# Victor Doroshenko

## Both instruments of SRG are now described

After the paper describing eRosita onboard SRG, now the paper describing the second instrument, ART-XC is out. If I'm not mistaken, that's the first Russian X-ray telescope with focussing optics, and the first focusing telescope doing an all-sky survey above 4\,keV at all. Sometimes overshadowed by eRosita in the eyes of international community, it's a very nice instrument in it's own and will certainly contribute to success of the SRG as a whole. If you are interested about it's science goals and performance, please read the paper linked above. From my side, warm congratulations to IKI collegues on getting the paper out!

## *Theseus* conference 2021 starts today (virtual)

Today the conference devoted to science possible with Theseus where IAAT is involved kicks in. There are many interesting talks in the program, be sure to check, and see you there if you're attending!

## SRG/ART-XC and NuSTAR observations of the X-ray pulsar GRO J1008-57 in the lowest luminosity state

The first proper paper by ART-XC onboard SRG is out!. GRO J1008-57 was the first source where we understood that accretion in quiescent BeXRBs might occur from a cold, non-ionized, low-viscosity accretion disk, much like in quiet periods of dwarf novae. The statistics of Swift/XRT and short Chandra pointing did not permit us at the time, however, to definitively prove that also the pulsar remains active, so we tried to get XMM data several times to do that, but without much success. Fortunately, the source was observed in quiescence by ART-XC telescope during the performance verification phase of SRG (hosting ART-XC and eRosita telescopes). This allowed to detect pulsations, and study the source properties in detail using the follow-up DDT NuSTAR observation

As you can see, our results show that pulsations from GRO J1008-57 in quiescence are detected both by NuSTAR and ART-XC, and the broadband X-ray spectrum resembles that of other low luminosity X-ray pulsars like X Persei, GX 304-1, 1A 0535+262 or SGR 0755−2933. Congratulations to ART-XC team on this result!

## SRG discovery of SRGA J124404.1-632232 = SRGE J124403.8-632231 - possible Galactic X-ray transient

We've just found a new Galactic BeXRB! See, i.e. telegrams 14357, 14361 and 14364. On Jan 27, 2021, during the third consecutive all-sky survey, the Mikhail Pavlinsky ART-XC telescope on board the SRG observatory discovered a bright X-ray source at RA, Dec = 191.0172deg, -63.3756deg (J2000) with a 90% error radius of 25". The position was later refined by looking into archival eRosita data, which allowed to pinpoint the optical counterpart (Be star). We've triggered NuSTAR observations and the results suggest that's indeed a new X-ray pulsar!

## SGR 0755-2933: a new High Mass X-ray binary with the wrong name

A new paper building upon our earlier work on accretion in magnetars has just been published. The soft gamma-ray repeater candidate SGR 0755-2933 was discovered in 2016 by Swift/BAT, which detected a short (~30 ms) powerful burst typical of magnetars. To understand the nature of the source, we analyzed follow-up observations of the tentative soft X-ray counterpart of the source obtained with Swift/XRT, NuSTAR and Chandra. As a result we found that based on the observed counterpart position and properties, it is actually not a soft gamma ray repeater but rather a new high mass X-ray binary. We suggest to refer to it as 2SXPS J075542.5-293353 while the true soft X-ray counterpart to the burst event remain a mystery. Presence of a soft counterpart is, however, essential to unambiguously associate the burst with a magnetar flare, and thus we conclude that magnetar origin of SRG 0755-2933 requires further investigation.

## X-ray pulsar XTE J1858+034: discovery of the cyclotron line and the revised optical identification

Some nice use of NOT and of course NuSTAR in work led by S. Tsygankov which just has been published. We present results of a detailed investigation of the poorly studied X-ray pulsar XTE J1858+034 based on the data obtained with the NuSTAR observatory during the outburst of the source in 2019. The spectral analysis resulted in the discovery of a cyclotron absorption feature in the source spectrum at ~48 keV both in the pulse phase averaged and resolved spectra. Accurate X-ray localization of the source using the NuSTAR and Chandra observatories allowed us to accurately determine the position of the X-ray source and identify the optical companion of the pulsar. The analysis of the counterpart properties suggested that the system is likely a symbiotic binary hosting an X-ray pulsar and a late type companion star of K-M classes rather than Be X-ray binary as previously suggested.

