191 research outputs found
X-ray eruptions every 22 days from the nucleus of a nearby galaxy
Galactic nuclei showing recurrent phases of activity and quiescence have
recently been discovered, with recurrence times as short as a few hours to a
day -- known as quasi-periodic X-ray eruption (QPE) sources -- to as long as
hundreds to a thousand days for repeating nuclear transients (RNTs). Here we
report the discovery of Swift J023017.0+283603 (hereafter Swift J0230+28), a
source that exhibits X-ray quasi-periodic eruptions from the nucleus of a
previously unremarkable galaxy at 165 Mpc, with a recurrence time of
approximately 22 days, an intermediary timescale between known RNTs and QPE
sources. We also report transient radio emission from the source, which is
likely associated with the X-ray eruptions. Such recurrent soft X-ray eruptions
from a low-mass black hole, with no accompanying UV/optical emission are
strikingly similar to QPE sources. However, in addition to having a recurrence
time that is times longer than the longest-known QPE source, Swift
J0230+28's eruptions exhibit slightly distinct shapes and temperature evolution
than the known QPE sources. The observed properties disfavor disk instability
models, and instead favor scenarios involving extreme mass ratio inspirals. Our
discovery reveals a new timescale for repeating extragalactic transients and
highlights the need for a wide-field, time-domain X-ray mission, which would
enable the exploration of the parameter space of recurring X-ray transients.Comment: Under review on Nature Astronomy. Main Section: 14 pages, 3 figures
and 1 Table. Methods: 32 pages, 11 Figures, 4 Table
Multi-Messenger Gravitational Wave Searches with Pulsar Timing Arrays: Application to 3C66B Using the NANOGrav 11-year Data Set
When galaxies merge, the supermassive black holes in their centers may form
binaries and, during the process of merger, emit low-frequency gravitational
radiation in the process. In this paper we consider the galaxy 3C66B, which was
used as the target of the first multi-messenger search for gravitational waves.
Due to the observed periodicities present in the photometric and astrometric
data of the source of the source, it has been theorized to contain a
supermassive black hole binary. Its apparent 1.05-year orbital period would
place the gravitational wave emission directly in the pulsar timing band. Since
the first pulsar timing array study of 3C66B, revised models of the source have
been published, and timing array sensitivities and techniques have improved
dramatically. With these advances, we further constrain the chirp mass of the
potential supermassive black hole binary in 3C66B to less than using data from the NANOGrav 11-year data set. This
upper limit provides a factor of 1.6 improvement over previous limits, and a
factor of 4.3 over the first search done. Nevertheless, the most recent orbital
model for the source is still consistent with our limit from pulsar timing
array data. In addition, we are able to quantify the improvement made by the
inclusion of source properties gleaned from electromagnetic data to `blind'
pulsar timing array searches. With these methods, it is apparent that it is not
necessary to obtain exact a priori knowledge of the period of a binary to gain
meaningful astrophysical inferences.Comment: 14 pages, 6 figures. Accepted by Ap
Evidence for a dynamic corona in the short-term time lags of black hole X-ray binary MAXI J1820+070
In X-ray observations of hard state black hole X-ray binaries, rapid
variations in accretion disc and coronal power-law emission are correlated and
show Fourier-frequency-dependent time lags. On short (~0.1 s) time-scales,
these lags are thought to be due to reverberation and therefore may depend
strongly on the geometry of the corona. Low-frequency quasi-periodic
oscillations (QPOs) are variations in X-ray flux that have been suggested to
arise because of geometric changes in the corona, possibly due to General
Relativistic Lense-Thirring precession. Therefore one might expect the
short-term time lags to vary on the QPO time-scale. We performed novel
spectral-timing analyses on NICER observations of the black hole X-ray binary
MAXI J1820+070 during the hard state of its outburst in 2018 to investigate how
the short-term time lags between a disc-dominated and a coronal
power-law-dominated energy band vary on different time-scales. Our method can
distinguish between variability due to the QPO and broadband noise, and we find
a linear correlation between the power-law flux and lag amplitude that is
strongest at the QPO frequency. We also introduce a new method to resolve the
QPO signal and determine the QPO-phase-dependence of the flux and lag
variations, finding that both are very similar. Our results are consistent with
a geometric origin of QPOs, but also provide evidence for a dynamic corona with
a geometry varying in a similar way over a broad range of time-scales, not just
the QPO time-scale.Comment: 19 pages, 14 figures, 3 tables, accepted for publication in MNRA
The RS Oph outburst of 2021 monitored in X-rays with NICER
The 2021 outburst of the symbiotic recurrent nova RS Oph was monitored with
the Neutron Star Interior Composition Explorer Mission (NICER) in the 0.2-12
keV range from day one after the optical maximum, until day 88, producing an
unprecedented, detailed view of the outburst development. The X-ray flux
preceding the supersoft X-ray phase peaked almost 5 days after optical maximum
and originated only in shocked ejecta for 21 to 25 days. The emission was
thermal; in the first 5 days only a non-collisional-ionization equilibrium
model fits the spectrum, and a transition to equilibrium occurred between days
6 and 12. The ratio of peak X-rays flux measured in the NICER range to that
measured with Fermi in the 60 MeV-500 GeV range was about 0.1, and the ratio to
the peak flux measured with H.E.S.S. in the 250 GeV-2.5 TeV range was about
100. The central supersoft X-ray source (SSS), namely the shell hydrogen
burning white dwarf (WD), became visible in the fourth week, initially with
short flares. A huge increase in flux occurred on day 41, but the SSS flux
remained variable. A quasi-periodic oscillation every ~35 s was always observed
during the SSS phase, with variations in amplitude and a period drift that
appeared to decrease in the end. The SSS has characteristics of a WD of mass >1
M(solar). Thermonuclear burning switched off shortly after day 75, earlier than
in 2006 outburst. We discuss implications for the nova physics.Comment: Accepted for publication in the Astrophysical Journa
Discovery, Timing, and Multiwavelength Observations of the Black Widow Millisecond Pulsar PSR J1555-2908
We report the discovery of PSR J1555-2908, a 1.79 ms radio and gamma-ray pulsar in a 5.6 hr binary system with a minimum companion mass of 0.052 M ⊙. This fast and energetic ( Ė=3×1035 erg s-1) millisecond pulsar was first detected as a gamma-ray point source in Fermi Large Area Telescope (LAT) sky survey observations. Guided by a steep-spectrum radio point source in the Fermi error region, we performed a search at 820 MHz with the Green Bank Telescope that first discovered the pulsations. The initial radio pulse timing observations provided enough information to seed a search for gamma-ray pulsations in the LAT data, from which we derive a timing solution valid for the full Fermi mission. In addition to the discovery and timing of radio and gamma-ray pulsations, we searched for X-ray pulsations using NICER but no significant pulsations were detected. We also obtained time-series r-band photometry that indicates strong heating of the companion star by the pulsar wind. Material blown off the heated companion eclipses the 820 MHz radio pulse during inferior conjunction of the companion for ≈10% of the orbit, which is twice the angle subtended by its Roche lobe in an edge-on system. © 2022. The Author(s). Published by the American Astronomical Society
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