121 research outputs found
Miniaturized High-Speed Modulated X-Ray Source
A miniaturized high-speed modulated X-ray source (MXS) device and a method for rapidly and arbitrarily varying with time the output X-ray photon intensities and energies. The MXS device includes an ultraviolet emitter that emits ultraviolet light, a photocathode operably coupled to the ultraviolet light-emitting diode that emits electrons, an electron multiplier operably coupled to the photocathode that multiplies incident electrons, and an anode operably coupled to the electron multiplier that is configured to produce X-rays. The method for modulating MXS includes modulating an intensity of an ultraviolet emitter to emit ultraviolet light, generating electrons in response to the ultraviolet light, multiplying the electrons to become more electrons, and producing X-rays by an anode that includes a target material configured to produce X-rays in response to impact of the more electrons
Ground calibration of the Silicon Drift Detectors for NICER
The Neutron star Interior Composition ExploreR (NICER) is set to be deployed on the International Space Station (ISS) in early 2017. It will use an array of 56 Silicon Drift Detectors (SDDs) to detect soft X-rays (0.2 - 12 keV) with 100 nanosecond timing resolution. Here we describe the effort to calibrate the detectors in the lab primarily using a Modulated X-ray Source (MXS). The MXS that was customized for NICER provides more than a dozen emission lines spread over the instrument bandwidth, providing calibration measurements for detector gain and spectral resolution. In addition, the fluorescence source in the MXS was pulsed at high frequency to enable measurement of the delay due to charge collection in the silicon and signal processing in the detector electronics. A second chamber, designed to illuminate detectors with either 55 Fe, an optical LED, or neither, provided additional calibration of detector response, optical blocking, and effectiveness of background rejection techniques. The overall ground calibration achieved total operating time that was generally in the range of 500-1500 hours for each of the 56 detectors. Keywords: Silicon Drift Detectors; X-rays; timing spectroscopy; calibrationUnited States. National Aeronautics and Space Administration (Contract NNG14PJ13C
Rapid accretion state transitions following the tidal disruption event AT2018fyk
Following a tidal disruption event (TDE), the accretion rate can evolve from
quiescent to near-Eddington levels and back over months - years timescales.
This provides a unique opportunity to study the formation and evolution of the
accretion flow around supermassive black holes (SMBHs). We present two years of
multi-wavelength monitoring observations of the TDE AT2018fyk at X-ray, UV,
optical and radio wavelengths. We identify three distinct accretion states and
two state transitions between them. These appear remarkably similar to the
behaviour of stellar-mass black holes in outburst. The X-ray spectral
properties show a transition from a soft (thermal-dominated) to a hard
(power-law dominated) spectral state around Lfew L, and the strengthening of the corona over time
100--200 days after the UV/optical peak. Contemporaneously, the spectral
energy distribution (in particular, the UV-to-X-ray spectral slope
) shows a pronounced softening as the outburst progresses. The
X-ray timing properties also show a marked change, initially dominated by
variability at long (day) timescales while a high frequency (10
Hz) component emerges after the transition into the hard state. At late times
(500 days after peak), a second accretion state transition occurs, from
the hard into the quiescent state, as identified by the sudden collapse of the
bolometric (X-ray+UV) emission to levels below 10 L. Our
findings illustrate that TDEs can be used to study the scale (in)variance of
accretion processes in individual SMBHs. Consequently, they provide a new
avenue to study accretion states over seven orders of magnitude in black hole
mass, removing limitations inherent to commonly used ensemble studies.Comment: Accepted version following referee comments. 2 new figures compared
to previous arxiv version (Figs 9 and 10). Data will be available from the
journal webpages, or upon request to the author
Bright X-ray and Radio Pulses from a Recently Reactivated Magnetar
Magnetars are young, rotating neutron stars that possess larger magnetic fields (B ≈ 10¹³-10¹⁵G) and longer rotational periods (P ≈ 1-12 s) than ordinary pulsars. In contrast to rotation-powered pulsars, magnetar emission is thought to be fueled by the evolution and decay of their powerful magnetic fields. They display highly variable radio and X-ray emission, but the processes responsible for this behavior remain a mystery. We report the discovery of bright, persistent individual X-ray pulses from XTE J1810-197, a transient radio magnetar, using the Neutron star Interior Composition Explorer (NICER) following its recent radio reactivation. Similar behavior has only been previously observed from a magnetar during short time periods following a giant flare. However, the X-ray pulses presented here were detected outside of a flaring state. They are less energetic and display temporal structure that differs from the impulsive X-ray events previously observed from the magnetar class, such as giant flares and short X-ray bursts. Our high frequency radio observations of the magnetar, carried out simultaneously with the X-ray observations, demonstrate that the relative alignment between the X-ray and radio pulses varies on rotational timescales. No correlation was found between the amplitudes or temporal structure of the X-ray and radio pulses. The magnetar's 8.3 GHz radio pulses displayed frequency structure, which was not observed in the pulses detected simultaneously at 31.9 GHz. Many of the radio pulses were also not detected simultaneously at both frequencies, which indicates that the underlying emission mechanism producing these pulses is not broadband. We find that the radio pulses from XTE J1810-197 share similar characteristics to radio bursts detected from fast radio burst (FRB) sources, some of which are now thought to be produced by active magnetars