## eRosita bubbles are featured @HEASARC!

Picture of the week at HEASARC is something for the mission!

One of the most surprising structures associated with our home galaxy, the Milky Way, are the gamma-ray emitting, bubble-like structures extending more than 25,000 light years above and below the center of the Galaxty. These bubbles were discovered by the Fermi Gamma-Ray Space Telescpe about a decade ago, and now affectionately known as the "Fermi Bubbles" by astronomers. Clearly the result of some remarkable outpouring of energy near the center of the Milky Way, the event (or series of events) that blew the "Fermi Bubbles" is still not entirely understood. A new all-sky X-ray map obtained by the eROSITA telescope on the Spektr-RG spacecraft may help solve this problem. The image above shows composite all-sky maps centered on the center of the Milky Way (so that the disk of the Milky Way stretches from left to right at the middle of the image, and the center of the Milky Way is at the center of the image). The gamma-ray map from Fermi is shown in red, while the X-ray map from eROSITA is shown in blue. The Fermi image shows the Fermi Bubbles emanating from the center of the Milky Way above and below the disk of the Galaxy. The X-ray image from eROSITA also shows similar X-ray bubbles above and below the Galaxy, but about twice as large as the Fermi bubbles, each having a diameter of about 45,000 lightyears. Analysis of the X-ray "eROSITA Bubbles" suggest that these bubbles are probably the remnants of an enormous but temporary burst of energy from Sgr A*, the 4-million solar mass black hole at the center of the Milky Way, about a Fermi bubble diameter away from us. The outburst might have happened if Sgr A* swallowed a massive cloud of gas (or perhaps some unfortunate star or group of stars) a few million years ago, and, for a million years or two, formed a disk of accreting matter and shot a particle beam into space perpendicular to the Galaxy's disk, blowing out material that we seen now as the Fermi/eROSITA bubbles. Sometime after this feeding frenzy, the monster black hole afterwards fell into its current sleepy state.

## X-ray reprocessing in accreting pulsar GX 301-2 observed with Insight-HXMT

Another nice work based on Insight-HXMT data. The last one for Long Ji while in Tuebingen :( Congratulations on new position, Long!

In the paper investigate the absorption and emission features in observations of GX 301-2 detected with Insight-HXMT/LE in 2017-2019. At different orbital phases, we found prominent Fe K$\alpha$, K$\beta$ and Ni K$\alpha$ lines, as well as Compton shoulders and Fe K-shell absorption edges. These features are due to the X-ray reprocessing caused by the interaction between the radiation from the source and surrounding accretion material. According to the ratio of iron lines K$\alpha$ and K$\beta$, we infer the accretion material is in a low ionisation state. We find an orbital-dependent local absorption column density, which has a large value and strong variability around the periastron. We explain its variability as a result of inhomogeneities of the accretion environment and/or instabilities of accretion processes. In addition, the variable local column density is correlated with the equivalent width of the iron K$\alpha$ lines throughout the orbit, which suggests that the accretion material near the neutron star is spherically distributed.

## The eROSITA X-ray telescope on SRG

Years of work, Vodka@Baikonur and hours of waiting for the news of doom which did not came. Well done MPE! Happy to be part of the team, and wish all the best for the mission! Now the "main" paper describing the telescope has been published

## An observational argument against accretion in magnetars

It turns out that one can easily distinguish accreting and non-accreting neutron stars based on their variability! The paper which we've just published settles a years-long dispute on whether persistent emission of magnetars could actually be powered by accretion, and the answer is "no". This conclusion is based on a simple fact that all accreting objects we know are variable (have noise in their power spectra), and all non-accreting objects (including magnterars) are not as is illustrated in figure below from the paper:

Surprisingly, the short and easy paper caused quite a bit of feedback with me getting messages from Ali Alpar, Sandro Mereghetti, Nikos Kylafis, and others. Some agreeing, and some disagreeing, but all friendly! Thanks for feedback and I really hope our result will be useful.

