Welcome everybody to today's colloquium. It's a pleasure for me to use Sure. Tongue. She graduated in China at Tsinghua University in Beijing. And then she came to the US for your PhD, which we did at Columbia University in New York. And afterwards became a postdoc, processed a postdoc and housing assignments fellow at Cambridge. And later she moved onto boston. Was Einstein Fellow inside of nasa habit fellowship before she came became 2020 then an assistant professor at College. And we are very happy to have her here with us today for the colloquium and then for seminar tomorrow. So thanks for coming and please start. And you're French. Firstly, I want to thank you for giving me this opportunity to share maybe six inches the width year. And you Thank you for coming to this talk. My talk today will folk song. My research interests are in counting for penetrants are very powerful particle accelerators in our Milky Way galaxy. So firstly, let me, let me just show you the outline of my talk. In the first portion of my talk, I will folk song, the galactic center, supermassive black hole, which is called statutes such terrorists a star or Saturday start for short, which is thought to be a likely, one of the most likely candidate for a PABA trunk in the galactic center region and space. It could be. I want to discuss how we can use multi-wavelength observations to approve a footprints, a supermassive black hole, serving as a public Trump. Next, I also want to discuss in what stage of Saturday start activity can it serve or most effective, effectively serve, serve as a strong particle accelerator. And in the second portion of my talk, I want to discuss some ongoing efforts on discovering and identifying other galactic penetrants outside of the galactic center. And I also want to lay out some of the efforts in multi-wavelength follow-up observations on some are very recently discovered potential public transacts. Now firstly, I feel obligated to use one slide to introduce what is the average home. Now we have fall off the experts on this topic in the audience. So a pebble Trump is a source that is capable of accelerating particles like protons or electrons up to the p, the energy range p, These tend to 15 electoral vote. And these extremely relativistic part charged particles are cosmic rays. Now the plot on the upper right corner is perhaps 11 of the most famous plot in modern physics, which shows you the energy spectrum of cosmic rays. And especially there's a feature around one TeV or sell and that, that is called the knee because this spectrum can be very well feet without broken power law. And that the break around slightly above one PDF of the PED is cut the knee. And the cosmic rays with energies below the knee are relieved of galactic a region. However, a century long owed question is, how are the cosmic rays produced? Where do they come from? What type of God Higgs sources conserve as a public Sean, deaf produce this cosmic rates that are very important questions remaining to be answered. And I want to know the answer. So where to find these penetrants? Now we have some clue from the earlier slide, right? The PABA, which produce cosmic rays up to 100 TeV or of galactic, our region, therefore, the entire galaxy, the Milky Way galaxy is our playground here. So for the non astronomers, let me just use this image to remind you of the spatial scales we are talking about in this. So this plot is a top-down view of our Milky Way galaxy, which is a spiral galaxy with a radius of about 15 kilo parsecs, when parsec equals 3.26 light years. And our solar system is located at somewhere between the center and the edge of the galaxy, about eight kiloparsecs from the galactic center. By a galactic center, I mean the dynamical center of the Milky Way, which harbors a supermassive black hole subject, start with a mess for 400 million times the mass. So that's the spatial scale we are talking about. And our private wrongs are just a hidden somewhere in this plot. We want to know where they are and what they are. Now why we are specifically interested in the galactic center region. That is because a couple of years ago, the has gamma ray observatory claim the discovery of a pepper tongue located within 10 parsec from the center of the galaxy. So let's look at how they show. I'll draw this conclusion. So this plot actually shows accumulated has data. What they see is that the TEEB gamma ray emission especially correlates with the distribution of code molecular gas along the galactic plane. So those reddish signal you are seeing here are the gamma ray signals and they correlate very well with code molecular gas. That is, that at this back actually points to the fact that these galleries signals are produced by hadronic process, which means proton-proton interaction. So some protons with very high-energy collide into the nucleus in these ambient CO2 gas and produce the gamma ray emission we are seeing here. And by fitting this gamma spectrum with the proton-proton interaction model, that has calibration concluded that it requires parent protons with allergies up to one TV. That is how the concluded that there must be a hadronic Republic Holmes somewhere in the galaxy center. Furthermore, by studying the spatial distribution of this gamma ray emission, they can conclude that this peloton should be located within 10 parsecs from that garlic center, overlapping with the position of sati start supermassive black hole. All right, 10 parsecs. My stance, very small spatial timescale for an astronomer. However, even within one parsec, the Galactic Center, we have like supernova remnants pose that we nebula and millions of stars and sun-like star forming regions. On top of that, we have the most peculiar object, the supermassive black hole. Now again, such a star is considered the most likely candidate for this galactic center Tevatron. So no matter whether you believe that the statute star is the answer to the nature of the galactic center public Trump. As far as we have a hadronic Tevatron in the galactic