 We don't even know a gamma ray source is an AGM without multi-wavelength observations. So multi-wavelength observations are absolutely critical to understanding this very important topic in gamma ray astrophysics. So, want to start off with this data sharing idea. This is the critical Fermi starting point. And in order to understand this, I have to take you back in time once upon a time. Astrophysicists used to treat data sort of the way mythological dragons treated gold. We sat on them, we polished them, we squeezed every result out and only then released data by published papers. Now, in the past, there were very valid reasons for being dragons. Communication was difficult. Multi-wavelength resources were limited. The gamma ray data analysis was time consuming. We used to analyze things by hand, individual photos. Systematic uncertainties were hard to quantify. And certainly we wanted a fair return. Anybody who builds an instrument wants a fair return for the time we invest building it. The term multi-wavelength, in fact, is almost unheard of before the emergence of the World Wide Web in the mid 1990s. I did some studies a while back. I went and looked at ads and looked for papers that included multi-wavelength in the title. Now, if you do that these days, you'll find dozens of papers that say multi-wavelength. But I went back to 1986. And in the entire year of 1986, there were two papers that mentioned multi-wavelength in the title. So I went back a decade earlier to 1976. And as you might not be surprised, the number of papers in 1976 that had multi-wavelength in the title was zero. So things have changed. The first four things I list here have really changed. Communication is now possible, electronic communication. We have lots of multi-wavelength resources. Gamma-ray data analysis, thanks to computing, is improved. Systematic uncertainties, which were hard to quantify. We now have good modeling for these. We understand all of these things. The last item, we still want a fair return for the time we invest building instruments. And that remains, but these other reasons for being dragons have largely disappeared. So with this in mind, we go back to the time when we were planning Glass to be launched. That is before it was named Fermi, it was called Glass. And the large area telescope collaboration included some dragons who were reluctant to share any data. And these were people who had no experience in doing multi-wavelength astrophysics. They just didn't know. But the collaboration members who had multi-wavelength experience argued, the project would be a scientific disaster if we could not fairly share the Gamma-ray data. NASA headquarters stepped in. NASA was paying a lot of the money for this and they could apply some pressure and they did. They pressured the team into a compromise. The data release would be limited in the first year after launch. A loud checkout and verification of how things worked. But after that, all the Gamma-ray data would become public immediately. Turns out this was a good idea because in the first year after launch, we learned a lot about how the lab worked, things that had not been anticipated before launch. But afterwards, this critical decision to make data public immediately has then been the driver for the Fermilat multi-wavelength program. So how is this implemented? The Fermi Science Support Center was set up by NASA. It's the central element in sharing. And I show you an image of the webpage that has this. What do you get there? This is where you can get the data. You can get the software to analyze the data. There's a help desk. There's a guest investigator program. There's a users group that provides advice and pressing outreach activities are all handled by the Fermi Science Support Center. So this is the way we implement the data sharing. Now, you may have noticed on that slide, there were a couple of links to Fermi blogs. This was an idea that we came up with to share information via blogs. And I show you an image from one of these about which sources were active in a given week. We thought this information might have been useful for multi-wavelength studies. In practice, almost nobody read the blogs. We haven't updated them in years. When you have all this experience, sometimes you learn about things that work and sometimes you learn about things that don't work. Blogs turned out to be one of those that weren't such a good idea. We did, however, decide as a Fermi-LAT collaboration, remember Fermi is two instruments, the LAT and the GBM. The LAT collaboration, we had a basic agreement. We deliver the gamma ray data. We deliver the software to analyze anyone to analyze the data. We provide documentation for that. We also agreed to provide some higher level data products that would encourage multi-wavelength studies. We said we would provide source catalogs in machine-readable form that would enable analysis and plotting. We would provide pulsar timing information. And we said, oh, well, we'll provide light curves daily and weekly for a selected set of about 20 sources, mostly AGM, and these were based on the LAT team's automated science processing. This is a set of software that runs automatically as soon as the data become available. The ASP generates this information. It's sent to the Science Support Center and these plots are generated. I show you an example here of one of them. This is the light curve over the history of Fermi for PKS0538 plus 134. This is one of the original 20. Now, all those red spots you see there are upper limits because we chose those 20 based on what we had seen during the Compton era. And back then, this was an active galactic nucleus that was gamma-reactive. Since then, it has not been very active. This is like those ads you see on television advertising you to buy something. Past performance is no