Mimoza Hafizi

Is the time lag-luminosity relation of gamma-ray bursts a consequence of the Amati relation?

Context. The lag-luminosity relation (LLR) provides a way of estimating GRB luminosity by measuring the spectral lags between different energy bands. Aims. We want to understand the origin of the LLR and test its validity. This appears especially important if the LLR is to be used as a distance indicator. Methods. We perform a linear analysis of the lag between two spectral bands. The lag is obtained as the time interval between the maxima of a given pulse in the two bands. Results. We get a simple expression for the lag, which shows in a very simple way how it is related to the spectral evolution of the burst via the variation of the peak energy and spectral indices. When this expression is coupled to the Amati relation, it leads to a LLR that agrees with the observational results only if the bursts spectral evolution is limited to a decrease in peak energy during pulse decay. However, when the variation of the spectral indices is also taken into account, the predicted LLR differs from the observed one. Conclusions. We briefly discuss some ways to solve this problem, such as a possible correlation between pulse spikiness and burst luminosity.

MICROLENSING SIGNATURE OF A WHITE DWARF POPULATION IN THE GALACTIC HALO

Microlensing and pixel-lensing surveys play a fundamental role in the searches for galactic dark matter and in the study of the galactic structure. Recent observations suggest the presence of a population of old white dwarfs with high proper motion, probably in the galactic halo, with local mass density in the range 1.3×10-4-4.4×10-3 M⊙pc-3, in addition to the standard galactic stellar disk and dark halo components. Investigation of the signatures on microlensing results towards the LMC of these different lens populations, with particular emphasis to white dwarfs, is the main purpose of the present paper. This is done by evaluating optical depth and microlensing rate of the various lens populations and then calculating through a Monte Carlo program, the probability that a lens which has caused a microlensing event of duration tE belongs to a certain galactic population. Data obtained by the MACHO Collaboration allow us to set an upper bound of 1.6×10-3 M⊙pc-3 to the local mass density of white dwarfs distributed in spheroidal models, while for white dwarfs in disk models all values for the local mass density are in agreement with observational results.

The lag and duration-luminosity relations of gamma-ray burst pulses

Institut d’Astrophysique de Paris, UMR 7095 Universit´e Pierre et Marie Curie-Paris 6 – CNRS, 98 bis boulevardArago, 75014 Paris, FrancePreprint online version: September 22, 2011ABSTRACTContext.Relations linking the temporal or/and spectral properties of the prompt emission of gamma-ray bursts (here-after GRBs) to the absolute luminosity are of great importance as they both constrain the radiation mechanisms andrepresent potential distance indicators. Here we discuss two such relations: the lag-luminosity relation and the newlydiscovered duration-luminosity relation of GRB pulses.Aims.We aim to extend our previous work on the origin of spectral lags, using the duration-luminosity relation recentlydiscovered by Hakkila et al. to connect lags and luminosity. We also present a way to test this relation which hasoriginally been established with a limited sample of only 12 pulses.Methods.We relate lags to the spectral evolution and shape of the pulses with a linear expansion of the pulse propertiesaround maximum. We then couple this first result to the duration-luminosity relation to obtain the lag - luminosityand lag - duration relations. We finally use a Monte-Carlo method to generate a population of synthetic GRB pulseswhich is then used to check the validity of the duration-luminosity relation.Results. Our theoretical results for the lag and duration-luminosity relations are in good agreement with the data.They are rather insensitive to the assumptions regarding the burst spectral parameters. Our Monte Carlo analysisof a population of synthetic pulses confirms that the duration-luminosity relation must be satisfied to reproduce theobservational duration – peak flux diagram of BATSE GRB pulses.Conclusions. The newly discovered duration-luminosity relation offers the possibility to link all three quantities: lag,duration and luminosity of GRB pulses in a consistent way. Some evidence for its validity have been presented but itsorigin is not easy to explain in the context of the internal shock model.Keywords.Gamma rays bursts: general; Radiation mechanisms: non thermal

A THEORETICAL CALCULATION OF MICROLENSING SIGNATURES CAUSED BY FREE-FLOATING PLANETS TOWARDS THE GALACTIC BULGE

Free-floating planets are recently drawing a special interest of the scientific community. Gravitational microlensing is up to now the exclusive method for the investigation of free-floating planets, including their spatial distribution function and mass function. In this work, we examine the possibility that the future Euclid space-based observatory may allow to discover a substantial number of microlensing events caused by free-floating planets. Based on latest results about the free-floating planet mass function in the mass range