## Spectral evolution of X-ray pulsar 4U 1901+03 during 2019 outburst based on Insight-HXMT and NuSTAR observations

The first result of the collaboration between Beijing, Tuebingen and Turku on Insight-HXMT data!

We report on detailed spectral analysis of emission from X-ray pulsar 4U 1901+03 using the data obtained by the Insight-HXMT and NuSTAR observatories during the 2019 outburst of the source. Thanks to the extensive coverage of the outburst by Insight-HXMT, we were able to investigate spectral evolution of the source as a function of flux, and compare these results to the previous reports with focus on the properties of a putative absorption feature at around 10 keV. In particular, we demonstrate that the broadband X-ray continuum of 4U 1901+03 can be well described with a two-component continuum model without absorption line at 10 keV, which casts doubt on its interpretation as a cyclotron line. The high quality of the data has also allowed us to perform both phase-averaged and phase-resolved spectral analysis as function of luminosity. Finally, we performed a detailed investigation of another absorption feature in the spectrum of the source around 30 keV recently reported in the NuSTAR data. In line with the previous findings, we show that this feature appears to be significantly detected, albeit only in the pulse-phase resolved spectra at relatively high luminosities. The non-detection of the line in the averaged spectrum is possibly associated with the observed strong variations of its parameters with the pulse phase.

## Switches between accretion structures during flares in 4U 1901+03

Another paper based on Insight-HXMT and led by Long Ji. We report on our analysis of the 2019 outburst of the X-ray accreting pulsar 4U 1901+03 observed with Insight-HXMT and NICER. Both spectra and pulse profiles evolve significantly in the decaying phase of the outburst. Dozens of flares are observed throughout the outburst. They are more frequent and brighter at the outburst peak. We find that the flares, which have a duration from tens to hundreds of seconds, are generally brighter than the persistent emission by a factor of ∼1.5. The pulse-profile shape during the flares can be significantly different from that of the persistent emission. In particular, a phase shift is clearly observed in many cases. We interpret these findings as direct evidence of changes of the pulsed beam pattern, due to transitions between the sub- and supercritical accretion regimes on a short time-scale. We also observe that at comparable luminosities the flares' pulse profiles are rather similar to those of the persistent emission. This indicates that the accretion on the polar cap of the neutron star is mainly determined by the luminosity, I.e. the mass accretion rate.

## The unusual behavior of the young X-ray pulsar SXP 1062 during the 2019 outburst

We present the results of the first dedicated observation of the young X-ray pulsar SXP 1062 in the broad X-ray energy band obtained during its 2019 outburst with the NuSTAR and XMM-Newton observatories. The analysis of the pulse-phase averaged and phase-resolved spectra in the energy band from 0.5 to 70 keV did not reveal any evidence for the presence of a cyclotron line. The spin period of the pulsar was found to have decreased to 979.48±0.06 s implying a ∼10% reduction compared to the last measured period during the monitoring campaign conducted about five years ago, and is puzzling considering that the system apparently has not shown major outbursts ever since. The switch of the pulsar to the spin-up regime supports the common assumption that torques acting on the accreting neutron star are nearly balanced and thus SXP 1062 likely also spins with a period close to the equilibrium value for this system. The current monitoring of the source also revealed a sharp drop in its soft X-ray flux right after the outburst, which is in drastic contrast to the behavior during the previous outburst when the pulsar remained observable for years with only a minor flux decrease after the end of the outburst. This unexpected off state of the source lasted for at most 20 days after which SXP 1062 returned to the level observed during previous campaigns. We discuss this and other findings in context of the modern models of accretion onto strongly magnetized neutron stars.

## Observations of GRO J1744-28 in quiescence with XMM-Newton

Following long-awaited observation of ultra-peculiar source, the "bursting pulsar", GRO J1744-28 with XMM-Newton in quiescence, we finally published a work reporting on the results.

