A. W. Shaw

Butler University

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C. O. Heinke

University of Alberta

John A. Tomsick

University of California, Berkeley

G. R. Sivakoff

University of Alberta

Arash Bahramian

International Centre for Radio Astronomy Research

Thomas J. Maccarone

Texas Tech University

P. Gandhi

University of Southampton

N. Degenaar

University of Amsterdam

R. Wijnands

University of Amsterdam

D. M. Russell

New York University Abu Dhabi

Richard M. Plotkin

University of Nevada, Reno

Cooperative Institutions

University of Southampton

Tokyo University of Science

Centre National de la Recherche Scientifique

University of Amsterdam

Tokyo City University

University of Crete

FORTH Institute of Computer Science

California Institute of Technology

National Institute for Astrophysics

American Association of Variable Star Observers

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Simultaneous NICER and NuSTAR Observations of the Ultracompact X-Ray Binary 4U 0614+091

The Astrophysical Journal (2023)

David Moutard R. M. Ludlam Javier A. García D. Altamirano D J K Buisson

Abstract We present the first joint NuSTAR and NICER observations of the ultracompact X-ray binary 4U 0614+091. This source shows quasiperiodic flux variations on the timescale of ∼days. We use reflection modeling techniques to study various components of the accretion system as the flux varies. We find that the flux of the reflected emission and the thermal components representing the disk and the compact object trend closely with the overall flux. However, the flux of the power-law component representing the illuminating X-ray corona scales in the opposite direction, increasing as the total flux decreases. During the lowest flux observation, we see evidence of accretion disk truncation from roughly 6 gravitational radii to 11.5 gravitational radii. This is potentially analogous to the truncation seen in black hole low-mass X-ray binaries, which tends to occur during the low/hard state at sufficiently low Eddington ratios.

X-ray binary

Black hole (networking)

10.3847/1538-4357/acf4f3

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Citations (5)

SCHWINGER'S RESULT ON PARTICLE PRODUCTION FROM COMPLEX PATHS WKB APPROXIMATION

International Journal of Modern Physics A (2000)

S. N. Biswas A. W. Shaw B. Modak

This paper presents the derivation of Schwinger's gauge invariant result of $Im \cal{L}_{eff}$ upto one loop approximation, for particle production in an uniform electric field through the method of complex trajectory WKB approximation (CWKB). The CWKB proposed by one of the author's \cite{bis:ijtp} looks upon particle production as due to the motion of a particle in complex space-time plane, thereby requiring tunneling paths both in space and time. Recently \cite{srini:iucaa,srini1:iucaa} there have been some efforts to calculate the reflection and transmission co-efficients for particle production in uniform electric field that differ from our expressions for the same. In this paper we clarify the confusion in this regard and establish the correctness of CWKB.

WKB approximation

Particle (ecology)

10.1142/s0217751x00001221

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Citations (7)

MAXI J1820+070 with NuSTAR I. An increase in variability frequency but a stable reflection spectrum: coronal properties and implications for the inner disc in black hole binaries

Monthly Notices of the Royal Astronomical Society (2019)

D J K Buisson A. C. Fabian D. Barret Felix Fürst P. Gandhi

MAXI J1820+070 (optical counterpart ASASSN-18ey) is a black hole candidate discovered through its recent very bright outburst. The low extinction column and long duration at high flux allow detailed measurements of the accretion process to be made. In this work, we compare the evolution of X-ray spectral and timing properties through the initial hard state of the outburst. We show that the inner accretion disc, as measured by relativistic reflection, remains steady throughout this period of the outburst. Nevertheless, subtle spectral variability is observed, which is well explained by a change in coronal geometry. However, characteristic features of the temporal variability - low-frequency roll-over and QPO frequency - increase drastically in frequency, as the outburst proceeds. This suggests that the variability timescales are governed by coronal conditions rather than solely by the inner disc radius. We also find a strong correlation between X-ray luminosity and coronal temperature. This can be explained by electron pair production with a changing effective radius and a non-thermal electron fraction of ~20%.

Corona (planetary geology)

Accretion disc

Black hole (networking)

Reflection

Electron temperature

Solar radius

10.1093/mnras/stz2681

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Citations (85)

Observations of the Disk/Jet Coupling of MAXI J1820+070 during Its Descent to Quiescence

The Astrophysical Journal (2021)

A. W. Shaw Richard M. Plotkin J. C. A. Miller‐Jones J. Homan Elena Gallo

Abstract Black hole X-ray binaries in the quiescent state (Eddington ratios typically ≲10 −5 ) display softer X-ray spectra (photon indices Γ ∼ 2) compared to higher-luminosity black hole X-ray binaries in the hard state (Γ ∼ 1.7). However, the cause of this softening and its implications for the underlying accretion flow are still uncertain. Here, we present quasi-simultaneous X-ray and radio spectral monitoring of the black hole X-ray binary MAXI J1820+070 during the decay of its 2018 outburst and of a subsequent reflare in 2019, providing an opportunity to monitor a black hole X-ray binary as it actively transitions into quiescence. We probe 1–10 keV X-ray luminosities as low as L X ∼ 4 × 10 32 erg s −1 , equivalent to Eddington fractions of ∼4 × 10 −7 . During its decay toward quiescence, the X-ray spectrum of MAXI J1820+070 softens from Γ ∼ 1.7 to Γ ∼ 2, with the softening taking ∼30 days and completing at L X ≈ 10 34 erg s −1 (≈10 −5 L Edd ). While the X-ray spectrum softens, the radio spectrum generally remains flat or inverted throughout the decay. We also find that MAXI J1820+070 follows a radio ( L R )–X-ray luminosity correlation of the form L R ∝ L X 0.52±0.07 , making it the fourth black hole system to follow the so-called “standard track” unbroken over several (in this case, four) decades in L X . Comparing the radio/X-ray spectral evolution(s) with the L R – L X plane, we find that the X-ray softening is consistent with X-rays produced by Comptonization processes in a radiatively inefficient accretion flow. We generally disfavor X-ray emission originating solely from within the jet, with the possible exception of X-rays produced via synchrotron self-Compton processes.

