Search for high-energy neutrinos from Einstein Probe detected Fast X-ray Transients using 14 years of IceCube muon track data

Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: I'm Vera.

Jocelyn: And I'm Jocelyn, here with Subrahmanyan.

Vera: We're looking at the paper: "Search for high-energy neutrinos from Einstein Probe detected Fast X-ray Transients using 14 years of IceCube muon track data".

Jocelyn: Here's the gist of the paper. The study performs a time-integrated search for high energy neutrinos from 27 fast X-ray transients detected by Einstein Probe using 14 years of IceCube muon track data, finding no statistically significant excess from any source.

Vera: Search Methodology The researchers employed the unbinned maximum likelihood method, which is used for IceCube point source searches We perform a time-integrated search for high energy neutrinos from 27 fast X-ray transients (FXTs) detected by Einstein Probe, using 14 years of publicly available IceCube muon track data.

Jocelyn: This analysis was performed using the SkyLLH software package.

Vera: The likelihood function is defined as L(ns, γ) = Y N i=1 ns N Si(γ) + 1 − ns N Bi i=2> The test statistics are constructed as TS = 2 log L(ˆns, γˆ) / L(ns = 0) For a large number of signal events, TS asymptotically follows a χ2 distribution for two degrees of freedom.

Paper summary: Vera: So, this paper is about searching for high-energy neutrinos coming from these fast X-ray transients, or FXTs, that Einstein Probe has spotted. Basically, they looked at twenty-seven of these transients using fourteen years of data from IceCube's muon tracks to see if they could find any neutrino signals <ref:2610.30722#pg1>.

Jocelyn: Right. The main claim here is that despite looking at all those sources and having a ton of data, they didn't find any statistically significant excess in the neutrino flux from any of these FXTs. It’s a null result for the search.

Subrahmanyan: That’s important because it tests some ideas about where these high-energy neutrinos come from, especially since FXTs are short bursts of X-ray radiation that can be at cosmological distances and have spectral characteristics somewhere between standard Gamma-ray bursts and X-ray flashes two <ref:2610.30722#pg1>.

Vera: Exactly. The authors use a method called the unbinned maximum likelihood method, which is what they use for IceCube point source searches, to make sure they are doing the search correctly across all those sources.

Jocelyn: And while they did that, they found something interesting about one specific source, EP240801a. They got a test statistic of four point seven seven for it, which gives them a local p-value of zero point zero four, but they say that’s not significant enough to be called a detection <ref:2610.30722#pg1,a local p-value of 0>.

Subrahmanyan: So, even when you have the largest value from one source, EP240801a with that TS of four point seven seven and its local p-value of zero point zero four, it still doesn't cross their threshold for significance > five sigma <ref:2610.30722#pg1>. That tells us that the neutrino emission from these FXTs, at least in the way they searched, is not coming out stronger than what we currently expect based on this data.

Vera: That leads them to calculate an upper limit on the differential muon neutrino flux. They found a ninety percent confidence level upper limit on this flux, and it’s most stringent for EP241021a <ref:2610.30722#pg1,confidence level upper limit on>.

Jocelyn: And that specific limit they set for EP241021a is E squared Phi of E equals two point one times ten to the minus ten in GeV cm minus two seconds to one, assuming a negative two spectrum at a pivot energy of one hundred TeV <ref:2610.30722#pg1>.

Subrahmanyan: That upper limit is pretty tight, and when you compare it to some of the early predictions from the choked GRB jet model, it’s about an order of magnitude lower. Since most FXTs have an isotropic energy Eiso less than ten to the power of five times thirty-three ergs, they say they can't rule out that specific model yet <ref:2610.30722#pg3>.

Vera: That’s a big caveat. They admit that because the physics of how these jets behave—whether they are off-axis or choked—is still being explored by simulations, they can't definitively rule out those models based on this observation alone.

