Gamma Ray Bursts: The Universe's Most Powerful Explosions

[00:00:11]it's it's [Music] hello and good afternoon the public outreach and education committee of the astronomical society of india welcomes you to the discussion series ask an astronomer i am sarita vik and joining me in the organization today are process dr christine karthik and virendra from asi poc our eminent astronomer today is professor kuntal research institute of observational sciences nanita and the theme for discussion is gamma ray bursts profunda will be giving a short presentation which will be followed by a discussion i invite our participants and viewers to note down their questions in the chat box i will first introduce our speaker process completed her phd from aries in the year 2008 following this she carried out research as a postdoctoral fellow in ayuka and sdsci usa she joined aries as a faculty member in december 2012 her main areas of research are high energy transients mainly gamma ray bursts supernova and electromagnetic counter parts of gravitational wave sources and big data from survey telescopes she is the pi of the international liquid mirror telescope at aries which is a joint collaboration between india belgium and canada it is expected to achieve a slide at the end of this year she is a recipient of the women in stem fellowship of the indo-us science and technology forum mia now requires process to begin her presentation thank you sarita for the introduction i will start sharing my screen now please let me know if it's visible it's visible i've made it full screen now can you see the slide in full screen mode yes so good evening everyone and thank you to asi poec for giving me this opportunity to talk about one of the most enigmatic cosmic explosions in the universe known as the gamma ray bursts uh to begin with as sarita mentioned that i work in aries aries is located in the picturesque location of the nanitan station we have another location 60 kilometers away from nanital which is known as devastal and it hosts the largest telescope in india which is the 3.6 meter devastal optical telescope so in future as and when the opportunity arises i would like to request all of you to visit this beautiful place and see the night sky using this uh hill station in nanital as well as in devasta so my main research interests are in a variety of cosmic transients but today i'll be speaking about gamma ray bursts i have kept the talk at a very uh basic level so just imagine if you could see something in the universe which is exploding and it's emitting as much energy as nearly the entire universe would emit in just a few seconds and the amount of energy emitted is typically of the order of 10 to the power of 51 ergs not only that this explosion is so enigmatic that all of the world's greatest observatories would start looking at this object when i say all the observatories it means that it is emitting the radiation in all the wavelengths across the electromagnetic spectrum right from high energy gamma rays to the low energy radio bands so you would be most likely seeing a stellar explosion the two biggest explosions which we've seen after the big bang are the supernova explosions and the gamma-ray bursts just to give an analogy between what a supernova and a gamma-ray burst is so supernova word has been derived from the latin word nova which means new and super means awesome so when we see a new supernova in the sky which means we are seeing a new awesome transient source in the sky on the other hand gamma ray bursts are sudden intense flashes of gamma rays that is why we call them as gamma ray bursts because they are coming from the bursts of stars and emitting intense flashes in the gamma-ray bands however the supernova are very frequent we typically see around 10 supernovae exploding every second in the universe on the other hand there is only typically around one grb which is detected every day on the other hand the supernova in a um in a galaxy like ours the milky way the number is around one to two per century which is very very rare the very closest supernova in our nearest neighboring uh um galaxy the lmc was 1987a which was about more than 30 years back now not even 30 about 40 years back and gives on the other hand if occurs in a galaxy like ours which is the milky way the rate is very very small it's very rare that a grv would occur in a galaxy like ours so typically these grbs would be would take around 10 to the power of 4 to 10 to the power of 6 years to be seen in a galaxy like ours the milky way so the rates of these cosmic transients is very small yeah i think uh yeah same here i think there is some internet issue at her side last slide you are muted i think you will disconnect it also for some time you're muted please unmuted can you hear me now yeah okay i'll start from this slide only so i i give you a brief idea about the explosion negative you need to share again it's not shared no i think you had got disconnected probably okay so the uh energetics involved in these explosions are typically of the order of 10 to the power of 51 earths in the case of a supernova explosion whereas in the case of a gamma-ray burst sarita you were saying something okay um whereas a gamma ray burst emits energy of the order of 10 to the power of 53 years which is about 100 times higher than that seen in a supernova explosion the energy emitted from the sun is typically 3 times 10 to the power of 33 earths per second excuse me i think there's a i think there's a echo somewhere is that right i can no that's fine that's fine go ahead uh can others hear me clearly yeah yeah we can okay so if sun has to emit this much of energy of 10 to the power of 51 earths it would require about 5 billion years which is 5 times 10 to the power of 9 years which is why these are the magnificent cosmic explosions seen in the universe now during the big bang only light elements were formed like hydrogen helium and lithium so where are we getting the heavier elements from the calcium which is present in our bones or the aluminum which is used in our day-to-day lives in making vehicles or any other things so these heavy elements are usually coming from the explosions in the universe like the supernova explosion because the massive stars have uh the heavier elements uh in the form of a onion shell structure which is the different elements in the