center, we should be expecting much wavelength and also multi messenger signatures from the existence of this Tevatron. Because it can produce like protons up to more than one TeV. And these PV protons, once they leave their source, now a large fraction of them would eventually collide into the molecular gas, which are abundant in the Gallic central region. And proton-proton interaction will take place. And we know that such an interaction, protein-protein interaction can produce very complicated secondary particles, including motor TEEB, neutrinos, which are of interest to as cube and other neutrino detectors. And it's like spectral energy distribution from a Gallic center at Peloton is shown here in this plot on the right. Now, another type of secondary particles produced by the proton-proton interaction are like neutral pions, which actually eventually decay into the gamma ray photons as observed by Hass, which we just saw in the last slide. Now among all these different source of secondary particles, I'm particularly interested in that 100 TeV electrons, which is another type of secondary particles produced by the proton-proton interaction. The reason why I'm interested in this population are the electrons, is because considering the magnetic field strength in the galactic center environment, the 180 EB electrons can produce detectable synchrotron emission that falls into the X-ray band. And that can be observable or detectable by currently operating X-ray telescopes. But the question is, out of all the different types of celestial bodies are sources in the Gallic center, which is very busy region. What's type of source can help us to probe 180 electrons. Here is my answer. So let me just, I'll walk you through this image. Now, this image is a radio image obtained by near a cat or radio observatory a couple of years ago. Now this is the image of the galactic center. Horizontal line is the galactic plane. And the spatial scale of this image is about 300 by 100 parsec. Now Saturday star, the supermassive black hole, somewhere in the middle. This image shows a lot of different features in the galactic center, like some supernova remnants, some giant molecular clouds, very busy region. But specifically, I want to draw your attention to these arguably most peculiar objects, which are very thin and the long filamentous jobs, structures you're seeing here. At least several of them, which are roughly like several of them are perfect looks as if they are perpendicular to the galactic plane. Right? So these features, they are actually pretty thin and as you can see it runs, it can extends their long up to 10 parsecs. They are caught magnetic filaments. You cannot really think of any object. You'll learn from standard astronomy textbook to explain what they are right now. They are, they are referred to as magnitude filaments because our best model for these type of source is that what we are seeing here is the radio synchrotron emission that tris the magnetic field lines within or distributed along the filament. So what's happening here is that charged electrons are charged. Charged, charged particles spiral around magnetic field lines within or along with the filaments and the produce the synchrotron emission. So that's why we see Synchrotron radio emission to up here SL, like thin and long structure. New York had actually reviewed more than 150 stash magnetic filaments are within 300 parsec of our galaxy. So let's just the pulse, the moment. And some use this table to summarize what we know so far for this type of source, the magnetic filaments. Now this type of source is most significantly detected in radio and x-ray band. Now, radio band, the justice that New York have told us, there are at least a 150 such filaments in the galaxy center. And they are as thin as one parsec. And that can be as long as 10000 parcels. Now thanks to the detection of radial polarization, we know that these emission are of synchrotron, a region, which means now the charged particles spiral around magnetic field lines and the produce radio synchrotron emission. Now in order to produce synchrotron emission falling into radio band. And we need to feed these filaments with a GeV electrons. Tim tonight, electro bolt electrons. Where do the electrons come from? They can come, they can be produced through direct particle acceleration and could be produced by powerful sources such as supernova remnants, star-forming regions, possibly nebula and so on. Now, some of these filaments are also detected in the higher energy band in X-rays. The literature from the literature, now we know there are about 20 x-ray filaments, but this number will be updated very soon as we will discuss later on in this talk, the X-ray filaments are of smaller spatial scales compared to the radio filaments, cakes then up to a few parsecs. Their polarizations have not been detected yet, because previously, we do not have such tool to measure X-ray polarization. But hopefully soon they can be detected considering newly launched x repolarization nations. And in order to produce synchrotron x-ray emission. Now we need to feed these filaments with TEEB electrons. Now TEV electrons could be more difficult to be directly produced through particle acceleration if there is a possibility and maybe even a more natural explanation, that is t, the electrons could be secondary products of the hadronic of presets. So you might find, already find the connection here. The TV electrons is what I'm mostly interested in probing in the galaxy's central region because they can be connected to the PABA trunks and the filaments, especially that they are x-ray emission. Of course also there, like Reader counterparts, can help us to probe this specific population of electrons. And that's why I'm interested in studying these are peculiar objects. So in this slide, I'm just showing you some of our efforts in improving and study the X-ray filaments and redo of elements as well in the galactic center. That the two images on the left and in the middle shows you the X-ray emission