Part history, part science, part roast: the afterglow era of gamma-ray bursts as lived in Caltech's GRB group from 1997, told through thirty years of emails with George Djorgovski: the 970508 redshift, host galaxies and offsets, the GRB–supernova connection, and how George taught a young student to be a spectroscopist.
“This is part history, part science, part roasting, so the good news is I have receipts: 30 years of emails, which I'll be referring back to throughout the talk.” – Joshua Bloom
“It was my only rejected paper ever, and needless to say, before I met George, I wasn't very happy with him.” – Joshua Bloom
“I realized as I looked back through all of these emails, he was indeed teaching me how to be a good spectroscopist.” – Joshua Bloom
Transcribed from the recording by machine (Whisper) and lightly edited for readability; may contain transcription errors.
Hi everyone. We heard about George in the high-redshift universe, but with GRBs it was high velocity, and that's maybe something that you haven't heard as much of today. The nights and days were very long in the early days of GRBs, and I want to tell you some of the stories from that. This is part history, part science, part roasting, so the good news is I have receipts: 30 years of emails, which I'll be referring back to throughout the talk. This is George telling me, 20 years ago, he didn't want to have a Festschrift, which I had to look up. But here we are with the GeorgeFest. So let's go.
For the GRB aficionados in the room, I thought I would just try to accumulate everything we've done in all of this time. It's now over 50 years since the first set of discoveries, and still going strong as a field, really separated by three different eras. The afterglow era is the one that Dale was referring to, and this is the one where we've spent most of our time here today. More recently, the multi-messenger era, which I'm sure many of you know about. Before that, GRBs were decidedly just high-energy events. You see just what a remarkable moment it was in satellite land, and what it's becoming again, for the study of the high-energy components of these events. So we are here, and I'm also surprised no one has talked about the GeorgeFests that have happened in the different time periods that people have been talking about.
You saw this from Dale in different forms: before the afterglow era started, there was this idea of isotropy on the sky, so it looked like they were evenly distributed across the sky, but it looked like there was a paucity of faint events, which looked like there was some sort of edge, which could be seeing to the edge of the observable universe or something like within our Galaxy. So that's where the great debate lay, in looking at these two data points. Looking at all of this here, this is the distribution of the two different classes of GRBs, separated in time and the peak energy of the events. These became called short, hard GRBs and long, soft GRBs, and there was an indication even before the afterglow era that there might be two populations of progenitors.
The afterglow era started in earnest in February of 1997. You heard about this from Dale: the discovery of this fading X-ray object that faded on timescales of hours, coincident with a gamma-ray burst. This allowed, for the first time, astronomers to pinpoint the location, not just at X-ray wavebands, but also to discover the first optical afterglow, and I had the privilege to be part of that discovery as well. Shortly thereafter, it was decided by the Italian Space Agency that they would create the first workshop and bring everybody to Elba Island in May of 1997 to start discussing what it meant for the afterglow era to begin. Little did we know that right before that, 970508 would happen, and you heard also from Dale just now about the discovery of the absorption redshift.
In terms of the speed at which this came out, you can see the UT time was the same as the date that it came out, so that was pretty fast. Some people had their initials there; George did not. Some people put their initials after their name. Some people said CIT, and then right after that they said CIT. And some people, like Dale, didn't have initials or his location. This was written very, very fast, but it was very, very right and very, very, very important.
I had been working on, as a master's student in Cambridge, the idea that if GRBs were cosmological, we could learn something about the progenitors. And I wrote a paper, which I submitted two days after 970508 (the first mention of 970508 on the arXiv, fortuitously; this had been ready to go), that if we could see the location of where optical afterglows were relative to galaxies on the sky, even if we couldn't associate them, with just six or seven optical transient events we'd be able to make some statements about the interconnectedness of them with their host galaxies, and hence learn something about the progenitors. I was very proud of this paper, and it sort of sat with the Monthly Notices for many months. And then, after the host galaxy of 970508 was finally discovered and it became very, very clear that GRBs were associated with galaxies very tightly, the paper was rejected. And it turns out George was the referee. It was my only rejected paper ever, and needless to say, before I met George, I wasn't very happy with him.
But we went off to Elba Island, and this is a scan of the final program. Shri was there, and I believe George was not, but a lot of the great work that had happened at Caltech over the last couple of weeks was presented. Shri accosted me, I think you can say, on the boat on the way to Pisa, handed me a stack of papers, and said, “We're going to start a GRB group. You're going to work on it.” And I said, “OK.” And that was my introduction, really, to Caltech.
I had the great pleasure of getting to write the News and Views that described some of the great discoveries of what was presented at that first meeting, and in particular the ending of the great debate: that indeed GRBs are cosmological. And you can see a letter (we did this with Mal Ruderman) from Nature, and he was faxing us the spectra and George's paper, which we got advance copies of so we could write the News and Views so it could come out at the same time. And for me, as a young student, it was a great honor to be able to write about this moment.
I joined Caltech in the fall semester of 1997, and George's mentorship of me began, and our collaboration began. So you can see how bad I was. I didn't know that O II is a forbidden line, so I forgot to put the brackets here. And I'm like, “Can you tell me about what's going on? And I hope to talk about this stuff when you get back. I'd particularly like to start learning” (we didn't have spell checkers back then) “about faint spectroscopy, and I'd like to get some references from you to get me going,” because I wanted to learn from George, as the spectroscopist, what I should know. “Also, I'd like to discuss my MNRAS paper, for which you were a referee.”