in the star and these are formed during fusion and these explosions are also the fundamentals of creation if you can see from uh this figure over here that most of these heavier elements the green one uh the the elements which are labeled with the green color they are coming from the exploding massive stars and that is why they are studying these explosions in the universe are of fundamental importance to us i will move on to uh the transients which are known as the gamma ray bursts why do we need to study gamma ray bursts uh the first important aspect would be that they are an indirect clue to the stellar evolution what kind of stars give rise to grps and in which galaxies do these bursts occur and indirectly what what can we know about the processes that are going on when a gamma-ray burst occurs in any of the galaxy however it is also related to the larger picture of the universe where we can get an indirect evidence about the star formation rate of the universe and they can be used as cosmological probes in cosmological studies i will discuss what are the milestones that have been achieved so far in the study of grbs so these were detected in the late 1960s by the u.s male veiler military satellites and this was a serendipitous discovery it was not a targeted discovery it was found that these are short intense pulses of gamma rays typically ranging from uh kev to mev ranges and they last for a very few seconds these are non-repetitive in nature coming from random directions in the sky so the figure which i am showing on the right shows the distribution of about 3 000 gamma ray bursts which were detected by the batsa satellite can be seen from the from this figure that they do not have a preferential location whereas there they can be located anywhere and they can come from any random direction in the sky as they are isotropically distributed they are located at cosmological distances and the redshifts that we know uh for the known grb so far the range of the redshift is from point zero zero eight to nine point four so the nearest grb known to us is located at a redshift of point zero zero eight whereas the farthest grb known to us is at a rate shift of nine point four because they are lying at cosmological distances they have huge energy output ranging between 10 to the power of 48 to 10 to the power of 54 herbs and this is directly related to the energetics of the burst when i say the the redshift of these gamma-ray bursts how do we measure the redshift of grbs so the redshift measurement is usually done from the spectrum the spectrum can be from the afterglow or from the host galaxy i will define these terms um after a few slides so the figures on the on this slide show the spectrum of typical gamma-ray bursts where the y-axis is the flux and the x-axis is the wavelength so by the presence of the emission or the absorption lines we can estimate the redshift of the gamma-ray bursts now we already said that these bursts are isotropic in nature we typically see one burst per day but these bursts are highly variable in nature the light curves of no two grbs are similar which means there is no similarity when you look at the burst phenomenology of a number of gamma ray bursts then you see that they have varying profiles they can either be a single pulse they can be a single pulse which is typically a broad pulse they can be multiple pulses they can be pulses which are very very narrow which means they are very short-lived so this indicates the highly variable nature of gamma-ray bursts indicating that there is no similarity between any two grp's predominantly two classes of gamma ray bursts the short burst and the long burst so the figure on the lower right over here shows the hardness ratio and the t90 distribution of the grp so i would like to focus you to focus only on the distribution of these dots on this figure where you clearly are able to see two classes of grbs the ones which have a duration greater than two seconds fall in the category of the long grps whereas the grbs which have a duration of less than two seconds fall in the category of short gamma ray bursts similarly the figure on the top shows the histogram and this also indicates a bimodal nature of the gamma ray bursts indicating two distinct classes of grbs when there are two distinct distinct classes of grps there there have to be two different models of origins of origin giving rise to these two different classes of grbs when the whole galaxy observations of these two classes of grbs were made they lie in different kind of host galaxies which again hints towards different projectors giving rise to these two classes of grps now these short grbs for which the duration is less than two seconds they also have a hard spectrum whereas the long grps for which the duration is greater than two seconds they have a soft spectrum so they are um generally known as short hard bursts or long soft bursts again in the case of uh long gamma ray bursts we have seen that they are associated with the code collapse supernova and there has been no supernova association seen for a short gamma ray burst however for a short gamma ray burst in very recent observations we have seen a kilonova emission which is coming from the r process nuclear synthesis of the neutron rich material the different evidences seen in the light curves or the spectrum of short and long gamma ray bursts have time and again indicated different progenitors or the different formation channels for these two classes of glps long grvs they are typically found in star forming galaxies they are close to the bright uv regions of the host galaxy whereas the short gamma ray bursts can occur in both late and early type galaxies and they have relatively larger offsets when i say relatively larger offsets it means that they occur away from the host galaxy so here in this uh picture i have shown the typical host galaxies of the long gamma-ray bursts which are generally star-forming irregular galaxies and the short gamma reverse can occur in a star-forming galaxy it can also occur in an elliptical galaxy and when i say relatively large offset so if you see the the blue uh picture of the host galaxy over here so the galaxy is over here whereas the cross wire here shows the location of the burst so this uh has been seen from uh very deep observations with the hubble space telescope and a detailed