Black hole (networking)

X-ray binary

10.3847/1538-4357/abd1de

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Citations (15)

Dirac current in expanding spacetime

Pramana (1999)

N G Sarkar S. N. Biswas A. W. Shaw

Stationary spacetime

Antiparticle

Time evolution

10.1007/s12043-999-0104-x

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Citations (1)

Using Chandra Localizations and Gaia Distances and Proper Motions to Classify Hard X-Ray Sources Discovered by INTEGRAL

The Astrophysical Journal (2021)

John A. Tomsick Benjamin M. Coughenour Jeremy Hare Roman Krivonos A. Bodaghee

Abstract Here, we report on X-ray observations of ten 17–60 keV sources discovered by the International Gamma-Ray Astrophysics Laboratory satellite. The primary new information is sub-arcsecond positions obtained by the Chandra X-ray Observatory. In six cases (IGR J17040-4305, IGR J18017-3542, IGR J18112-2641, IGR J18434-0508, IGR J19504+3318, and IGR J20084+3221), a unique Chandra counterpart is identified with a high degree of certainty, and for five of these sources (all but J19504), Gaia distances or proper motions indicate that they are Galactic sources. For four of these, the most likely classifications are that the sources are magnetic cataclysmic variables (CVs). J20084 could be either a magnetic CV or a high-mass X-ray binary. We classify the sixth source (J19504) as a likely active galactic nucleus (AGN). In addition, we find likely Chandra counterparts to IGR J18010-3045 and IGR J19577+3339, and the latter is a bright radio source and probable AGN. The other two sources, IGR J12529-6351 and IGR J18013-3222, do not have likely Chandra counterparts, indicating that they are transient, highly variable, or highly absorbed.

Cataclysmic variable star

10.3847/1538-4357/abfa1a

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Citations (8)

Hitting a New Low: The Unique 28 hr Cessation of Accretion in the TESS Light Curve of YY Dra (DO Dra)

The Astronomical Journal (2022)

Katherine Hill Colin Littlefield P. Garnavich Simone Scaringi Paula Szkody

Abstract We present the Transiting Exoplanet Surveying Satellite light curve of the intermediate polar YY Draconis (YY Dra, also known as DO Dra). The power spectrum indicates that while there is stream-fed accretion for most of the observational period, there is a day-long, flat-bottomed low state at the beginning of 2020 during which the only periodic signal is ellipsoidal variation and there is no appreciable flickering. We interpret this low state to be a complete cessation of accretion, a phenomenon that has been observed only once before in an intermediate polar. Simultaneous ground-based observations of this faint state establish that when accretion is negligible, YY Dra fades to g = 17.37 ± 0.12, which we infer to be the magnitude of the combined photospheric contributions of the white dwarf and its red dwarf companion. Using survey photometry, we identify additional low states in 2018–2019 during which YY Dra repeatedly fades to—but never below—this threshold. This implies relatively frequent cessations in accretion. Spectroscopic observations during future episodes of negligible accretion can be used to directly measure the field strength of the white dwarf by Zeeman splitting. Separately, we search newly available catalogs of variable stars in an attempt to resolve the long-standing dispute over the proper identifier of this system.

Intermediate polar

10.3847/1538-3881/ac5a51

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Citations (7)

A 420-day X-ray/optical modulation and extended X-ray dips in the short-period transient Swift J1753.5−0127

Monthly Notices of the Royal Astronomical Society (2013)

A. W. Shaw P. A. Charles A. J. Bird R. Cornelisse J. Casares

We have discovered a ∼420-d modulation, with associated X-ray dips, in Rossi X-Ray Timing Explorer-All Sky Monitor/Monitor of All-Sky X-ray Image/Swift-Burst Alert Telescope archival light curves of the short-period (3.2 h) black hole X-ray transient, Swift J1753.5−0127. This modulation only appeared at the end of a gradual re-brightening, approximately 3 yr after the initial X-ray outburst in mid-2005. The same periodicity is present in both the 2–20 and 15–50 keV bands, but with a ∼0.1 phase offset (≈40 d). Contemporaneous photometry in the optical and near-infrared reveals a weaker modulation, but consistent with the X-ray period. There are two substantial X-ray dips (very strong in the 15–50 keV band, weaker at lower energies) that are separated by an interval equal to the X-ray period. This likely indicates two physically separated emitting regions for the hard X-ray and lower energy emission. We interpret this periodicity as a property of the accretion disc, most likely a long-term precession, where the disc edge structure and X-ray irradiation are responsible for the hard X-ray dips and modulation, although we discuss other possible explanations, including Lense–Thirring precession in the inner disc region and spectral state variations. Such precession indicates a very high mass ratio low-mass X-ray binary, which even for a ∼10 M⊙ BH requires a brown dwarf donor (∼0.02 M⊙), making Swift J1753.5−0127 a possible analogue of millisecond X-ray pulsars. We compare the properties of Swift J1753.5−0127 with other recently discovered short-period transients, which are now forming a separate population of high-latitude BH transients located in the galactic halo.

Swift

X-ray transient

Black hole (networking)

Precession

10.1093/mnras/stt763

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Citations (21)

The Swift Bulge Survey - in search of the faintest X-ray transients

HEAD (2017)

A. W. Shaw C. O. Heinke Arash Bahramian Thomas J. Maccarone J. A. Kennea

Swift

Source