guarantee of future results. But nevertheless, we provided these. So, having done that, what did the LAT team decide to do? Here's our basic idea. We take advantage of the fact that Fermi is an all-sky monitoring. We see the entire sky typically every three hours. We're gonna use that capability to offer useful information to the multi-waveline community. We realize, well, that data are public. Most astrophysicists don't wanna spend their time learning how to do gamma-ray data analysis because it's different from X-ray or optical or radio analysis. Second, by example, and by direct communication, we promote the idea to other astrophysicists of making at least some data public in useful formats. Nothing useful here, nothing unique here. It's useful, I hope, but nothing unique. The various other groups were already doing this. The third step is we encourage other scientists to work with us on multi-wavelength and now multi-messenger projects. Not everyone's gonna choose to do that. And we accept that risk, but we make this offer in the interest of maximizing the scientific return from the Fermilat data. We've set up Memorand of Understanding with many groups to work on joint projects. Now, I should emphasize here, we didn't do this because we're particularly altruistic. This is really a case of enlightened self-interest. See, Fermi being funded by NASA every three years has to go through a senior review. We have one coming up next year. And which NASA is gonna decide whether or not they continue to fund Fermi or whether they dump it in the ocean. And the more scientific papers and the more presentations at conferences that use Fermi data and Fermi results, the easier it's gonna be for us to convince NASA to keep funding the project. So this basic strategy of sharing is based on our interest in getting the maximum science but also in keeping the project going. So our three applications of these principles then are monitoring, pre-planned campaigns and rapid responses to events of interest. All right, monitoring. Well, as I already said, AGN are highly variable. And one of our goals then is to compare variability patterns. Differences in variability patterns or lack of those shed light on where and how the radiation is produced. What we'd like to do is use analysis tools like this, discrete cross-correlation function. They can extract needed information but only if you have simultaneous or at least contemporaneous data available at multiple wavelengths. So monitoring of AGN without regard to the level of activity provides the data needed to carry out such analysis. Well, we get that automatically with Fermi and what we wanna do is encourage other groups to do the same thing. But we get this with Fermi, how do you provide this data in a form that's useful to those other groups? We have a number of ways of doing this. We have quick look data. This is our monitored source list. This goes back to what we originally promised NASA that we would have these 20 sources and we would put out daily and weekly monitoring of these using the automated science processing. We've now expanded this. We add a new source to this list every time the lat daily flux above 100 MeV reaches one times 10 to the minus six. We have now over 180 AGN are part of this list. I show you an example here. PKS0346-27 was only added in early 2018. Before that it hadn't been bright enough. It has however remained fairly active since then. What you see here in this diagram in the upper is the entire history from the beginning of the lat mission and down below it's the most recent one year data segment. And you can see not very many red spots these are actual detections. These light curves are updated daily. Now these are automated processing. It's not a full likelihood analysis. And so the calibration is not perfect but it gives a good idea of the activity and it does so daily. So if you have a source you might be interested in monitoring you can go to the Fermi website and say, oh yes, this is one Fermi has been monitoring. And by the way it's data activity in gamma rays has been pretty solid for the last year. Maybe we'll go ahead and monitor this one. That's the idea. Another way is using aperture photometry light curves. Remember we have only 180 monitored source light curves. These 30 day light curves are done for all known Fermilat sources. Energy is above 100 MEV based on one degree apertures. No background subtraction however. But again, you can see the pattern over the entire course of the Fermi mission. And I show you the light curve on 30 day intervals for this same AGM that we looked at on the previous slide. And you can see it was basically dead zero for years and years and years and then suddenly became active. These light curves are updated weekly. In addition to this, it also produces a power spectrum for all these light curves, any power spectrum between 65 days and the length of the light curve. So this is another way of getting an estimate of what activity is taking place on an AGM. A third way is something called the Fermi All Sky Variability Analysis, FABA. This is photometric analysis of all that data. It's done on weekly interviews, intervals, studying two energy ranges, 100 to 800 MEV and 800 MEV to 10 GEV. And this is available like those others on the Fermi Science Support Center website. Provides a list of flares every week. And the thing is, it is not limited to known sources. What it does is to take the sky and energy as seen in a recent week and compare that to a reference gamma ray sky accumulated over several years and look for differences. And therefore it can indeed detect new flaring sources. It can also generate light curves, photometric analysis for any point in the gamma ray sky. This is once again the same AGM using the FABA light curve generator. And if you have a source you're interested