morphological study of the host galaxies and the locations of the gamma ray bursts in these host galaxies has been performed now when we talk about two different classes of gamma ray bursts the long and the short we are directly hinting towards different progenitor channels which are giving rise to these two classes of grbs either ways there is a collapse taking place which is leading to the formation of a of a black hole or a magnetometer they can either come from the collapse of a massive star or from the merger of two compact objects in one of my previous slides i had shown that the long gamma ray bursts are associated with a supernova which means which has an indirect evidence stating that long gamma ray bursts are coming from the collapse of massive stars whereas the short grps are thought to be coming from the merger of two compact objects it could either be a merger of two compa two neutron stars or a merger of a neutron star and a black hole and this was recently established by the gravitational wave detection uh in the year 2017 with the event uh gw 1708 17 and the associated short gamma ray burst 1708 17 a this is by far the best the best observations uh taken so far for any short gamma ray burst and the uh gravitational wave signal and this has put strong evidences to date about the short grbs coming from the merger of two compact objects so what happens is that you have in the case of a long gamma reversed if you see uh the figure on the left panel of this figure you have a long gamma ray burst which is a burst that has a duration of greater than two seconds a massive star the core of the massive star collapses and the material is expelled out there is a supernova explosion also the central remnant object would be a black hole and there is an accretion disk formed from where jets would be launched and this causes the the gamma ray burst similarly in a short gamma ray burst you have two compact objects which are spiraling inwards and eventually colliding again when they collide there will be a central uh black hole formed and again the scenario would be the same as a long gamma ray burst where the accretion disk formed from the uh jet form would give rise to the jet and resulting which would eventually result in the formation of a gamma ray burst so there are strong evidences for the two different progenitor channels seen for the long and the short classes of gamma ray bursts now how do we explain this once there is a central compact object the accretion disk has formed and jets are launched so those jets are traveling through the ambient medium only when those jets are directed towards the line of sight of an observer we will be able to see that gamma ray burst so the most promising model to explain the scenario is known as the grb fireball model and this is also one of one of the most simplistic models that has been developed where the burst takes place over here releasing energies of the order of 10 to the power of 51 to 10 to the power of 54 earth then the material comes out and it is interacting and colliding there is shock formation the internal shocks over here in this region they are giving rise to the burst and this is where we are seeing the burst photons in the gamma-ray energy band now as the ejecta slows down it interacts with the surrounding local or the ambient medium and once this interaction takes place so this is the these are the external shocks over here when these external shocks interact with the ambient medium or the surrounding medium they give rise to an afterglow and this afterglow radiates across the different uh wavelengths of the electromagnetic spectrum right from x-rays to radio to optical via the synchrotron radiation process now there are different energetics involved in the burst so when we are detecting the burst uh photons we are basically measuring the fluids to calculate the total gamma-ray isotropic energy we just do uh uh applying this uh we know the fluence so we just do four pi dl square times the fluence and that will give you an estimate of the total gamma isotropic energy the second important factor over here is collimation the collimation angle theta which i will come to later in after a few slides so clearly as you can see from this figure there are two phases one is the burst phase and one is the after glow phase there are clear differences between the burst phase and the afterglow phase the burst phase typically follows a band function this is just an empirical power law function to the spectrum and this is the typical prompt energy spectrum ah which we see in the case of gamma ray bursts and these are two power laws and where these two powers join this is the peak energy of the prompt emission of the prompt emission seen in a gamma ray burst however the after glows are long lasting counterparts as i said earlier that the afterglows emit across different wave bands of the electromagnetic spectrum so this is typical afterglow uh scene in the optical band so this after this year we happened in on 29th march uh 2003 i will just mention over here the nomenclature when when we say 0 30329 it means it occurred this burst occurred in the year 2003 in the month of march and on the 29th day of march so the first image which was taken a few days after um the burst and this is the optical counterpart of this grp seen in the optical wavelengths and about a month later this was still visible but it had significantly faded so the fading nature indicates that the after glows fade over time um again i mentioned that these are low long lasting low frequency counterparts seen across different wave bands of the electromagnetic spectrum in one of my earlier slides i had mentioned that the gamma ray bursts are located at cosmological distances and the nearest red shift nearest grb is located at a redshift of 0.008 if you convert this this was typically at a distance of 37 mega per sec and these are the conversions which i have just put down here for for your ease and when you convert this to the light travel time it means that the light travel time corresponding to a z of 0.008 is about 0.112 giga year which means that this ah the burst took these many years the radiation from the burst took these many years to reach us however the high rate shift grb which is at a location at a red shift of 9.4 is nine eight nine six three mega power six away from us and the