# Victor Doroshenko

## Mapping the circumnuclear regions of the Circinus galaxy with the Imaging X-ray Polarimetry Explorer

And another IXPE paper. We report observation of the closest and X-ray brightest Compton-thick active galactic nucleus (AGN), the Circinus galaxy. This time significant polarization is detected at a level of ~30 per cent (at 68 per cent confidence level) for the neutral reflector, with a polarization angle of 18 deg, roughly perpendicular to the radio jet. A comparison with Monte Carlo simulations of the polarization expected from the torus shows that the neutral reflector is consistent with being an equatorial torus with a half-opening angle of 45°-55°. This is the first X-ray polarization detection in a Seyfert galaxy, demonstrating the power of X-ray polarimetry in probing the geometry of the circumnuclear regions of AGNs, and confirming the basic predictions of standard Unification Models.

## Mean opacities of a strongly magnetized high temperature plasma

Valery Suleimanov has finally pushed a very important theoretical paper required for proper modeling of any emission from X-ray pulsars. All relevant physical processes one could include are included and calculated opacities are made available in tabular form for everyone who wants dive into sea of complex radiatively-dominated physics within X-ray pulsars emission regions!

## The X-ray Polarization View of Mrk~421 in an Average Flux State as Observed by the Imaging X-ray Polarimetry Explorer

Another exciting result from IXPE! A pretty large polarization from Mrk~421 (factor of three larger than in other bands) was surprisingly detected This, and lack of polarization variability, basically means that observed X-rays come from shocks near the very engine of the Blazar whereas emission in other bands comes from further away! A solid result showcasing the power of X-ray polarimetry. Congratulations to Laura Di Gesu, the first author of the paper by IXPE collaboration.

## A strangely light neutron star within a supernova remnant

And another Nature astronomy paper, this time without a large collaboration. In fact, it comes in the same volume as the Her X-1 paper, which might be the first case when someone published two first author papers in the single volume of Nature astronomy :) The essense of the paper is summarized by the title and figure below We have basically re-visited the old friend, the CCO in HESS J1731-347 in order to use all available constraints on spectrum and distance to the source and improve estimates of mass and radius of the neutron star. Which turned out to be, well... strangely light! It might be the lightest neutron star known, in fact. Or even some more exotic object! The paper attracted quite a bit of press-coverage including press release by the Univeristy, getting me some interviews/outreach (New scientist, Sky&Telescope,phys.org and more) and ranking high in Altmetric:

## Angling for x-ray pulsar geometry with polarimetry

And now my share of the X-ray polarization pie. A very important source for me (did my Bachelor on it!) and for the IAAT: Her X-1. In the latest Nature Astronomy paper we use phase-resolved X-ray polarimetry to constrain the geometry of the pulsar.

We also add old optical polarimetry to the mix to show that the spin axis of the neutron star is misaligned with its orbital axis - a condition required to allow its free precession (suggested previously to explain observed changes of the pulse profile shape over the 35d super-orbital variability cycles). More observations are coming next year, and of course more observations of X-ray pulsars by IXPE too! The paper attracted quite a bit of attention, so we also have a press-release from the University and NASA

## Timing properties of the X-ray accreting pulsar 1A 0535+262 studied with Insight-HXMT

Another paper on observations of 1A~0535+262 with Insight-HXMT in 2020. This time we looked at evolution of the pulse profiles: The observed changes indeed seem to confirm our previous spectra-based suggestion that the source did transition to super-critical state in this outburst!

## X-ray polarization detection of Cassiopeia A with IXPE

And now the first Imaging polarimetry result from IXPE! Jakko Vink has just published an ApJ paper describing first infant steps towards the true spatially-resolved analysis of supernova remnants. We're still talking about two sigma detection in individual pixels, but overall significance is of course much higher. And that's the first time X-ray polarimetry was used to probe magnetic field structure in an SNR! Observed polarization appears to be in line with what radio (still bringing some new insights), which is also a good thing!

## Constraints on the magnetic field structure in accreting compact objects from aperiodic variability

Juhani Mönkkönen has finally managed to wrap-up and push his main PhD paper through all the hurdles of complex data analysis and refereeing process. I were lucky to be with Sergey Tsygankov at the beginning of the project which we initially wanted to do ourselves, but failed miserably :) So, the more important is the conclusion, congratulations! The basic idea of the paper is that properties of the aperiodic variability appear to be similar and (unsurprisingly) driven by same physics in many kinds of compact objects, and can be used to probe interaction of the magnetosphere with accretion flow. In particular, Juhani used the observed power spectra of Her~X-1 to argue for presence of strong multipole field components in the source.

## Polarization constraints on the X-ray corona in Seyfert Galaxies: MCG-05-23-16

And another IXPE paper! This time on Seyfert galaxy MCG-05-23-16. We report on the first observation of a radio-quiet Active Galactic Nucleus (AGN) using polarized X-rays. Half a megasecond of IXPE time plus some XMM and NuSTAR. Only an upper limit on the polarization, but that's valuable as low polarization might indicate a lamp-post and a conical geometry of the corona (and if it's a slab, then some rather strict constrains on inclination must be satisfied). Stay tuned for more IXPE papers!

