, it provides a ``cut off'' radius. For current estimates of rho_c and velocity dispersion of galactic structures, this is around five times larger than the virialization radius. The effects of the Lambda$--field can hence be ignored in structure formation models, but could be significant in the dynamics of superclusters in the linear regime or of gravitational clustering at large scales.

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arXiv2002

Chemical Evolution Models of Local dSph Galaxies

We calculate chemical evolution models for 4 dwarf spheroidal satellites of the Milky Way (Carina, Ursa Minor, Leo I and Leo II) for which reliable non-parametric star formation histories have been derived. In this way, the independently obtained star formation histories are used to constrain the evolution of the systems we are treating. This allows us to obtain robust inferences on the history of such crucial parameters of galactic evolution as gas infall, gas outflows and global metallicities for these systems. We can then trace the metallicity and abundance ratios of the stars formed, the gas present at any time within the systems and the details of gas ejection, of relevance to enrichment of the galaxies environment. We find that galaxies showing one single burst of star formation (Ursa Minor and Leo II) require a dark halo slightly larger that the current estimates for their tidal radii, or the presence of a metal rich selective wind, which might carry away much of the energy output of their supernovae before this might have interacted and heated the gas content, for the gas to be retained until the observed stellar population have formed. Systems showing extended star formation histories (Carina and Leo I) however, are consistent with the idea that their tidally limited dark haloes provide the necessary gravitational potential wells to retain their gas. The complex time structure of the star formation in these systems, remains difficult to understand. Observations of detailed abundance ratios for Ursa Minor, strongly suggest that the star formation history of this galaxy might in fact resemble the complex picture presented by Carina or Leo I, but localized at a very early epoch.

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arXiv2001

Cosmological Origin of the Lowest Metallicity Halo Stars

We explore the predictions of the standard hierarchical clustering scenario of galaxy formation, regarding the numbers and metallicities of PopIII stars likely to be found within our Galaxy today. By PopIII we shall be referring to stars formed at large redshift ($z>4$), with low metallicities ($[Z/Z_{\odot}]<-2.5$) and in small systems (total mass $\simlt$ $2\times 10^{8} M_{\odot}$) that are extremely sensitive to stellar feedback, and which through a prescribed merging history (Lacey & Cole 1993) end up becoming part of the Milky Way today. An analytic, extended Press-Schechter formalism is used to get the mass functions of halos which will host PopIII stars at a given redshift, and which will end up in Milky Way sized systems today. Each of these is modeled as a mini galaxy, with a detailed treatment of the dark halo structure, angular momentum distribution, final gas temperature and disk instabilities, all of which determine the fraction of the baryons which are subject to star formation. Use of new primordial metallicity stellar evolutionary models allows us to trace the history of the stars formed, give accurate estimates of their expected numbers today, and their location in $L/L_{\odot}$ vs. $T/K$ HR diagrams. A first comparison with observational data suggests that the IMF of the first stars was increasingly high mass weighted towards high redshifts, levelling off at $z\simgt 9$ at a characteristic stellar mass scale $m_s=10-15 M_\odot$.

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arXiv2000

Euler, Hipparcos and the five dwarfs : Reconstructing star formation histories in the Local Group

The unprecedented quality of recent colour-magnitude diagrams of resolved stellar populations in nearby galaxies requires state-of-the-art techniques to infer the star formation histories which gave rise to the observed distributions. We have developed a maximum likelihood technique, which coupled to a variational calculus allows us to make a robust, non-parametric reconstruction of the evolution of the star formation rate. A full Bayesian analysis is also applied to assess whether the best solutions found are also good fits to the data. Applying this new method to WFPC2 observations of five dSph galaxies of the Local Group, we find a wide variety of star formation histories, with no particular epoch being dominant. In the case of the solar neighbourhood observed by Hipparcos we infer, with an unprecedented resolution of 50 Myr, its star formation history over the past 3 Gyr. The surprising regularity of star formation episodes separated by some 0.5 Gyr could be interpreted as the result of interactions with two spiral arms or the Galactic bar. These bursts possibly trigger the formation of massive star clusters which slowly dissolve into the galactic field.

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arXiv1999

Star Formation Histories of the Galactic Satellites

Late accretion models for formation of the Galactic halo require that many Galactic satellite galaxies have been cannibalised into the halo field. Comparison of the metallicity and age distribution function of stars in the surviving satellites with the apparently exclusively old stars in the field halo can constrain the importance of any such process. We have developed a new objective technique to determine star formation histories in dSph galaxies. We apply this technique to the surviving Galactic satellites, deducing an approximately uniform distribution of ages for the constituents, quite unlike the halo field stars. Thus, late accretion did not play a substantial part in Galactic halo formation.

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arXiv1999

Objective Determination of Star Formation Histories

Objective determination of a star formation history from a colour-magnitude diagram, independently of assumed parametric descriptions, is necessary to determine the evolutionary history of a galaxy. We introduce a new method for solving maximum likelihood problems through variational calculus, and apply it to the case of recovering an unknown star formation history, SFR(t), from a colour-magnitude diagram. This method provides a non-parametric solution with the advantage of requiring no initial assumptions on the SFR(t). As a full maximum likelihood statistical model is used, one may exploit all the information available in the photometric data which can be explained by available isochrones. Extensive tests of the method show it to be reliable under noise conditions comparable to those appropriate for real observations. Implications for future surveys, such as the GAIA mission are significant: accurate determination of the evolutionary history of the Milky Way is limited only by adequate spectroscopy and/or photometry to provide a determination of stellar metallicity and temperature for a sufficient sample of stars of luminosities at and above an old main-sequence turnoff.

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