light travel time is about 13.2 gigahertz it means it took this much of time the light or the radiation from this gamma ray bus took this much of time to reach us so these are the two um the two typical examples of uh the radiation arriving us from the gr bays which are located at two different redships the nearest and the farthest gamma ray bursts known so far the after glows uh they follow a non-thermal spectrum where f neo is directly proportional to nu to the power of minus beta and this is a a general afterglow spectrum where which is constituted of a broken power laws at different regimes of the electromagnetic spectrum when we ah talk about the light curve of an afterglow as you can see in this figure the x-axis is a time and the y-axis is flux when the flux is plotted against time we see that the flux generally fades with time in some cases it can be rising also and then fading but generally in majority of the cases we see a fading flux from afterglow from from from x-ray radio or an optical afterglow and which indicates that this is a time varying light curve where f nu is directly proportional to t to the power of minus alpha where alpha is the decay index with which the light curve is decaying or falling now when we have introduced these two terminologies about uh the burst and the afterglow as we also saw in the fireball model that there were two distinct boundaries for the burst phase and the afterglow phase there are differences between the burst and the afterglow phase so the burst phase would see emission only in the gamma-ray band whereas the afterglow is a multi-wavelength phenomena where the radiation is seen across different bands of the electromagnetic spectrum the burst is very short-lived of the order of of a few seconds sometimes it can also be a fraction of a second sometimes it can go up to thousands or two thousand seconds but generally not more than that so these are very the burst phase is very short-lived the afterglow is a long lived longer wavelength counterpart and the after glows can be seen right from of from one day to a few days to a few years it also depends upon the the nature of the burst as well as the band in which we are observing the burst phase will give you an estimate of the prompt emission and the isotropic gamma ray energy the afterglow phase as i said earlier also is a non thermal synchrotron radiation it indicates time varying flux and typically one hour after the burst we have seen after glows ranging from 16 to 24 magnitudes in the r band they follow a power law decaying this where f nu is directly proportional to t to the power of minus alpha and one very important ah estimate that can be made from the after glow is to estimate the redshift of the burst the redshift at which the burst was located so this is again done from the spectroscopy or ah sometimes indirectly from the photometry also but this is why the after glows are so important um we can only study the after glows because the bursts are very short-lived we really cannot study them for a very long time or observe them for a long time and extract information out of it but since these afterglows can be seen across different bands and also for a longer time we have to follow each one of these ah dedicatedly to um to actually see the history of the burst or the progenitor star which has given rise to this or any gamma ray burst now we already saw in the fireball model and even during the progenitor scenario that when once the accretion risk is formed jets are launched which means that the grbs are highly beamed and they have opening angle of a few degrees as you can see from this figure this is coming out in the form of a narrow conical beam and this is the opening angle which we estimate indirectly and once this opening angle we call as theta j this is expanding the material is expanding and its coming outwards as this theta j it falls or it becomes less than one upon gamma their gamma is the lorentz factor one thing which i forgot to mention is that initially during the burst phase the lawrence factor is very very high these are highly relativistic events and the uh the lawrence factor is of the of the order of 10 to the power of 3 so once the ejector has slowed down and the jet opening angle it falls below 1 upon gamma the observer the observer is located somewhere over here the observer will be able to see larger fraction of the emitting surface when the observer sees larger surface ah larger fraction of the imaging emitting surface then the flux drops and when the flux drops this is seen as an achromatic jet break feature in the light curves when i say that this is an achromatic feature it means that it will be seen at the same time at all bands of the electromagnetic spectrum and this time at which ah this flux dropping occurs is known as the jet brake time now these jet brake times are very important to estimate the energetics of the burst primarily they can estimate the extent of collimation so with the information of the generate time we can calculate this collimation angle theta j as can be seen from this expression over here that theta j in a constant density medium is directly related to the jet break time the other inputs are the red shift the isotropic energy and the number density i won't go into the details of this on the other hand if you have an estimate of this uh tj so if you look at this figure over here and here the histogram of the um distribution of the isotropic gamma ray energy is shown in the top panel you see that e gamma iso it varies from roughly around 10 to the power of 52 to 10 to the power of 54 earth however if you connect it for the collimation angle theta j then these bursts have collimated corrected energy typically around 10 to the power of 51 eggs this picture looks very simplistic that for all grbs if we can estimate the generic time we will indirectly know about the collimation and hence we can estimate the true energetics of the burst the true energetics meaning the collimated character energy but this is not always true it is now with the detection and discovery of more and more gamma-ray bursts we are seeing the absence of achromatic jet bricks there are different behaviors seen in the x-ray and the optical light curves as you can see in the two figures on the right over here that this is a very bumpy light curve it shows a flaring nature it