## Limits on X-ray Polarization at the Core of Centaurus A as Observed with the Imaging X-ray Polarimetry Explorer

So, it's official - IXPE opened a new observational window in form of X-ray polarimetry and started to pump-out results! Well, in this particular case the main result is not detection, but that's just a beginning!

## Insight-HXMT Discovery of the Highest-energy CRSF from the First Galactic Ultraluminous X-Ray Pulsar Swift J0243.6+6124

The Swift J0243.6+6124 is one of my favorite sources, linking normal pulsars with PULXs and one where we found solid evidence for presence of radiatively-pressure-dominated accretion disk normally reserved for BH systems. Now with the help of Insight-HXMT it was possible to detect also the highest-energy-to-date cyclotron line in spectrunm of the source! The results are reported in Chinese-led paper where we also discuss implications of the discovery. Basically, it does not simply imply that the field is strong, it also means that the field is strongly non-dipolar!

## Explosion on a White Dwarf Observed

Sometimes Nature just throws it to your face! When the event was detected by eRosita near time analysis team, everyone was rather surprised by the mere appearance of it. It did not take long to associate what we saw with the flurry of ATELs reporting the flash of YZ Reticuli. Which was big, because it was clear that we observed the ignition flash of a novae, something which was (unsuccessfully) searched for more than thirty years! Which made the work to make it to the nature cover!

A very lucky coincidence taking into the account the comparatively narrow field of view of eRosita and the fact that the flash is such a short event (hours). Indeed, evein if eRosita is an all sky survey, instantaneous field of view is just shy of two moons in diameter, and given point on the sky is scanned only twice a year, but this time it was in the right place at the right time!

Of course, detection was just a beginning of the very complex work led by Ole König from FAU, which allowed us not only to recover the time and place of event, but also the decently constraining X-ray spectrum from the extremely piled-up data. The phenomenon of "pile-up" occurs in X-ray CCD detectors when the source is too bright and more than one photon hits given pixel between the readout cycles, so while we still know the total energy received, we loose track of the number of photons carrying it and thus can not recover their energy distribution. The only thing is - we now can! Ole modeled the response of the instrument to extremely bright source taking this effect into the account, which allowed to recover the true spectrum of the source, and measure its temperature (327000K), luminosity (52000 times the Sun!) and radius of 50000 km for the white dwarf. This implies that eRosita has indeed detected the "naked" photosphere just after the released energy reached its surface and before the main expansion of the envelope, i.e. a Nova in fireball phase. The results are published in Nature today. The full press releases from Uni Tübingen and FAU, as well as journalist's take on the paper from Sky&Telescope and inverse.com are also in place :)

## A nice review on X-ray pulsars

Alexander Musthukov and Sergey Tsygankov just posted a nice review on X-ray pulsars on arxiv which is going to become a part of the upcoming Handbook of X-ray and Gamma astronomy by Springer I am currently editing. Feel free to recommend to your students!

## Phase dependent evolution within large luminosity range of 1A 0535+262 observed by Insight-HXMT during 2020 giant outburst

In 2020 1A~0535+262 surprised us with a quite spectacular outburst, which allowed to exploit Insight-HXMTto it's fullest and part of the results appear now in our paper led by collegues from IHEP. We focus on the two-dimensional dependence of the cyclotron resonance scattering features (CRSFs) along the outburst time and at different phases. The basic result seen also in figure below is that CRSF fundamental shows different time- and phase-dependent behaviors compared to harmonic. Based on these results, we speculate that when the source enters the supercritical regime, the higher accretion column can significantly enhance the harmonic line at a narrow phase through an "anti-pencil" beam at a higher energy band. At the same time, it will also affect the behavior of the fundamental line.

## Hard X-ray luminosity functions of cataclysmic variables: Joint Swift/BAT and Gaia data

We further elaborated on our work with Valery on intermediate polars to get better constrains on their luminosity funciton and numbers in Galaxy. It turns out the Mily Way is full of accreting white dwarfs we don't see because they are far away and faint as we discuss in our newest paper!

in the image above you can actually see those 340000 thousand (well, less, because it's only part of the Galaxy visible) objects along with about eighty which we actually know. Those are shown as red lines to reflect the uncertainty on distance to those which remains even after Gaia EDR3 release became available.

## Luminosity dependence of the cyclotron line energy in 1A 0535+262 observed by Insight-HXMT during 2020 giant outburst

Another nice result by Insigh-HXMT and Chinese collegues (with a little help from the friends)! It seems that our results on V 0332+53 are not all that strange, and other BeXRBs might exhibit similar behavior! The abstract is below

Basic idea is that we were finally able to see anti-correlation of line energy with luminosity in this source, and moreover, also transition from positive to negative correlation, and transition from sub- to super-critical regime!