A simple theory of lags in gamma-ray bursts: Comparison to observations

[10^{-5}, 10^{-2}]M_{\odot}

The astrometric signal of microlensing events caused by free floating planets

, we calculate the optical depth towards the Galactic bulge as well as the expected microlensing rate and find that Euclid may be able to detect hundreds to thousands of these events per month. Making use of a synthetic population, we also investigate the possibility of detecting parallax effect in simulated microlensing events due to free-floating planets and find a significant efficiency for the parallax detection that turns out to be around 30%.

Predictions on the detection of the free-floating planet population with K2 and spitzer microlensing campaigns

Context. Lags observed between the light curves of a gamma-ray burst (GRB) seen in different energy bands are related to its spectral evolution. Moreover the lags have been found to correlate with burst luminosity, therefore providing a potential distance indicator. Aims. We want to quantify the nature of the link between lags and spectral evolution to better understand the origin of the lag-luminosity relation and evaluate its interest as a distance indicator. Methods. We directly relate the lag of a pulse to the spectral parameters (peak energy E p , low and high energy indices, α and β, and their time derivatives) evaluated at pulse maximum. Then, using a Yonetoku-like relation we obtain a theoretical lag-luminosity relation that is confronted with data. Results. We first apply our model to the initial pulse of GRB 130427A, for which high quality data are available, to check quantitatively whether the measured lags are consistent with the observed spectral evolution. We then use a Monte Carlo approach to generate a sample of synthetic lags, which we compare to an observed sample of Swift bursts. The dispersion of both the observed and modelled lag-luminosity relations appears large, which questions the value of this relation as a reliable distance indicator.

Estimating Finite Source Effects in Microlensing Events due to Free-Floating Planets with the Euclid Survey

Astrometric observations of microlensing events can be used to obtain important information about lenses. During these events, the shift of the position of the multiple image centroid with respect to the source star location can be measured. This effect, which is expected to occur on scales from micro-arcseconds to milli-arcseconds, depends on the lens-source-observer system physical parameters. Here, we consider the astrometric and photometric observations by space and ground-based telescopes of microlensing events towards the Galactic bulge caused by free floating planets (FFPs). We show that the efficiency of astrometric signal on photometrically detected microlensing events tends to increase for higher FFP masses in our Galaxy. In addition, we estimate that during five years of the Gaia observations, about a dozen of microlensing events caused by FFPs are expected to be detectable.

On the duration-luminosity relation of GRB pulses during the prompt emission

The K2’s Campaign 9 (K2C9) by the Kepler satellite for microlensing observations towards the Galactic bulge started on April 7, 2016, and is going to last for about three months. It offers the first chance to measure the masses of members of the large population of the isolated dark low-mass objects further away in our Galaxy, free-floating planets (FFPs). Intentionally, this observational period of K2 will overlap with that of the 2016 Spitzer follow-up microlensing project expected to start in June, 2016. Therefore, for the first time it is going to be possible to observe simultaneously the same microlensing events from a ground-based telescope and two satellites. This will help in removing the two-fold degeneracy of the impact parameter and in estimating the FFP mass, provided that the angular Einstein ring radius ΘE is measured. In this paper we calculate the probability that a microlensing event is detectable by two or more telescopes and study how it depends on the mass function index of FFPs and the position of the observers on the orbit.

A Test on Distribution of Opening Angles of Beamed GRBs

In recent years free-floating planets (FFPs) have drawn a great interest among astrophysicists. Gravitational microlensing is a unique and exclusive method for their investigation which may allow obtaining precious information about their mass and spatial distribution. The planned Euclid space-based observatory will be able to detect a substantial number of microlensing events caused by FFPs towards the Galactic bulge. Making use of a synthetic population algorithm, we investigate the possibility of detecting finite source effects in simulated microlensing events due to FFPs. We find a significant efficiency for finite source effect detection that turns out to be between 20% and 40% for a FFP power law mass function index in the range [0.9, 1.6]. For many of such events it will also be possible to measure the angular Einstein radius and therefore constrain the lens physical parameters. These kinds of observations will also offer a unique possibility to investigate the photosphere and atmosphere of Galactic bulge stars.