shows a plateau it doesn't show any feature of an achromatic jet break similarly in this figure these are the two light curves in the optical and the x-ray bands both of them behave differently so the main criteria to estimate the jet break time is that it has to be achromatic meaning occurring at the same time at the different energy dance so as more and more complexities were seen in the light curves it became very difficult to estimate the true energetics of the bursts so this poses a very important question about the energetics about how the energetics can be calculated in such a case and how and whether they can be trusted or not a similar behavior was also seen in very high energetic grbs which were detected by the fermi uh large area telescope so fermi operates in the wave band in the energy band of 20 mev to 300 gev the red triangles over here show the most energetic grbs which were ever detected by the fermi large area telescope and none of these energetic grbs have actually shown a break so how do we calculate their true energies in the recent times we have also seen drvs which have emitted in the very high energies we know three grv so far which have vhe emission and detected i will just talk about briefly about one of them drp 1901 14c which is located right over here the isotropic energy of this burst was about 3 times 10 to the power of 53 whereas the e peak was around 1000 kv and the vhe emission for this grv was detected by the magic telescope so you can see from this figure that this again lies towards the very high energetic grps there was a multi-wavelength campaign for this uh gamma rebirth because this was the first ever grb detected with sub tv emission and um in this figure the light curve or the evolution of flux with time at the entire electromagnetic spectrum is shown ranging from uh tv bands to gev bands to kv in the x-rays and the optical and uh the radio bands so this was one burst and this is one burst so far which with the tv emission detected and sub tv actually uh the highest energy photon detected in this grp was around 300 gev and it was uh very well followed up with about two dozen telescopes across the world okay the three primary instruments are the missions which are detecting the grbs and giving us the localization of gamma ray bursts um till date are today i mean which are operational today i have not listed the ones which were operating earlier um the main is the swift gamma-ray burst mission which has three instruments on board that the burst alert telescope which operates in the energy band 15 to 150 kv and is primarily responsible for detecting the gamma-ray burst in the gamma-ray bands the x-ray telescope operating at 0.2 to 10 kv and the ultraviolet optical telescope operating in the ultraviolet and optical bands 170 to 650 nanometer and this is the mission which gives us precise locations of about a few arc seconds that can be followed up with the ground waste telescopes the fermi gamma ray mission has two instruments on board the large area telescope um operational between the energy range 20 mev to 300 gb and the gamma ray burst monitor operational between 8 kv to 40 mav and our own indian multi wavelength observatory the astrosand it has five instruments on board but the primary primary one which is uh detecting the gamma ray bursts is the coded is the cadmium zinc telluride imager which is a coded mask detector which operates in the energy range of 10 to 150 kv so these um three instruments give us information about the gamma ray bursts which are occurring in the sky and one or another gamma ray burst would be occurring right now in the sky and if any of these instruments picks it up we will get an alert that a gamma ray burst has gone off at such and such location however once the burst phase is over we really need to follow the afterglow in the different bands of the electromagnetic spectrum and india plays a very crucial role in observing these after glows at the optical bands as you can see from this figure over here there are some telescopes marked at um different locations around the globe and why india plays a critical role is that it is exactly in the middle of the 180 degree wide longitude band between eastern australia over here and the canary islands so when the observations are not possible at the at these two places due to night time hours can be successfully carried out from india and we have been studying gamma ray burst after glows from india for the last two decades and in many cases we have reported the earliest discovery or the earliest observations of the optical afterglows of gamma ray bursts and different telescopes are used to monitor the optical afterglows in india and these are the different these are the prime telescopes which i have listed over here which are used for monitoring the optical after glows of gamma ray bursts these gamma ray bursts are also observed at other wavelengths particularly with gmrt at the radio wavelengths below 1.4 gigahertz and as present i already mentioned earlier that astrosat the caesar ti imager on um is also detecting the gamma ray bursts i would like to end my presentation over here and would like to point out that ah grbs are not very simplistic in nature eglv poses a challenge to the standard model i presented some of those complexities in the light curves especially related to the energetics of the bursts and whether there could be any alternate uh methods to estimate the true energetics of the grbs and this is still a growing field with the discovery of new grps and in the recent times with the discovery of the association of the short gamma ray bursts with the gravitational wave detection has opened a completely new window as well as the detection of the very high energy emission seen in gamma ray bursts has also opened a new window in in the electromagnetic spectrum to study these gamma ray bursts so thank you for listening to this i'm open to any discussion and questions thank you for explaining about these karma rippers maybe perhaps we can begin uh uh discussion amongst ourselves can you hear me i can hear you yeah the screen yeah yeah that's fine so i was just uh wondering that these events you do not know beforehand uh where they will occur right so just for the curiosity of the people who are participating how how do the observers know where for example or even the telescopes