and radio emission of a specific x-ray filament located at about 10 parsec from suggests that this filament is called static II. And by studying is radio to X-ray data, we concluded that to produce the synchrotron emission, we see that it's nice to, we need to like continued continuously inject. Electrons with energies up to 100 to 200 TV into this filament. At the same time, there's no obvious particle accelerators. For example, supernova remnant partnered with Nebula or a stellar black holes anywhere nearby or within this feature. That's why we concluded that the origin of the 100 to eat the electron could be most likely come from like, like secondary upper particles of like hadronic process. And a and b has to be more specific, produced by a collision of PEP protons with ambient gas. Now the image on the right is another sample of a filament we read better recently studied, which just want to show you how thin and along this feature can be. So even before has collaboration discovered a galactic have a charm. So in the earlier work of my eyebrows, like proposed this model, trying to uniformly explain the origin of radio and x-ray filament, as well as the gamma ray emission we see from the molecular clouds along the galactic plane. So here is this model I proposed. If, let's start from the left of this image. If we have a very powerful cosmic ray accelerator in the GL extension, which can accelerate protons into TeV to PEP range. And this protons after travelling a while, eventually the word heat into the code. Molecular cloud distributed on surrounding the supermassive black hole. Now within the molecule account, the proton-proton interaction take place and the produce different types of secondary particles. Now, one type of them are the electrons ranging, with energies ranging from GED to t0 be a certain fraction of these electrons. They can actually survive and eventually escape from the molecule are correct. So if a magnetic structure with locally enhanced magnetic field compared to the environment, if they can trap these escaped electrons. And those electrons can spiral around the magnetic field lies within the filament and the produce the radio to x-ray band emission as we see from the filaments. So that is the model I proposed a couple of years ago, hoping to just uniformly expand radio filament, extra filament and a gamma ray emission, we see founder molecular clouds. But as, as you may have seen here, there are a lot of things we, we cannot really qualitatively claim here. We need to really be careful and really do a very careful calculation to try to stop some missing links here. That is indeed an ongoing effort between me and my collaborators. Now they're specifically, we can just stop like break down this big problem into several steps and use simulations and theoretical calculations to help us to solve this problem and to test whether this hypothesis is correct or not. Now the first physics process is the diffusion and the cool off of the GED to PDB protons in the galactic center region, especially considering the magnetic field strength in the galactic center environment. Now a second, the process we need to be careful about and workout is the proton-proton interaction. And what is the spectrum of the secondary particles is specially on secondary electrons. Now, a third process we want to better understand is the diffusion and escape process of the electrons. The electrons of what energy can evacuate, survive and escaped from the Cloud. And what fraction of those electrons can survive and escape. That's the question that we have some ACLU But still working on. The last US that is more straightforward, which is just a, to calculate the synchrotron emission of those are electrons within the filaments. So between me and my collaborators, we have forming a plan to serve each of these steps or with help of simulations. That's how we're like theoretical. Like I first, I'm trying to solve this problem. On the other hand, we also need to collect more data to better understand the population of the 180 to 200 TEEB electrons in the galactic center. Therefore. My other collaborators and I have been focusing on proposing like new observations or I'd analysis on archival data to perform a systematic study of both X-ray and radial magnetic filaments. Which can eventually help us to test whether, I'm sad, restart a supermassive black hole maybe can be a public trunk in the galactic center. We have already just laid out now what criteria we have here to identify the filaments that is of the strongest interest to us from their morphology, from their spectroscopy. And also some, some signature, a stat chest X-ray polarization, which could be, which could be enabled by some very new and even next-generation of x repolarization missions such as x p0 and SDP. So our like pipe finder project has already returned some very promising results is that from like archival x-ray observations by multiple instruments with our ID like a discovered a new extra filaments and can almost doubled our current knowledge of the X-ray films. And these specially with the help of a mirror cat results like which again reviewed more than 100 and probably more on would, would, would. Like I'm drawing the current list of radio filaments. We also have a better guidance on where these filaments are, how they distributed in the galactic center. And that better understand their connection to pow for sources in the galactic center. That's why we are entering a stage where we have all the helpful To started filaments, which syrups at very good probes for about a 100 to 200 TB electrons in the galactic central region. Now, however, the question is that maybe some of them already know saturday star, which is actually the least active supermassive black hole known to us. The question might be consulted, start really serve as an efficient particle accelerator that could produce TEV energy protons. Now again, it is a least an active supermassive black hole is not doing a lot of things. Current days, especially is volumetric