We detected the source at a luminosity level of $\sim10^{34}$erg/s with an X-ray spectrum that is consistent with the power law, blackbody, or accretion-heated neutron star atmosphere models. The improved X-ray localization of the source allowed us to confirm the previously identified candidate optical counterpart as a relatively massive G/K~III star at 8 kpc close to the Galactic center, implying an almost face-on view of the binary system. Although we could only find a nonrestricting upper limit on the pulsed fraction of ∼20%, the observed hard X-ray spectrum and strong long-term variability of the X-ray flux suggest that the source is also still accreting when not in outburst. The luminosity corresponding to the onset of centrifugal inhibition of accretion is thus estimated to be at least two orders of magnitude lower than previously reported. We discuss this finding in the context of previous studies and argue that the results indicate a multipole structure in the magnetic field with the first dipole term of ∼1010 G, which is much lower than previously assumed.

## eROSITA finds large-scale bubbles in the halo of the Milky Way

Now finally out of the embargo! The detailed press-release is available at our webpage, but I must also briefly summarize it here: basically eRosita has just found gigantic "bubbles" encompassing famous Fermi Bubbles, so even larger than those! In eRosita all-sky survey they look like that

Here the false-color map the extended emission at energies of 0.6-1.0 keV highlights the structure extending from teh Galactic center upwards and downwards. The upper part was actually already known in X-rays as a North Polar Spur (NPS), but it was believed to be a local structure associated with an old SNR or somethign similar. Now it became clear that NPS together with the newly seen part form structure analogous to Fermi bubbles, which we called eRosita bubbles. That thing is huge and required huge amount of energy to blow! The source of energy is still uncertain, so lots to be done in the near future.

Schematic view of the eROSITA (yellow) and Fermi bubbles (purple). The galactic disk is indicated with its spiral arms and the location of the Solar System is marked. The eROSITA bubbles are considerably larger than the Fermi bubbles, indicating that these structures are comparable in size to the whole galaxy. As you could imagine, we went to Nature to publish this discovery.

From my side, I'd like to add that the latest touches to the images now published were prepared in August, just before we went off to vacations in Italy. Basically, I were running circles between the laptop and stuff scattered all around the house and partly in the car already! Now it has payed off, which I'm really happy about.

PS. Vacation was nice too!

## Advances in Understanding High-Mass X-ray Binaries with INTEGRAL and Future Directions

We've just published a large review on HMXBs. Large team of authors led by Peter Kretschmar who managed to find time to do that somehow!

High mass X-ray binaries are among the brightest X-ray sources in the Milky Way, as well as in nearby Galaxies. Thanks to their highly variable emissions and complex phenomenology, they have attracted the interest of the high energy astrophysical community since the dawn of X-ray Astronomy. In more recent years, they have challenged our comprehension of physical processes in many more energy bands, ranging from the infrared to very high energies. In this review, we provide a broad but concise summary of the physical processes dominating the emission from high mass X-ray binaries across virtually the whole electromagnetic spectrum. These comprise the interaction of stellar winds with the high gravitational and magnetic fields of compact objects, the behaviour of matter under extreme magnetic and gravity conditions, and the perturbation of the massive star evolutionary processes by presence in a binary system. We highlight the role of the INTEGRAL mission in the discovery of many of the most interesting objects in the high mass X-ray binary class and its contribution in reviving the interest for these sources over the past two decades. We show how the INTEGRAL discoveries have not only contributed to significantly increase the number of high mass X-ray binaries known, thus advancing our understanding of the population as a whole, but also have opened new windows of investigation that stimulated the multi-wavelength approach nowadays common in most astrophysical research fields. We conclude the review by providing an overview of future facilities being planned from the X-ray to the very high energy domain that will hopefully help us in finding an answer to the many questions left open after more than 18 years of INTEGRAL scientific observations.

## INTEGRAL View on cataclysmic variables and symbiotic binaries

After a long while and lots of work by Alexander Lutovinov, Valery Suleimanov and others (including my modest contribution), a gigantic review on CVs and Symbiotic binaries is finally published.