in, whether or not it's ever been detected by Fermi, you can look using FABA at the gamma ray light curve for that point in the sky. So all these different ways of doing this. Now, all these methods produce useful information, different methods, different time scales. None however, produce calibrated quantitative flux values. This is where we've got something new coming. Publication quality Fermi light curves. And this is coming soon. Actually it's something we promised to NASA in the last senior review. This is a full maximum likelihood analysis including all the backgrounds, all the nearby sources. This is a project led by Daniel Kaczewski. It's gonna be publication quality light curves for all the variable sources in the 4FGL data release two catalog. There are 1500 of these. And they're gonna come out with time bins of three days, one week and one month. I show you an example here of a three day light curve for one of these AGM. And the plan is to update these light curves regularly making them useful. Quantitative multi wavelength analysis is gonna be possible using these light curves because they are going to be the publication quality light curves and you can extract the data and then construct your own discrete cross correlation functions with whatever data you might have at other wavelengths. All right, so this is something new. What about time resolved energy spectrum? Our monitoring data don't include any information about time resolved energy spectrum. Flux values are not particularly sensitive to the spectral shape. The automated science process does construct a power loss spectrum for each source typically with large error bars because it's over short time intervals. But we know that many lat sources are not well represented by the functional form. So we've decided not to make those public. It wouldn't be scientifically valid. The basic issue here is that on short time scales the lat data are usually limited by photon counting statistics. So what is needed is a way to define what is a relevant time interval over which you might want to construct an energy spectrum. We're not gonna do that. The effort is just beyond it but what we do have are ways to define quantitatively those time intervals that might be of interest. And there are a couple of ways of doing this. One shown on the left is the Bayesian block method developed by Jeff Skargle. This was actually developed before Fermi launched. Jeff is however part of the Fermi lat team and has emphasized how it can be used for Fermi lat data. Basically a Bayesian block is a time interval over which you can say, this is not a statistical fluctuation for this flux. And this is where it changes to where it is a significant change. And you can see the Bayesian block shown in blue here breaks down intervals. Although you can see up and down on this curve beyond the blocks these are not statistically significant the Bayesian blocks gives you that. A complementary approach developed by Benwell a lot and his colleagues is to use adaptive binning. That is you expand your light curve interval to a long enough time that you get a significant detection. And then you've got something where you have enough statistics then to construct a light curve. Something useful that we provide. All right. Now, so far what I've talked about is just Fermi lat data. What about multi-wavelength monitoring? There are lots of groups who've already made monitoring data available for quite a few sources. And I list a bunch of those there. Fermi project and the Fermi lat team have been encouraging programs like this. Some of these have been supported by the Fermi guest investigator program. And then if the sources happen to be monitored the data readily available when interesting events occur or if you wanna do some analysis where you want to simultaneous data because remember you can always get the Fermi lat data. Now, we obviously do not do multi-wavelength monitoring but what we can do is make links to many of these resources available to anybody who wants it. And I show you here a couple of links to lists of groups that do multi-wavelength monitoring or multi-wavelength monitoring at particular wavelengths. And so if you find a source you're interested in this is a way to go to something provided by Fermi that tells you where you might look. Another thing we do to encourage cooperation is it's our policy to make sure we refer to the source of any data, even if the data are public because we know there have been groups who have made their data public, had their data used and had no reference to it. That's not fair. It goes back to the idea of getting fair return for your efforts. And the lat team policy is to make sure that we have at least invite the observers to participate in our papers if they want to, or at least make sure that we have very clear references, web links, references to papers, the people who've done the work. One particularly useful link, especially if you're interested in blazars is one that Matt Lister, he's with Mojave Radio Monitoring. He has a list of which observatories are monitoring which blazars. And this list is updated regularly and it can be searched. I show you a segment of it. What I did was I listed it by the most heavily monitored AGM. Take a quick look at it for a moment. If you look down this list, you'll see that the most monitored blazars over multi-wavelength are all Gamma Ray AGM. So if you're working on a blazar, here's a good reference source to go to. All right. What about planned campaigns? After all, monitoring programs cover only a fraction of the known Gamma Ray AGM and different monitoring efforts cover different sources. So multi-wavelength campaigns on specific sources are a good way to collect the data. The obvious comment here, organizing a multi-wavelength campaign is a lot of work. Starting