Probing spectral and timing properties of the X-ray pulsar RX J0440.9+4431 in the giant outburst of 2022-2023
The X-ray pulsar RX J0440.9+4431 went through a giant outburst in 2022 and
reached a record-high flux of 2.3 Crab, as observed by Swift/BAT. We study the
evolution of different spectral and timing properties of the source using NICER
observations. The pulse period is found to decrease from 208 s to 205 s, and
the pulse profile evolves significantly with energy and luminosity. The
hardness ratio and hardness intensity diagram (HID) show remarkable evolution
during the outburst. The HID turns towards the diagonal branch from the
horizontal branch above a transition (critical) luminosity, suggesting the
presence of two accretion modes. Each NICER spectrum can be described using a
cutoff power law with a blackbody component and a Gaussian at 6.4 keV. At
higher luminosities, an additional Gaussian at 6.67 keV is used. The observed
photon index shows negative and positive correlations with X-ray flux below and
above the critical luminosity, respectively. The evolution of spectral and
timing parameters suggests a possible change in the emission mechanism and
beaming pattern of the pulsar depending on the spectral transition to sub- and
super-critical accretion regimes. Based on the critical luminosity, the
magnetic field of the neutron star can be estimated in the order of 10
or 10 G, assuming different theoretical models. Moreover, the observed
iron emission line evolves from a narrow to a broad feature with luminosity.
Two emission lines originating from neutral and highly ionized Fe atoms were
evident in the spectra around 6.4 keV and 6.67 keV (higher luminosities).Comment: Published in Monthly Notices of the Royal Astronomical Societ
Bright X-ray and Radio Pulses from a Recently Reactivated Magnetar
Magnetars are young, rotating neutron stars that possess larger magnetic fields (B ≈ 10¹³-10¹⁵G) and longer rotational periods (P ≈ 1-12 s) than ordinary pulsars. In contrast to rotation-powered pulsars, magnetar emission is thought to be fueled by the evolution and decay of their powerful magnetic fields. They display highly variable radio and X-ray emission, but the processes responsible for this behavior remain a mystery. We report the discovery of bright, persistent individual X-ray pulses from XTE J1810-197, a transient radio magnetar, using the Neutron star Interior Composition Explorer (NICER) following its recent radio reactivation. Similar behavior has only been previously observed from a magnetar during short time periods following a giant flare. However, the X-ray pulses presented here were detected outside of a flaring state. They are less energetic and display temporal structure that differs from the impulsive X-ray events previously observed from the magnetar class, such as giant flares and short X-ray bursts. Our high frequency radio observations of the magnetar, carried out simultaneously with the X-ray observations, demonstrate that the relative alignment between the X-ray and radio pulses varies on rotational timescales. No correlation was found between the amplitudes or temporal structure of the X-ray and radio pulses. The magnetar's 8.3 GHz radio pulses displayed frequency structure, which was not observed in the pulses detected simultaneously at 31.9 GHz. Many of the radio pulses were also not detected simultaneously at both frequencies, which indicates that the underlying emission mechanism producing these pulses is not broadband. We find that the radio pulses from XTE J1810-197 share similar characteristics to radio bursts detected from fast radio burst (FRB) sources, some of which are now thought to be produced by active magnetars
STROBE-X: X-Ray Timing and Spectroscopy on Dynamical Timescales from Microseconds to Years
The Spectroscopic Time-Resolving Observatory for Broadband Energy X-rays
(STROBE-X) probes strong gravity for stellar mass to supermassive black holes
and ultradense matter with unprecedented effective area, high time-resolution,
and good spectral resolution, while providing a powerful time-domain X-ray
observatory.Comment: Accepted for Publication in Results in Physic
Thermonuclear X-ray Bursts with late secondary peaks observed from 4U 1608-52
We report the temporal and spectral analysis of three thermonuclear X-ray
bursts from 4U 1608-52, observed by the Neutron Star Interior Composition
Explorer (NICER) during and just after the outburst observed from the source in
2020. In two of the X-ray bursts, we detect secondary peaks, 30 and 18 seconds
after the initial peaks. The secondary peaks show a fast rise exponential
decay-like shape resembling a thermonuclear X-ray burst. Time-resolved X-ray
spectral analysis reveals that the peak flux, blackbody temperature, and
apparent emitting radius values of the initial peaks are in agreement with
X-ray bursts previously observed from 4U 1608-52, while the same values for the
secondary peaks tend toward the lower end of the distribution of bursts
observed from this source. The third X-ray burst, which happened during much
lower accretion rates did not show any evidence for a deviation from an
exponential decay and was significantly brighter than the previous bursts. We
present the properties of the secondary peaks and discuss the events within the
framework of short recurrence time bursts or bursts with secondary peaks. We
find that the current observations do not fit in standard scenarios and
challenge our understanding of flame spreading.Comment: Accepted for publication in the Astrophysical Journa
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