that are operating how is it known that a grp is there in a certain direction because you are not looking in all the directions at the same time for example so however be able to first detect these events at all yeah so the primary detection in the gamma ray bands is coming from swift or fermi or astrosat in some cases these are large area detectors the burst alert telescope or the gamma-ray burst monitor on fermi are large area detectors which are covering a huge portion of the sky there are satellites going around and covering a huge portion of the sky and if they detect anything which is above their energy threshold they're operating typically in the energy band which is about 150 kav or more than 150 kav so if they pick up any signal which is uh greater than their threshold they have some on board algorithms where they process the data and localize whether it is a burst or not and they will give you a precise location of uh whether the array and declination of the burst and once the satellite has confirmed the nature of the burst or whether it is a burst or not it will give information to a gamma-ray burst coordinate network there is a gamma-ray burst coordinate network which is operational throughout the globe and it disseminates information to the community and that's how we know that a gamma ray burst has taken place and this is all very very rapid a burst being detected by a mission and the information reaching us sometimes happens in just a few seconds i see but uh some of the burst as you said the long uh gamma ray bursts they last till few tens of seconds is it possible for the other instruments to catch up also because what happens in swift is that because swift has two other telescopes the xrt and the uvot which are co-lined so once a burst is detected by that it will give information to xrt and uvot and these two instruments would co-align in the direction of the burst because bat is a large area detector but uvo t and xrt are also in the same field of view but not necessarily exactly at the same position where the burst has been detected so these will co align in the direction of the burst and if there is a new source like in uv or an optical or an x-rays you can easily identify a new source which is the afterglow right and that will refine the coordinates the array and deck of the burst and we are given actually very precise coordinates the coordinates which are given to the to the community are very very precise coming from swift uv ot just about with an error bar of around a few seconds just two or three seconds in most of the cases okay thank you are there any questions is there any typical time difference that is happening between the main burst and the afterglow that we observe uh yeah so the it depends upon so the um when i said that the uh the the duration of the burst which is primarily responsible for classifying the grbs the long ones lasting for two seconds for greater than two seconds and the short ones lasting less than two seconds so this two second or the burst duration is calculated from the burst phase in the gamma-ray band okay so once that burst phase is over you will start to see the afterglow that that's like proceeded immediately after the burst immediately after the burst and in some cases after glows it's not necessary that afterloads could be it would be detected in all gamma ray bursts okay but in majority of the gamma ray burst about um 80 percent uh gamma ray bursts have an x-ray after glow um about 60 percent have optical afterglow and only about 30 percent have radio after glows okay so any specific reason for not seeing afterglow in almost 20 percent of the yeah so it could be that suppose in x-rays it was detected by some other mission which did not have uh like if fermi detects and if swift is not looking in that direction then fermi usually requests a t over to swift and it may take time to execute the to so sometimes you may miss the x-ray afterglow in optical generally it is thought that um suppose we have we are not in uh i mean our telescopes are not able to observe at that location or they could be very very faint for our telescopes or for the existing optical telescopes that we have or it could be lying in high dense uh regions with very high extinction and which is why the optical after close could be obscured and we are not able to look for any optical emission from those okay so it's more related to the uh observational uh restrictions rather than any physical phenomena happening okay okay okay and one more thing i want since uh pointed out the discussion regarding the alerts and how do we get the alerts and all that for the burst happening are there any false alert also that happened very few uh they retracted it immediately very very few okay okay the number is very less yeah i think uh i'm over to you sarita okay uh may i ask yeah yeah let's go ahead so uh uh when you mentioned about uh the long grp is that these are mainly collapsing originated from collapsing stars and then short grbs uh they are occurring mainly because of the merger of say two neutron star or a neutron star in a black hole so is it possible that i mean or are there any such mergers where uh one component was a white dwarf and the other was uh say a neutron star or black hole which is generated energy the only evidence we have is from the merger of two neutron stars from the gravitational wave signal gw170817 we have not seen any associated counterparts or the electro magnetic counterparts from a neutron star or a black hole merger so far okay so there is only one clear-cut observation evidence okay so uh i mean but then uh what are the other uh possible origins for say short grbs neutron star black hole or two neutron stars are the proposed scenarios for uh the merger of compact objects and mergers of compact objects giving rise to short duration gaming bursts okay and where you had mentioned about the distances so you had mentioned the redshifts and then you had given the equivalent distance so like uh the nearest one was uh i think around some 37 megaparsec something like that yeah so any uh specific reason like uh because that distance is i mean it although it's not uh a very large distance astronomically or cosmologically speaking but still i mean it's a uh quite a big distance in which there must be quite many galaxies so i mean what is the reason behind like uh