with deciding on a source and then finding the resources to make the observation. We tried a couple of times as lack collaboration to organize a system. We have one I show you here called our VIP list, our very important project list. And we decided we would signal these out as ones that we thought ought to have multi-wavelength campaigns. Turns out this sort of top-down planning didn't turn out to be particularly useful. And the reason is back to this idea that a multi-wavelength campaign is a lot of work. Our experience is that multi-wavelength campaigns are largely driven by individual scientists who have a vested interest in particular source. And this often happens because they have access to observing facilities that can provide essential information. They have some prior experience with these sources and because these sources have some interesting properties. I'll show you an example here. This is a work on 4C plus 01.02 and was actually listed on the previous slide. This group has time on the South African Large Telescope, salt, it can do polarimetry. It's interesting because it's a fairly distant place on Redshifted 2.1 and has shown significant gamma-ray variability in the past. So results were presented of just a couple of weeks ago at the Fermi Symposium on this source because this group had chosen to do this. This is the best way to do it. You can't do it top down. Give you some idea of what's involved. The most extensive multi-wavelength campaigns involving gamma-rays have been those on Markerian 421 and Markerian 501. I show you a listing of the participating instruments on one of these campaigns. It's a tremendous effort. And my congratulations go out to David Ponek and the other participants for keeping up is a long-running planned campaign on these two AGM. And this is what's involved in this. So what can we do? Well, we can provide support for these planned campaigns. You go back to the FSSC webpage, you can find a number of links. You can find, for example, the observatory status, our timeline, targets of opportunity, which were possible. We no longer do those because of the solar array issue we had a couple of years ago. But you can also tell Fermi if you're planning a multi-wavelength campaign. And you can do it confidentially if you want. Basically, the idea is you tell us when you're doing your multi-wavelength campaign and we can make sure the project operators don't schedule a calibration that would take Fermi offline for a few hours. So this is something that we can do to support these planned campaigns. We can also provide information about individual sources. That is, we've seen a number of these sources and here's a link to a list of what we call friends of the sources. These would be the contact persons. People who've worked on these sources on behalf of the Fermi Large Area Telescope Deliberation. These are all the AGM that have been bright enough to have wanted a Latt Astronomers Telegram. And so if you decide you're interested in one of these, you can go to this link and say, oh yes, if I wanna do a study on PKS 1502 plus 106, the Latt person to talk to is Stefano Ciprini. So that's the information that we can provide. Well, third area, rapid response. Now, we view the entire sky. So it's, one thing we can do is to alert the multi wavelength community to interesting activities. That's critical to making the best use of our data. And so we use a variety of ways to report AGM activities. One way is using GCN notices. Now, if you're familiar with GCN, you're probably familiar with it in terms of gamma ray bursts, but they can also be used to report activity for AGMs. And we have done this, we do this regularly. They're generated automatically, again, using the automated science processing. And it's fairly conservative. What we do is we look at the data for all the monitored sources every day and ask is today's flux five sigma above the previous two weeks average. And if so, a GCN notice goes out. This is often the fastest way of getting information about a flaring source seen by the Fermilat. These are also fixed format notices and therefore they are machine readable. So you can set up your software to read one of these and say, ah, yes, it's interesting. I wanna point my telescope at it. So it's a useful way to do it. Second approach, we have what we call a flare advocate program. These are volunteer members, the lateral liberation. They take one week intervals running a set of analysis scripts, primarily written by Dennis Bastieri and Sarah Boussone. They run daily and it generates a report about all bright sources seen on both six hours or daily timescales. And then sent to the full team of flare advocates, show you an example of one here. The flare advocates then prepare astronomers, telegrams and the one that came out today on BLLAC activity. The flare advocate then typically becomes the friend of the source who would follow up on any multi wavelength campaign. Well, the flare advocate reports include approximate spectrum and lists of gamma rays with energies above 10 GEV, whose rival directions are consistent with locations of sources. Now, if a spectrum is hard enough, or if there are enough gamma rays above 10 GEV of possible interest, we can send an email to the TEV facilities with whom Latin has memorandum of understanding. Here's an example of a recent message. The way we have this set up is the flare advocates generate the report. And so there's no confusion about whether it comes from the Fermilat is I'm the one who sends it. And so I have this list of people who received this message. And this is again about BLLAC, which has been active for some time. And this goes out and alerts the TEV community. We have something else that we decided to try called the GAMA MW mailing list. This is a moderated archived mailing list. You have to be