luminosity is several orders of magnitude dice Eddington luminosity. However, now another branch of my research is focused on reconstructing the activity history of stuff. Now we have a lot of observational evidence pointing to the fact that such a star could be much more active in the past compared to each nowadays activity. And perhaps exotic star could use to serve as a PABA trunk when it used to be more active in the past. Now, I'm not going into like, like a technical details, but I want to highlight how we can play this game in terms of our reconstructing Saturday start activity of history in the past. We are going to make use of the X-ray emission from malignant pals surrounding Saturday star. And then we can measure their extra room nasty. So these monocular cows emit X-ray emission. Now, the code molecules themselves cannot emit x-ray mission, right? They must be reflecting. Incoming X-ray emission is like just all light equal from some external source. And it is believed that they are reflecting past x-ray outburst from the supermassive black hole, sadness them. Therefore, by marrying the x-ray intensity of molecular clouds at different distances from such a star, we can infer what such a star's luminosity was in different epochs in the past. So that's the basic ideas of this game. Now a quick answer. If you'll look at the image on the upper left upon them by studying the emission from different molecular clouds located at different distances from studies done, we can infer, for example, status stars luminosity level a couple of 100 years ago. Or even like up to a couple of thousands years ago, if you've studied molecular clouds farther away. And the conclusion is that we are indeed seeing a trend of such a star has a higher activity and have, have like higher or brighter x-ray burst in the past a couple of 100 years for even a couple of thousand years ago, during which is activity level can be six or more authors higher dice, current activity. Now to be specific, we can use like either elect for fluorescence physics process which was developed by the group of Sunni, have an trust of many years ago. And our group specifically also developed a, an independent analytical methods to rely on like Compton scattering to calculate Saturday starts luminosity from the x-ray intensity metric from molecular clouds. Now again, no matter which method you use here, then conclusion is that Saturday start indeed used to be more active even just a couple of 100 years ago. Now for example, I want to show you a project I worked with field Rogers, who is a fourth year PhD student at MIT, should see our lead this project and has done a very nice job analyzing a specific molecular cloud. Cuts such B2C located at 100 parsec from statutes time. So film studies like the archival X-ray data from this molecular cloud and risk constructed is like light curve in the past two decades. Conclusion is that the actual luminosity has been consistently decaying in the past two decades. And the using the method I mentioned in the earlier slide, we can calculate the luminosity or active the level of such a star like just about 110 years ago. And the concluded that it is five orders of magnitude higher than the current luminosity of such stuff. Now we just submitted this paper a couple of months, it a couple of months ago. Now I don't want to go into details here, but I want to let you know that. Another takeaway message is that it is indeed a pretty complicated story because we figured that different portions of this multi-cloud actually have different time variability, which is pretty interesting. And that could be explained by the fact that there's not only one single x-ray outburst from such a stark, the different time variability from different substructures. This molecular cloud could be telling us like the story of multiple, like an x-ray outburst from Saturday start a couple of 100 years. Now, another example of our efforts in this direction is a bard College student, Natalie Jones, who just graduated and is now working with me as a researcher. So she's leading this project, starting the X-ray variability of another molecule of cloud called the bridge, located at about 20 parsecs from Saturdays done. And she figured that the x-ray intensity from the rage molecular cloud has been increasing over the past two decades or so, which is vastly different from what we stop, satrapy to moniker Cloud, then we could be seeing the illumination, illumination stage for this clouds, meaning the light, extra light from such a star is sweeping through this pretty current molecular cloud, and therefore is making this molecular cloud becoming brighter and brighter. Therefore, we are seeing very different stage of such light equal event now, which again, that can tell us more about the details of Saturday stars activity a couple of 100 years ago. Now specifically, at this is my best the knowledge to what a Saturday star was doing from the behavior of these two moniker class. So this coordinate shows like horizontal line is the projected plan. Vertical is the line of sight. Line of sight observers is to the bottom. And we are looking at Gothic center when looking at our course. So start B2, located closer us in front of the galaxy your plan, while the brain located behind the project at the, the, the guy feel plane. Now, we trust the location of this molecule accounts and we are also able to measure x-ray intensity of these molecular clouds. Then we can calculate like Saturday stars luminosity, for example, 100 years ago as revealed by satrapy 2, which is about ten to 39 ergs per second. And we can also calculate its luminosity level even longer ago, like 400 years ago, as revealed by the breach molecular cloud, which is about like maybe half that value, 0.4 times 10 to 13. Per second. So these all tell us about like a, just a couple of 100 years ago. Now static star could be much more active compared to nowadays. And it's often natural to think that at a higher, at a more active stage surgery stock could be more effective in