Mar 24, 2021
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!
Mar 23, 2021
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!
Mar 11, 2021
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!
Jan 29, 2021
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!
Jan 26, 2021
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.
Jan 18, 2021
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.
Jan 18, 2021
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.
Dec 04, 2020
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.
Oct 07, 2020
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
Sep 29, 2020
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.
Sep 29, 2020
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.
Sep 29, 2020
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.
Sep 29, 2020
This one had made our heads scratch for a while...
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.
Sep 28, 2020
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.
Sep 12, 2020
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!
Sep 07, 2020
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.
Aug 24, 2020
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.
Aug 21, 2020
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.
Aug 04, 2020
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.
Apr 01, 2020
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