a member of it. You have to sign up for it. And the messages are archived and you can post messages to it or you can read it. Seemed like a good idea. In fact, it was prompted by a multi wavelength meeting. And it was actually the group from Veritas that suggested it. And we set this up still exists but it really has not been extensively used. It's not one of those great ideas. Nevertheless, it does still exist. And because it's a moderated list, it avoids the spam that creeps into many mailing lists. Well, what about the future? Well, with the emergence of observatories like the Zwicky transient facility in the Vera Rubin Observatory, that's the new name for LSST, they're gonna generate vast numbers of transient notices. It's gonna be necessary to have new systems of data sharing because the flood of results are gonna overwhelm anything like I've just described. So brokers are needed. Here's a link if you're interested in finding out how these brokers work. We think of transients as primarily dealing with Gambery Burse, but we know that GCN's already used to distribute information about AGN players and neutrino awards. There are two data management systems under development. And they're outlined in the next two slides. These are courtesy of Judy Rackison. First one is called Time Domain Astronomy Coordination Hub, TEC. And you'll see down at the center, this is a brand new version of GCN. And it will allow things to come in from places like ZTF, LSST, and all these other multi-wavelength, multi-messenger resources to send them out. I'm happy to say Scott Barthelme is still involved. Scott has kept GCN going for many, many years. And the fact that he's involved is some assurance that this new tax system will continue in the capability that we need. Another one is called SkimUp, scalable cyber infrastructure to support multi-messenger astrophysics. And this is sponsored by the US National Science Foundation. And it's primarily trying to deal with these huge floods of data from the optical telescopes. I don't know much about the details. It's really just under development. But what I can tell you is the TAC and SkimA groups are working to develop compatibility. And this is the future of sharing information. So at this point, I've shown you a whole list of things. How can the system produce results? I want to show you an example that illustrates how a lot of these pieces fit together. It's a familiar result, but the background might be interesting to you. It all started with a GCN circular, shown below. This announced a high-energy neutrino event seen by the ice cube observatory. And it announced this was a tracked event, which meant it had a pretty good localization. And it had a high level of confidence that this was an astrophysical neutrino. They set out this circular. And people did the usual follow-up. Everybody who's interested in neutrinos goes in looks and number of ATLs and number of GCN circulars appeared, listed some candidate objects or upper limits. Nothing was compelling as a counterpart. Few days later, I was in Amsterdam, attending a meeting about, of all things, transience. And we received this message from one of our Flare advocates, Yasuyuki Tanaka. And he said, we followed up on this. We did an optical observation and found there's this QSO that's brightening. And it's in the 3FGL catalog and it's a blazor. Well, it's not one of the monitored sources. So we had to use FABA to determine its light curve. And we discovered that it's flaring. So what should we do? Should we submit an ATL? Should we call the TEV people? Because it has a relatively hard power law index. So you see all these elements, the catalog, FABA, the ATL, email, all of these things. And Yasuyuki picked up on it. So next steps. The Flare advocates went to work on the ATL. I sent the message to the TEV community. And of course, since I was at a meeting at Transience, I talked to people at the meeting. There were people there from the TEV community, from SWIFT, from the ice cube community. And we put out this ATL. Yasuyuki was the lead author. Sara Boussone and Dan Kochetski were involved in this because they were already working on trying to calculate the chance probability of this occurring just at random, having a flaring blazor. We already knew it had brightened in the optical. We already knew it had brightened in the radio. So we sent out this notice. Well, a couple of days later, still at the meeting in Amsterdam, Anna Frankowiak sent me this message. She sent it across the room because she was attending the same meeting. Why don't we get the Fermilatin ice cube teams together and do a joint paper? We have an MOU with ice cube. So I then sent off a note to the leaders of the ice cube group, inviting them to consider doing a joint paper. Meanwhile, over on the right, you see the magic team had gotten the message. They had done more observations and they had made a VHE detection of this same blazor. So you put all these pieces together and the outcome was a multi-messenger paper in science. The U.S. National Science Foundation at a press conference got extensive press coverage. It was on the cover of science. It really got a lot of press. But considering this, this was essentially a three-sigma result. But its impact was significant primarily because all of these groups cooperated. If it had come out from just one of these groups, it probably would have gotten buried. But this is the way the system works. And I would say all this planning, we did all these options of sharing information, not every time, but all in all has worked pretty well. So I put up my summary and emphasize this point at the bottom. Fermi-Lat collaboration continues to work. Fermi is still in operation. ATL came out today and we look forward to continue cooperative efforts in the CTA era. And I thank you all for listening.