Jocelyn: The paper also points out that for a more robust constraint, they would need to do a triggered search, which means looking at narrow time windows around when these FXTs happen. That’s future work for them to increase the sensitivity.

Subrahmanyan: I agree with that. A temporal analysis using narrow time windows around the events could definitely improve the sensitivity because it focuses the data on a specific short period rather than integrating over fourteen years <ref:2610.30722#pg3>.

Vera: So, to sum up this paper, the main point is that they searched for high-energy neutrinos from twenty-seven FXTs using IceCube track data and found no statistically significant excess, but they did get a very stringent upper limit on the neutrino flux.

Jocelyn: And that upper limit for EP241021a is two point one times ten to the minus ten GeV cm minus two seconds to one for an E negative two spectrum at one hundred TeV <ref:2610.30722#pg1>.

Subrahmanyan: This means that while we haven't found a definitive source of high-energy neutrinos from these FXTs, the constraints they put on the neutrino flux help guide theoretical models for how these transients might produce particles <ref:2610.30722#pg3>.

Vera: So, as they conclude, they didn't find any statistically significant excess for any of the FXTs studied in this paper.

Jocelyn: And the most stringent upper limit they derived is for EP241021a, which is two point one times ten to the minus ten GeV cm minus two seconds to one for an E negative two spectrum at a pivot energy of one hundred TeV <ref:2610.30722#pg1>.

Subrahmanyan: It’s a tight constraint on the neutrino flux, which is what we need to compare against models like the choked jet model mentioned in their work <ref:2610.30722#pg3>.

Vera: That's the result of this time-integrated search for high-energy neutrinos from Einstein Probe detected Fast X-ray Transients using fourteen years of IceCube muon track data <ref:2610.30722#pg1,search for high-energy neutrinos from Einstein Probe detected Fast X-ray>.

Conclusion: Vera: So, we've been looking at this paper about searching for high-energy neutrinos from these fast X-ray transients, or FXTs, using fourteen years of IceCube muon track data from Einstein Probe.

Jocelyn: Yeah, it’s titled "Search for high-energy neutrinos from Einstein Probe detected Fast X-ray Transients using fourteen years of IceCube muon track data," and it comes from a team that's been looking at the sky for a long time.

Subrahmanyan: So, the authors used this unbinned maximum likelihood method to search for any statistical excess in the neutrino events. They didn't find anything significant coming from any of those twenty-seven FXTs.

Vera: Right, and that's what they did, looking at the data they had. The main result is that there isn't a clear signal of high-energy neutrinos coming from these specific transients right now.

Jocelyn: And the authors give us an upper limit on how much neutrino flux could be coming from these sources, which is a very important constraint for theorists.

Subrahmanyan: That upper limit they set for EP241021a is two point one times ten to the minus ten GeV cm minus two seconds to one, assuming a negative two spectrum at a pivot energy of one hundred teraelectronvolts.

Vera: That's a specific number, and it gives us an idea of how faint the neutrino emission could be if it exists in that source. It’s like setting a very low bar for what we expect to see.

Jocelyn: And when you compare that to some of those earlier models, like the choked GRB jet model, this upper limit is about an order of magnitude lower than those initial predictions.

Subrahmanyan: That’s interesting because it means that while we haven't found a signal yet, these constraints help narrow down what kind of physics we should expect to see in those transient events.

Vera: It shows that if there are neutrinos coming from these FXTs, they probably aren't as bright as some of the most optimistic early models suggested.

Jocelyn: And the team does mention that for a more powerful search, they need to use a triggered search with narrower time windows around when these events happen.

Subrahmanyan: That makes sense; focusing on specific times could definitely boost the sensitivity and give us better constraints on those underlying physical processes.

Vera: So, in the end, this paper confirms what we already knew—no significant detection yet—but it sets a much tighter constraint for future searches.

Jocelyn: And that constraint is crucial for guiding our understanding of high-energy phenomena associated with these fast X-ray transients.