"We report on a detailed spectral analysis of the transient X-ray pulsar 1A 0535+262, which underwent the brightest giant outburst ever recorded for this source from November to December 2020 with a peak luminosity of $1.2\times10^{38}$ erg s$^{−1}$. Thanks to the unprecedented energy coverage and high cadence observations provided by Insight-HXMT, we were able to find for the first time evidence for a transition of the accretion regime. At high luminosity, above the critical luminosity $6.7\times10^{37}$ erg s$^{-1}$, the cyclotron absorption line energy anti-correlates with luminosity. Below the critical luminosity, a positive correlation is observed. The 1A~0535+262 becomes, therefore, the second source after V~0332+53, which clearly shows an anti-correlation above and transition between correlation and anti-correlation around the critical luminosity. The evolution of both the observed CRSF line energy and broadband X-ray continuum spectrum throughout the outburst exhibits significant differences during the rising and fading phases: that is, for a similar luminosity the spectral parameters take different values which results in hysteresis patterns for several spectral parameters including the cyclotron line energy. We argue that, similarly to V~0332+53, these changes might be related to different geometry of the emission region in rising and declining parts of the outburst, probably due to changes in the accretion disk structure and its interaction with the magnetosphere of the neutron star."

## First public data release for eRosita!

For those of you who wondered what eRosita data is like, you can now take a look! MPE, ourselves and other core institutes of the German consortium have prepared press-releases to celebrate this major milestone.

The early data release (EDR) contains almost 100 individual observations of 29 distinct fields taken before the start of the all-sky scans. They cover a wide range of different astronomical objects, from galactic neutron stars to clusters of galaxies (see figure below) and showcase the potential and versatility of the eROSITA telescope for imaging, spectroscopy and time domain analysis. Arguably the most interesting part of the released data is the final equatorial depth survey (eFEDS) which covers over 140 square degrees of the sky to depth of $\sim10^{-14}$ erg/cm$^2$/s comparable to final sensitivity which eRosita will achieve over equatorial areas in the ongoing all-sky survey.

The image above gives a glimpse on this dataset. The top panel shows all sources, color-coded by the energy of the photons. The inset is a zoom onto a "supercluster", i.e. a conglomeration of clusters of galaxies about to merge. In bottom panel, all stars and AGN (i.e. all the point sources) have been filtered out from the eFEDS X-ray image. What is left is the diffuse emission from clusters and groups of galaxies, the matter in between, and the halo of the MilkyWay in front. As such, it gives a visual illustration of the large-scale structure that is almost impossible to gain otherwise.

More details are available at MPE website and of course in the papers describing science we were able to do with the released data already and now making a splash on arxiv!

## Discovery of the 5 keV cyclotron line followed by three harmonics in Swift J1626.6-5156

After a long careful analysis and writing up of the results, another joint DE/RU paper inspired by SRG and led by IKI collegues is finally accepted. We mostly focus on analysis of NuSTAR data obtained following our TOO request, and report discovery of the four cyclotron lines in the broadband X-ray spectrum of a transient Be/X-ray binary Swift J1626.6-5156. Besides the mere number of harmonics detected, the funny part is the energy of the fundamental of 4.9 keV, the lowest among all X-ray pulsars! This implies that the source has the weakest confirmed magnetic field among all X-ray pulsars $B\sim4\times10^{11}$G and makes it a prime target for the upcoming X-ray polarimetry missions covering soft X-ray band such as IXPE and eXTP. For other pulsars the region around the resonanse (typically above 20 keV) is not observable by those missions, and there lots of interesting things are expected to happen in terms of polarimetric properties! Up to now our only hope was GRO J1744-28 which also has a line at 4.5 keV, and has also shown a smallish outburst recently, so now we have two transients to watch for once IXPE launches this autumn. The more the merrier, obviously! Especially considering that Swift J1626.6-5156 appears to exhibit more or less regular (albeit low luminosity) outbursts.

The contribution of SRG to the discovery was minor but essential as the initial alert by MAXI only indicated some elevated activity, so stronger justification was needed to trigger NuSTAR which ART-XC and eRosita were able to provide as coincidentely SRG scanned over the source just a couple of days after the first reports of activity thereby unambigously confirming detection.

## A splash of papers in support for THESEUS

Theseus is going to revolutionize GRB science, but is also highly relevant for many other science cases. In support for the mission we've prepared a splash of papers discussing a wide range of science topics where Theseus can shine:

## Disk versus wind accretion in X-ray pulsar GX 301-2

Another interesting work led by Chinese Insight-HXMT collegues about my favorite source related to the spin-up episode we discused earlier. GX 301-2 provides a rare opportunity to study both disk and wind accretion in a same target. We reported on Insight-HXMT observations of the spin-up event of GX 301-2 happened in 2019 and compare with those of wind-fed state, which allowed to detect interesting pulse profile changes. Inspection of the Fermi/GBM data also confirmed Insight-HXMT results. We discuss possible differences between disk and wind accretion that may explain the observed different trend of pulse profiles.

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