accelerating particles as well. So want to provide a recap for what have discussed so far. So has collaboration discover that there is the public from a hadronic Tevatron in the galactic center. And the most likely candidate is a supermassive black hole, satyr stuff. All right, therefore, with the existence of a public firm in the galactic central region, we should be expecting to find a multi-wavelength observation of proof for the creases. Fmp habitat. For example, as I demonstrated, the radio and x-ray filaments could be ideal probe to help us to do so. And also because it is very inactive nowadays, I think Saturday star could serve as the pepper trunk when it was more exit in the past. As reviewed by many observational evidence. At the same time, there are also the at least the several open questions that remains to be answered. And they are important questions waiting to be answered. For example, what is the acceleration mechanisms for supermassive black holes? Now, if such a star is indeed this peloton was the detailed physics process is involved in accelerating protons are ions into P B range. And where does it happen? Around the black hole? Is it in accretion flow or in the Jets? Next, can we really find multi-master signatures for the Galactic Center Republicans? If we can indeed someday detect like commodity be neutrinos and that can be associated with such a star, then that would be like the smoking gun and undeniable evidence for such a star to serve as the pouch. Now lastly, which is also very important question, is, for a supermassive black hole, what stage of, at what stage of its activity cycle can serve as the Tevatron ieee effective in accelerating particles. So these are the questions that I remains to be answered. That is all I want to discuss about my current efforts in studying galactic center supermassive black hole, static star. Essay candidates for the galaxy centre publication. That's still have some time left. I want to briefly introduce some of our ongoing efforts in discovering and identifying other galaxy patron's outside of the galactic center. Now this is an example of our ongoing I first starting another peculiar system which is called SS for 33 or WE 50, which is very complicated system, including consisting a micro quasar in the center and also a supernova remnant system. So this is a pretty actually fema source in a high-energy sram physics community. Especially hoc detected TED emissions from some lobes on the two, into two directions from the micro quasar. Assess for 33. And my collaborators and I have collected more data in x-rays and other wavelength to, to try to better study the interactions here of some extra probably originating from a very high-energy particles produced by these particles and how they interact with the ambient material of materials. So much women's legs spectral energy distribution study is ongoing for this source. Another example and also very, very interesting news is that the lawful, which is a gamma ray Cherenkov Observatory in China. And the loss of collaboration now published a paper just a couple of months ago. And day. Now claim that the discovery of a dozen a PD or like a PB, cos may create a production sites or to have a trunks. They could be electronic or hadronic. Patron's. What's most exciting about this discovery is that before or losses. So other emissions. Work below the TEV energy threshold, maybe into like a couple of 100 of TED. Like what has the writers about magic? But Laszlo, Right now, it's dense. It's window up to a slightly more than one Pb. Thanks Lisa for now. And day detected photons up to 1 for TED. Now, in this way, they actually are able to claim that they discovered very, very high-energy, ultra high energy gamma ray source this, and they are the like very likely PE, the cosmic ray production sites. Now displeased of 12 sources. Some are better known and with counterparts in other equivalent, but some of them are completely new source. So this is very, very exciting and also want to point out that this discovery of 12 sources are only based on a relatively small chunk of data in the very early stage of the operation of the lasso observatory even before it was fully constructed. Therefore, what do you tell law tells us that is, what it tells us is that galaxy could be full of penetrance. So these 12 is not like are all the penetrants. There could be just the tip of an iceberg of many, many have a trans in the galaxy and we are entering a like a golden age of starting the gala, take a penetrants and hopefully CAN in the near future really understand what is the source nature of these penetrants. But to do that, and we need to perform like multi-wavelength follow up on these newly discovered a potential pebble strong candidates aiming to identify their nutrient. And just want to show you one. Will my collaborator perform at Lucky University of us miscounting and I were thinking about just a follow up, especially and new law sole source was this telephone number name and that that is of particular interests because firstly, is newly detected, the newly report it. And when we have very little knowledge of the source and gamma ray production mechanism is far from being known. We even don't know whether, we definitely do not know whether the emission is hadronic or that tonic. Therefore, that is why we want to propose follow-up observations. Answer these questions. For example, X-ray emission can help us to effectively distinguish between hadronic and lap tonic and processes. If we can rule out lepton a mechanism, then we should be more confidence to claim that this is a hadronic puppetry source. Now, these are just some very early, I first, but I believe more substantial work is ongoing and especially the lasso wood. Wood is on his way to discovering more and more PABA tones. And two gave us are just the more interests, interesting sources to study. Now therefore, I just want to summarize what have discussed so far. I'm firstly, they discussed about the galactic center, supermassive black hole, such