That was October 5th, 1997, and then within a month I'm asking George to change plots and update his plots for a paper that we're writing. I don't know how that happened so quickly, and I don't think that would fly today if my students did that to me, asking me to change plots. But George says, “I'm too tired to do this data reduction now, and why are you still working on this paper? Don't we have to write proposals?” Things are starting to accelerate.
And then, as you heard from Dale, the great discovery at an incredibly high redshift for the time, 3.42. I'm telling George on December 17th, “We've got a spectrum. Shri's pulling it over.” George wants to know where the data is; he's ready to go. And then, like, 10 minutes later I get an email from him saying, “We're having a party at our house tonight, just a reminder for you to come over.” I guess we were welcoming Mike Pahre at the time. And I'm like, “George, I'm at Keck. I'm not coming. We're working on something.” And then one of my favorite emails from George is: “Please do not say a word to anyone about this new redshift possible line. Shri would be very, very pissed if the word gets out. Not even to Bob Goodrich at this point. Know more tomorrow.” He knew that we had to keep this secret and keep it close to the chest.
We wound up getting HST imaging in the end, and this really began my interest in the host galaxies of GRBs, which became a big part of my thesis, as I'll talk about in a moment. As we're learning about the redshift distribution and the cosmological nature, we're also getting afterglows, studying these things at multiple wavebands. And the model of afterglows starts to emerge, in concert with some of the theory that's being developed here at Caltech. And some of the papers that were important at the time were for understanding things like collimation. This is really, I think, the first really good example of jetted emission, where you have a power-law decay and it actually breaks off, indicative of a jet, because it happens at all wavelengths at the same time. This is a really important paper that Fiona led in 1999.
So all of these things are emerging as we're learning about the physics of the afterglow. I'm getting interested in the host galaxies of GRBs, and also the locations (back to my original paper) of where GRBs are happening relative to the predictions from different types of progenitors. At the time, merging neutron stars seemed to be a pretty good candidate, but it looks like (and this is one of the chapters in my thesis that George and Shri are part of and had a huge role in) it just doesn't match up with the long, soft gamma-ray bursts and where they ought to be if they come from merging neutron stars.
Around the same time, we're also learning from a GRB from 1998 that there appeared to be a weird bump, which we explained in a paper as being due to a supernova. That was early evidence, photometric evidence, for the connection of GRBs to supernovae and the death of massive stars. And the spectroscopic confirmation that people point to is from 2003, where you see the overlay of the spectrum of a GRB afterglow, which is actually just the supernova that follows it, compared with another broad-line Type Ic supernova. What people probably don't know is that we also had a spectrum right around this time that George worked on, and we decided not to put it in the Nature paper. The only thing I could find is a viewgraph from an old talk, where we compared the spectrum of a (this was a Type Ia) in the UV to a binned-up version of what we had from Keck. We didn't have enough confidence in it, and we also didn't have a redshift of the GRB. And so I think we were being quite circumspect in not putting this out as part of the evidence, because the photometric evidence itself, I thought, was quite compelling and exciting.
I did a little sleuthing to look at the work that George and I did together. So I'll first point you to this one right here, going from 1997 up to 2006. I left for my postdoc here and then went off to Berkeley after that. Everything in purple are papers that George and I wrote. There were 38 papers in total on GRBs during my time here, and then there were another 47 papers that I wrote without George over that time. He happened to write a few papers on GRBs without me, a few before I got here and then a few after I left. But it was a very special and amazing time for us. We wrote a lot of circulars (these are not refereed papers, but they're communications), and we wrote a lot of emails to each other, a lot, thousands. I looked through all of them; I had my Claude look through all of them. And if you just look at the citations, it's clear that George did well by working with me, because these are the citations he got from just his papers on GRBs without me, and these are the ones that we did together.
I asked Claude, “What are the sauciest, spiciest emails that you can find from George to me?” And I found three here that I can show; there are many I cannot. Here's one where he's, again, being quite paranoid, rightfully so: “By all means keep this absolutely quiet. The enemy has ears and does not sleep either.” In another one he's actually kind of crapping on our HST proposal, where we're saying GRBs may tell us something about cosmology, and he says, “It's clear to me we can learn absolutely nothing new and interesting about cosmology from GRB hosts. To pretend otherwise is to invite being shot down, deservedly, by the TAC. Thus spake your local pundit.” And then the one I think is awesome: I had an ad hominem attack on me, from a well-known person, after I wrote a paper, and I was pretty upset. And George wrote this back to me: “What I had in mind was responding with something like ‘On the ontological emptiness of the [redacted] model as a social construct, whereby we offer a detailed post-structuralist neo-Marxian critique in the context of being and nothingness as related to the crypto-Freudian undertones,’ you know, that kind of thing, or its dialectical opposite. George.” Yeah. So this is definitely not ChatGPT, because you can see the year when this came out, right? But it sounds like it.
I went through lots of emails, some that I had printed out, and finding charts that he faxed me when I was at Keck as backup charts. George didn't like to send images; he would just send locations of where galaxies were: if this was an image, this is what the sky would look like. And I realized, as I looked back through all of these emails, he was indeed teaching me how to be a good spectroscopist. He was telling me how to do the setups, how to take flats, and “don't worry if you don't get the calibrations tonight, you can do it tomorrow.” I have troves and troves of these, and they're a real joy to read. And one of the things that I love is he's telling me what to observe or not to: “Just sticking with spectroscopy also makes the observing simpler. And do we really need any pictures anyway?”
So with that, I'll say thank you. This is another slide from another talk of mine. It was a large effort, and it was a heady time and an exciting time in the history of GRBs, and I'm so thankful for the time that I got to spend with George and learn from him. So thanks, George. Thank you.