grbs occurring so far away yeah so um these uh again i did not mention one thing that the high red shift scenario is mostly for um long grps okay what has been observationally seen so far is that the short grps are lying at lower red shifts because they are also coming from the thought to be coming from the mergers so these um neutron stars would be more in like uh um nearby globular clusters and uh these um these are the primary locations of neutron stars right okay whereas the long grbs are coming from the collapse of uh massive stars and star forming their origin is in star forming galaxies the uh short grb's mean median redshift is only around 0.5 whereas the median rate shift of long grp is around 2.5 the mass limits of these stars that are forming long and short grbs yeah so short geoviews are coming from the compact object mergers in one case where the um in gw 1708 17 where the two two neutron stars merged there the neutron star masses were calculated from the gravitational wave signals they were around the lower one was around 1.1 solar mass and the higher mass neutron star was around 1.6 or 1.7 solar masses and typically the long gamma ray bursts they are coming from uh stars which are massive uh which are massive and have masses greater than about 20 solar masses it could be even more even higher around 40 solar masses okay so typically like in order of magnitude difference is there their masses um they're completely different right one is yeah yeah at ten times so normally uh code collapse occurs for stars which are greater than eight solar masses and that's how kurukala supernovae also originate right but uh the mass limit of grbs coming from collapsers is much higher than the normal good collapse events the normal collapse supernovae okay so we can uh easily um i mean say that they're coming from um stars which are more massive than 20 certain masses for long glbs okay and is there any uh distinction between them based on the type of stars like o-type stars or type three uh they are generally evolution stars massive world phrase stars wr stars are one of the main primary uh thought to be the primary progenitors of long gamma rivers [Music] yeah so maybe we will take up the questions that were asked by our participants from zoo uh rohit mundo wants to know the uh he says that low metallicity stars being progenitors of long grps has the massive explosion similar to supernovas how are they so bright than supernovas can we know previously if a star will go off in a grb or supernova if yes then how so i will answer the first part uh before uh that uh long grbs are coming from uh massive um from mess i just answered this question actually when mama asked that gervais are coming from much more massive stars as compared to supernovae they are not coming from stars which are of similar mass as the collapse supernovae and which is why they are bright brighter than supernovae and they emit much more energy as compared to the normal collapse supernovae and the other thing is that these are highly relativistic events so all stars cannot all massive stars cannot give rise to grbs okay they have to be special kind of massive stars which are rotating lying in low having low metallicity to conserve the angular momentum only those stars would give rise to gamma ray bursts all right so that is the first part of the question that why grbs are have emit much more energy as compared to supernovae and if we can know previously whether a star will go off in like a grp or supernova it's very very difficult to know because we have so many massive stars in the universe and to track each of uh to track the evolution of each of those massive stars as a is not a very easy task i would say so it is rather impossible to know beforehand which star would become a a grb or a supernova however in the recent times there are there are groups who are um who have undertaken uh projects um to map uh the the massive stars to the stellar deaths i am not uh very clear about that but to my understanding there is no prior way to know which star will immediately go off or should we be looking at we will have a rough idea when we know the evolutionary stage of a particular star we will have a rough idea but we cannot precisely say that today the star would go off and give rise to a grp order supernova [Music] okay so the next question is by rayard who but who wants to know he says the energy released has to go through the interstellar medium and many energy loss phenomena to reach our detectors then how are we estimating the high energy released initially so there are two different things actually because these um bursts are um occurring at cosmological distances which means it is an external galaxy so we are not dealing with ism there we are not dealing with the interstellar material there we are dealing with the circumstellar material there and as soon as the burst goes off once the burst goes off we are detecting the gamma-ray photons which are being captured by the space telescopes okay so the burst photons are being recorded by the space missions where there is no attenuation uh going on due to the circumstance material or the interstellar material at a very later stage once it is uh reaching us those sort of effects could only affect the afterglow not the initial prompt emission when the gamma ray burst when the gamma-ray photons have been recorded by the missions you can directly convert it using four pi dl square times the fluence so what the detectors are recording is the fluence which is earth per centimeter square per second so if a burst has a typical fluence of 10 to the power of 6 10 to the power of minus 6 earth per centimeter square per second you can calculate the gamma ray isotopic energy which is e gamma iso it's nothing but it just something around the four pi uh spherical area the four pi dl square times the fluence okay so the next question i have a couple of questions by churchill the first is you highlight the cause of large grbs to be hyphen away but what causes the generation of this tremendous amount of gamma-ray photons in the progenitor in the first place and what special scenario is happening over there so these are very highly relativistic events when the gamma ray burst occurs the lorenz factor is very very high these are rapidly rotating stars which are giving rise to gamma ray bursts and once this