Accreting white dwarfs (WDs) constitute a significant fraction of the hard X-ray sources detected by the INTEGRAL observatory. Most of them are magnetic Cataclysmic Variables (CVs) of the intermediate polar (IP) and polar types, but the contribution of the Nova-likes systems and the systems with optically thin boundary layers, Dwarf Novae (DNs) and Symbiotic Binaries (or Symbiotic Stars, SySs) in quiescence is also not negligible. Here we present a short review of the results obtained from the observations of cataclysmic variables and symbiotic binaries by INTEGRAL. The highlight results include the significant increase of the known IP population, determination of the WD mass for a significant fraction of IPs, the establishment of the luminosity function of magnetic CVs, and uncovering origin of the Galactic ridge X-ray emission which appears to largely be associated with hard emission from magnetic CVs.

## Measuring the masses of magnetic white dwarfs: a NuSTAR legacy survey

A follow-up of our work with Valery Suleimanov on IPs has just been published. This time led by our US collegues (Thanks Aarran!). NuSTAR is a wonderful instrument, and much better than BAT when doing precise measurements for fainter objects! This allowed to correct couple of outliers we've got in earlier work with Valery Suleimanov, and overall, improve accuracy for mass measurements.

The hard X-ray spectrum of magnetic cataclysmic variables can be modelled to provide a measurement of white dwarf mass. This method is complementary to radial velocity measurements, which depend on the (typically rather uncertain) binary inclination. Here we present results from a Legacy Survey of 19 magnetic cataclysmic variables with NuSTAR. We fit accretion column models to their 20-78 keV spectra and derive the white dwarf masses, finding a weighted average M${WD}$=0.77$\pm0.02M\odot$, with a standard deviation $\sigma=0.1M_\odot$, when we include the masses derived from previous NuSTAR observations of seven additional magnetic cataclysmic variables. We find that the mass distribution of accreting magnetic white dwarfs is consistent with that of white dwarfs in non-magnetic cataclysmic variables. Both peak at a higher mass than the distributions of isolated white dwarfs and post-common-envelope binaries. We speculate as to why this might be the case, proposing that consequential angular momentum losses may play a role in accreting magnetic white dwarfs and/or that our knowledge of how the white dwarf mass changes over accretion-nova cycles may also be incomplete.

## Insight-HXMT Firm Detection of the Highest-energy Fundamental Cyclotron Resonance Scattering Feature in the Spectrum of GRO J1008-57

A nice Insight-HXMT result was just published by our Chinese friends (but we're also involved). Large effective area of HE detector makes such things possible!

We report on the observation of the accreting pulsar GRO J1008-57 performed by Insight-HXMT at the peak of the source's 2017 outburst. Pulsations are detected with a spin period of 93.283(1) s. The pulse profile shows double peaks at soft X-rays, and only one peak above 20 keV. The spectrum is well described by the phenomenological models of X-ray pulsars. A cyclotron resonant scattering feature is detected with very high statistical significance at a centroid energy of E${cyc}$=90.32$^{+0.32}$\$ keV, for the reference continuum and line models, HIGHECUT and GABS respectively. Detection is very robust with respect to different continuum models. The line energy is significantly higher than what suggested from previous observations, which provided very marginal evidence for the line. This establishes a new record for the centroid energy of a fundamental cyclotron resonant scattering feature observed in accreting pulsars. We also discuss the accretion regime of the source during the Insight-HXMT observation.

## A splash of eRosita ATELs

The mission works as intended and constantly gives new results! Several ATELS on pulsars in magellanic clouds have just been posted.

eRASSU J052914.9-662446: SRG/eROSITA discovery of a second Be/X-ray binary in the LMC

X-ray pulsations from the recently discovered Be/X-ray binary eRASSU J052914.9-662446 in the LMC

eRASSU J050810.4-660653: SRG/eROSITA discovery of a new Be/X-ray binary in the LMC

SRG/eROSITA observations of a bright X-ray outburst from Circinus X-1

SRG/eROSITA discovery of a bright supersoft X-ray emission from the classical nova AT 2018bej in the Large Magellanic Cloud

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