a star as the Galactic Galactic Center Peloton. And we also have many ongoing efforts which are largely in their early stage to try to discover and to identify other gala take a penetrants. So on my last slide, I just want to summarize a little bit and also provide other bit of outlook on this research. Now firstly, we really wanted to search for mulch wavelength and the multi messenger footprints of the god. If you've thens Republic trunk, no matter whether that is the supermassive black hole or not. And perhaps sleep. More important and more exciting is to know where other galactic PEBO trunks are, how they are spatially distributed in our galaxy. And what are they, What's source nature they carry? Now again, I believe we will have very exciting news. It may perhaps just the, in the next 30 years through, I like multi-wavelength observation of these newly discovered penetrance. With that, I will stop here at the end. I'd be happy to answer your questions. Okay. Thank your LOTRO. This really interesting talk. We have. First question, why Mariana? The chat already and others. Yeah, please raise your hand or put your question in the chat. So, Mariana, you've got a very thank you fool. Do you fly you do with this document, the x. So for this production, for this P, V and T be prudent. Yes. So let me go back to that slide. Do you mean this one? Yes. So I was just curious how to interpret this. Does it means literally that there are two different mechanisms. The folk theory from the one produces PV prudence and another TB problems. Or is there, is technically different opinions. So, or there is a, some sort of energy scale here between them because since you only observe any of the extra filaments and the radio filaments, presumably Ezra know filaments and say, you know, let's say obstacle. So how does this work? That you only either have soup, It's ICP or TV is the right interpretation here. I just want to understand the picture here. Of course that's very good question. And I I buy things like TB and pmi courthouse. I I'm not suggesting that like the Gallic center pepper Trump only produce like TED protons as a distinct, distinctive population and produce PDB pronounced with another mechanism separately. I myself, I think. Still this is the open question. I'm more leaning forward that a continuous spectrum of protons, let's say from TV all the way to PV are produced. But indeed a why and how and the why are they are produced? They are still open questions. The reason why I'm like highlighting TV and a PV protons is because the filaments are only detected in radio band, an extra exam. So radial filaments costs for TV protons. To produce GED electrons. An x-ray filaments cost for TB protons. So that's the two population we can study using the filaments. But again, that does not mean that we don't have protons in-between these two energies. It's just that we don't have off signals from the filaments in-between video and the X-ray energy that allows us to study the protons with energies in between it. Up. Thank you. Does it means can you tell based on which observation capabilities are available? Can you confirm or no filament through different regions or just maybe I'm just not observing them in a different region. Again, that's another very good question. The galactic center is very like closes the money, monitored by all the different wavelengths. But for example, like optical band in between radio and x-ray. That is, that would be very interesting. However, at Galaxy, Galaxy center is not observable in optical band due to the high extinction. On, on top of that, the filaments right now are most significantly detected in radio and x-ray band, for example, day they are not really. If you look at a YUV image of the guy center, we don't see significant filaments. Our on the data we have in hand right now. Just two very quick question. Do you really understand the possible mechanism of how the supermassive black hole can become more or less active with time because, I mean, you could study evidence of this, probably did, but what's the mechanism of what's happening? That's wonderful question. Firstly, there are many different models and regarding the activity cycle and what trigger maybe to a supermassive black hole 2 to be less active or or be more active. I think there are different models trying to explain that. But maybe I can provide an example. Back when one example for a supermassive black hole too, perhaps stop and they become more active and produce some sort of outburst. Could be like a disruption of small objects, maybe an asteroid or even a planet, or get more gas clouds that moves closer to the black hole. So for perhaps it cannot survive and sort of feed to the black hole. And such event, the title, title disruption could give rise to a huge outburst from supermassive black hole. The maybe could also trigger the black hole to go into a more active stage. Because we are seeing like Saturday star is the Gallic central supermassive black hole is one case here where we have evidence that it could be past, there could be past outburst. Though we also have another, a cloak here. The nearby supermassive black hole in Andromeda Galaxy cut, I'm 31 star. Now we also notice that it's used to have an outburst back in 2006, a very sudden increase in activity. The author that either and trust to untrust a more active stage compared to its previous quiescent stage. But like I said, one possibilities, perhaps there is some like so-called like a tidal disruption of some small objects, but at same time now different, Whoops. What happened like a different theories or models trying to explain is trying to explain this. Thank you so much. Thanks for the question. Extreme. Yeah, thanks for the torture. Thanks to the very national culture. If I understand correctly, looking at you from what you're saying about the actual variability, the, the very, the x-rays are basically being reflected from the molecular cloud is accurate. Exactly. So X-rays from a Saturday star travel to the molecular clouds