explosion takes place it emits in very high energies and because of uh the the very high lorenz factor involved the internal uh the internal shocks are created the shells are moving outwards the slower moving uh the faster moving shells they will slowly catch up with the shells in the outer regions of the ejecta and once that happens it gives rise to the internal um shock and in this internal shock scenario because the photons have a very high energy and they are highly relativistic in nature we are seeing the very high energetic photons over here once the ejecta cools down and this is also the optically thick phase this phase is uh around the burst is the optically kicked phase once the ejector cools down and enters the optically thin regime we are able to see uh the less energetic counterparts which are known as the after groups so the the second question is how far do you think primordial as a short grbs due to binary neutron star and neutron black hole mergers could help improving the very early universe and what what idea about the before burst central engine can be obtained from the observed grp data um to be honest i don't know the answer to the first part because short grbs are only a handful so far and the only confirmed case of its actual association with a compact object merger is from the gravitational wave signal so i will not attempt to answer the first part of the question i will attempt to answer the second part of the question which is about the idea before the burst uh central engine can we get from the observed grp data yes so once the burst goes off the all the information is lost what we have recorded is the gamma-ray flux or the fluence okay and then we are observing the after glow so what we have recorded is the afterglow we we try to construct a sed from the observed data and fit that sed to the different [Music] galaxy models different stellar population models and try to estimate the star formation rate and hence infer the different uh what kind of star would have given rise to such a uh to up to any particular burst so it is like back tracing the history of the progenitor star and the burst from whatever observations we have taken in the afterglow phase so those are the questions from soon are there any questions from youtube participants uh yeah there's one question some of them are already answered by kuntal already so there's one question that is not answered yet that says that is there any difference between the duration of after glows for the short grb and the long grp yes um there are there is i forgot to mention that so short uh gamma ray burst optical after glows typically fade very rapidly and they are also fainter as compared to the after glows of longer memory bursts what about the intensities of the after uh intensities of the gamma-ray bursts themselves that also is relatively uh weaker in the case of short-term short-term as compared to long grvs so it will be anyway difficult for us to be able to observe them if they are coming very from very far or very distant uh yeah but uh shajiris are typically lying at lower red shifts yes so we should be able to see them more if the relative intensity of the gamma ray photons was the same but the gamma ray emission itself in short short grps is weaker as compared to long grps that's all about the questions from youtube as of now i would like to ask one question uh at this stage that is there any like uh the after globe is uh coming because of the uh interaction with the medium also for example like you have a shock front that is created and there's another burst and you see an afterglow that in the diagram that you showed so is there any pattern that we see in the intensity of the afterglow like if there is any other interaction happening with maybe like a shock front and again creation of a shock front at a later stage yeah so what i presented was a very simplistic picture of internal shocks giving rise to the burst photons and the external shocks when interacting with the surrounding medium giving rise to the afterglow emission which is radiating via the synchrotron uh process there also could be inverse quantum effect but that is not seen very frequently okay so i asked this question for the very simple reason that if there are any burst happening in a region where there is a lot of extinction so can the interaction with the medium of the burst photons we observed in some other by some other means uh like you just now mentioned inverse compton or radiation or something else is why i asked this question that yeah if it is anyway happening at a very in a region which is at uh very high extinction the optical afterload would certainly be obscured yeah it would be very very difficult to um detect it in the optical bands but in x-rays that won't be the case if there isn't there is a detected um x-ray optical so sometimes you know these grvs are also known as dark grps there is a class of drbs which is known as dark grps where there is a x-ray afterglow detected but there is no optical after flow detected which means it is dark in the optical and this is one proposed scenario for a dark grp that it could either be lying in a highly extincted region or it could be like very very far away which we and outside the capabilities of the observational facilities that we have okay so the high rate shift grb limit nine point something that we have observed yeah is it it has to do with the the early formation of galaxies also yeah so that is um very it's occurring in the uh in the early universe and that red shift has been estimated using um photometry not through it is not a spectroscopic redshift but a confirmed spectroscopic red shift at 8.3 has been um estimated for a long grp okay okay yep uh that's all from my side and from the youtube questions uh yeah so thanks i think uh we will now be uh closing we are at the end of this session and we thank uh for giving an insight into the phenomena of gamma ray bus to summarize professor puntal explained about the types of grps the nature of emission and progenitors giving rise to these different types and also giving for giving us an overview about uh the different uh instruments and the satellites which are carrying out these measurements on uh behalf of asi poc we thank you and we also thank our participants in zoom and youtube for participating in the program thank you very much you