and get reflected. Okay. So you wouldn't expect but variability to be reflected in the gamma ray emission from the molecular clouds. That's a wonderful question. But the fact is that I think that's very interesting whether the gamma ray emission can varies. However, first of the facts, as far as I know that I don't think that any gamma ray observatory report variability on the gamma ray. Emission is not reported, but we really have very little sensitivity to measure it much, most of the variability searches have been from Sagittarius, a star itself. It's only been monitored for the past 15 years or so with much lower sensitivity than you have in the tray. So it could be the well, yeah, I guess my follow-up question. May I ask? Are there I want to answer because I think that's very interesting question. Firstly, I complete a great If there is the variability of gamma emission can be detected data with you don't be huge. That will be very interesting. But another reason, I'm like a little bit hesitant in directly comparing an X-ray variability and the gamma ray variability is that they could, they could be different a region. So here the X-ray emission we see are like reflecting the X-ray outburst from Saturday star, and that cost, the marginal cost to have variable x remission. However, the gamma ray emission we are looking at from the molecular cows, they, they are secondary effects of proton-proton interaction that could be of different origin. They are like neutral pions decayed into the gamma ray photons. So if you believe that it is closely connected to the hadronic proton-proton interaction that I don't think we expect to see any variation for, for, for the camera image. If that is dominating process, I guess. But that's only true if you also assume that the, the proton population is constant. If the cosmic ray proton density near the galactic center is related to episodes of higher activity from the central source than maybe you would see variability. It'll be interesting to think about how are the times that it would be the same as the x-ray and link the two together? Yes. That's a good point. Thanks, Henry. Yeah. Hi. I have a slight a two-part type question. First is the second Cloud. How are you determining its distance from us? And how are you having methods in the future to pinpoint its distance exactly? And the second part is, how did you know the increase in radio x-rays from that cloud is the same radio waves or a fingerprint we look for are how do we know that the x-ray signals are the same? Okay, the first question, that's very good question. I do have a note here that the line of stack location of these two molecules house is highly uncertain. I think it's, it's very sharp observation that the line of sight distance is very important here, especially in constraining now luminosity saturday star because it's proportional to distance square. Now, any uncertainty associated with distance indeed introduce a very large error squared for, for our estimation of Saturday stars activity. That indeed the uncertainty in the distance, especially in the line of sight distance, introduce the biggest error here in our calculation of Saturday stars luminosity. Now here I'm just giving you our best estimation of the line of sight distance for these to monitor accounts from, like largely I think relies on redo. Observations. But again, the idle, idle bar, I have to admit is indeed very high. And we factor that in when we calculate the luminosities. And your next question is that how we can improve, write them constraints on line of sight, distance. And that is, on one hand, that's difficult. But on the other hand, it's not like not doable. For example, I had bought all hope for X-ray polarization detection on this molecule accounts. And from the linear polarization, I'm sorry, that the polarization signal we, I expect to see from section like a geometry and this reflection that can actually help us to coun, country and hopefully better constrain the line of sight, a distance. But by how much? I don't know at this stage. Could you please remind me another question? We side the how to associate the X-ray signal to monochrome house. Is the Zagat such a star that how do we know that the reflection from the two clouds are the same, are from the same source. What is the fingerprint we look for? Great question. Does he is I think one of the also important question that triggered at years of a discussion, who study like this, like x-ray reflection scenario? Firstly, study star is believed to be the most likely source because according to our measurements of the extra variability of this multi-cloud, first, did the outburst or the luminosity level of this extra source should be very high, up to ten to 39 ergs per second. And we don't know any other sources in the Gallic center which can have an outburst of this high level, 2030 networks ergs per second. And that last for as long as ten years. Now, we can have some trends in sources, can have a brief outburst, but in order to produce a, a very long outbursts, as long as 10 years, which is required by some of those Manichean account. And at this level, the supermassive black hole is our best candidate here. And rethink that gives out why both Monica cows are illuminated by Saturday star because both requires a luminosity up to ten to 39 ergs per second. Now indeed, if somebody Kirchoff's requires a lower level outburst and without shorter time, dad could we cannot at this moment a completely rule out the fact that those molecular cloud could be illuminated by other powerful sources, for example, and magnetism. Thank you. That was very helpful. Yes. Thank you for the question. Let me have question. So you explained how you can maybe for salvation through brushed past luminosity of Sagittarius a star. Can you also estimate the cosmic ray flux potentially accelerated by such tenuous a stop the time and potentially still Uppsala today. That is they question. I mean, I've been keeping thinking of so like I like I discussed the in the, in the talk. Now indeed, I think it's more natural to think that wouldn't Saturday starting to be more active. It could be also be more efficient in accelerating particles into high-energy. But let's say when it was more active at the same time to produce these cosmic rays. And indeed, we want to find the traces of these accelerated particles which exist in the galactic center environments. So indeed, when one example is like earlier is that maybe some 100 to 200 TB electrons. And for, for this population, my view, my belief is that filament is the best approach. But for other, I think he's just depends on the population of their target particle. For the electrons, we can try to look for some source of like maybe with some synchrotron emission and to probe the electrons. But for like protons, it's harder to directly probe using observations. Because the protons can do a cool, cool off of the cooling of timescale of protons are artists too long. It's very, very challenging to, to, to find them as far as I know, to find the, any observational evidence for, for high-energy protons. Okay, thanks. So two more questions and then really close to colloquium. So for a startup. How about neutrinos? Wouldn't such a system produce neutrinos? Hadronic publish on, which we believe to be the case for the Gallic center of habit home. We'll produce neutrinos and two. And it is believed that it will produce like multi TEV neutrinos. I saw some calculation. I can actually add a reach this question to the TD collaboration in their discovery papers according to their calculation. But we have experts Brown from IceCube collaborations here has people from their accommodation. They think after years of ice cube observation and there could be a marginal detection should put out. I'm curious about IceCube experts, comments on this. There is no evidence of any, any I scoop detecting anything from galactic center unfortunately, so far array, but I saw some abstract and some work that I'm, perhaps the algorithm can be improved. And I don't know what, what do you think that if more data comes? Is there a hope of detecting excess neutrinos? All we have, we have sitting on my analyzed 10 years of data. So I think that, I mean, the, the analysis is going on. So IceCube collaborations working on techniques that increase the sensitivity towards the galactic center can achieve, we don't have for any results yet because the data are blinded. And if that unbinding would really increase the sensitivity enough to see something it's too early to say. Let's see. Yeah, when we looked for the lasso sources and all that. So it's all coming out loud relay. Thanks. Thanks for the comments. Okay. Last question for you. Hi. Thanks for the clock ran in that clock. So I have a quick question on the same thought. Actually, it, we really consider that the galactic center was active before than put that there. And then the continuation of formation of the Fermi level. One of the republic was working factor. Yeah, if I'm a robot, one of the contributing factor for the formation of new bubbles. Yeah. Indeed, if you look at this image I'm showing on this slide, right? Yeah. If you believe the Fermi bubbles is associated with the supermassive black hole, is. It actually suggests that started Star used to be even more active roughly a couple of million years ago. Actually reaching it, it's adding to Luminosity, becoming an active, the so-called actually got a new play, the Aegean. The most active black homes, however, I believe, is largely an open question whether Fermi bubbles is indeed produced by the supermassive black hole. If that's my personal preference, either either leaning forward to to, to, to hopefully to see some connection. But you form a double spot and a supermassive black hole. But dear, no theorist has brought up other possibilities, like from Davos could be created by other energetic events in the guide extent, we still cannot rule out those possibilities. Okay? But they acted in the past or activity. If the supermassive black black call scenario is leading to a deformation or a contributing factor than the past act more activity could be. Anything significant contributing factor, right? I see what you mean here. So if you look at this trend, it is indeed a snps that the Fermi bubbles pointing to more active stage of studies are, is consistent with what we know from molecular clouds, right? It was just the more and more active. If we go back in time. When the tool like when when when I said earlier stage. But this is again, one possibility. One. Yes, sure. Is more, I can, I think, connected to the activity cycle of supermassive black holes. Excellent question. So thank you again for answering all the many questions that we had. And thanks again for for the talk so that if my treasure, Thank you. Thank you all again for joining this time. And then we will see again, Tamar. And yes, I look forward to seeing you tomorrow. Have a good evening.
Hunting for Galactic PaVatrong | Shuo Zhang, Bard College, 2/21/2022
From Federica Bianco February 21, 2022
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Cosmic rays with energies up to a few PeV (10^15 PeV) are believed to be of Galactic origin. However, after a century since the discovery, the origin of Galactic cosmic-rays still remains a mystery. In 2016, H.E.S.S. observatory discovered a PeVatron within 10 parsecs from the center of our Galaxy, pointing to the supermassive black hole Sgr A* as a likely candidate. As one of the least active supermassive black holes nowadays, Sgr A* could serve as a powerful particle accelerator during its active stage in the past. In this talk, I am going to outline our efforts on searching for multi-wavelength observational evidence for the Galactic center supermassive black hole serving as a PeVatron. In addition, I will discuss how we can reconstruct the activity history of Sgr A*. Lastly, I will introduce ongoing efforts in discovering and identifying Galactic PeVatrons outside of the Galactic center.
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