The Swift-BAT Hard X-Ray Transient Monitor Krimm, H. A.; Holland, S. T.; Corbet, R. H. D. ...
The Astrophysical journal. Supplement series,
11/2013, Letnik:
209, Številka:
1
Journal Article
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The Swift/Burst Alert Telescope (BAT) hard X-ray transient monitor provides near real-time coverage of the X-ray sky in the energy range 15-50 keV. The BAT observes 88% of the sky each day with a ...detection sensitivity of 5.3 mCrab for a full-day observation and a time resolution as fine as 64 s. The three main purposes of the monitor are (1) the discovery of new transient X-ray sources, (2) the detection of outbursts or other changes in the flux of known X-ray sources, and (3) the generation of light curves of more than 900 sources spanning over eight years. The primary interface for the BAT transient monitor is a public Web site. Between 2005 February 12 and 2013 April 30, 245 sources have been detected in the monitor, 146 of them persistent and 99 detected only in outburst. Among these sources, 17 were previously unknown and were discovered in the transient monitor. In this paper, we discuss the methodology and the data processing and filtering for the BAT transient monitor and review its sensitivity and exposure.We provide a summary of the source detections and classify them according to the variability of their light curves. Finally, we review all new BAT monitor discoveries. For the new sources that are previously unpublished, we present basic data analysis and interpretations.
ABSTRACT Gamma-ray binaries consist of a neutron star or a black hole interacting with a normal star to produce gamma-ray emission that dominates the radiative output of the system. Only a handful of ...such systems have been previously discovered, all within our Galaxy. Here, we report the discovery of a luminous gamma-ray binary in the Large Magellanic Cloud, found with the Fermi Large Area Telescope (LAT), from a search for periodic modulation in all sources in the third Fermi LAT catalog. This is the first such system to be found outside the Milky Way. The system has an orbital period of 10.3 days, and is associated with a massive O5III star located in the supernova remnant DEM L241, previously identified as the candidate high-mass X-ray binary (HMXB) CXOU J053600.0-673507. X-ray and radio emission are also modulated on the 10.3 day period, but are in anti-phase with the gamma-ray modulation. Optical radial velocity measurements suggest that the system contains a neutron star. The source is significantly more luminous than similar sources in the Milky Way, at radio, optical, X-ray, and gamma-ray wavelengths. The detection of this extra-galactic system, but no new Galactic systems, raises the possibility that the predicted number of gamma-ray binaries in our Galaxy has been overestimated, and that HMXBs may be born containing relatively slowly rotating neutron stars.
ABSTRACT We report the Fermi Large Area Telescope (LAT) detections of high-energy (>100 MeV) γ-ray emission from two recent optically bright classical novae, V1369 Centauri 2013 and V5668 Sagittarii ...2015. At early times, Fermi target-of-opportunity observations prompted by their optical discoveries provided enhanced LAT exposure that enabled the detections of γ-ray onsets beginning ∼2 days after their first optical peaks. Significant γ-ray emission was found extending to 39-55 days after their initial LAT detections, with systematically fainter and longer-duration emission compared to previous γ-ray-detected classical novae. These novae were distinguished by multiple bright optical peaks that encompassed the time spans of the observed γ-rays. The γ-ray light curves and spectra of the two novae are presented along with representative hadronic and leptonic models, and comparisons with other novae detected by the LAT are discussed.
We report on the long-term average spin period, rate of change of spin period and X-ray luminosity during outbursts for 42 Be X-ray binary systems in the Small Magellanic Cloud. We also collect and ...calculate parameters of each system and use these data to determine that all systems contain a neutron star which is accreting via a disc, rather than a wind, and that if these neutron stars are near spin equilibrium, then over half of them, including all with spin periods over about 100 s, have magnetic fields over the quantum critical level of 4.4 × 1013 G. If these neutron stars are not close to spin equilibrium, then their magnetic fields are inferred to be much lower, of the order of 106-1010 G, comparable to the fields of neutron stars in low-mass X-ray binaries. Both results are unexpected and have implications for the rate of magnetic field decay and the isolated neutron star population.
We report the identification from multiwavelength observations of the Fermi Large Area Telescope (LAT) source 4FGL J1405.1−6119 (=3FGL J1405.4−6119) as a high-mass gamma-ray binary. Observations with ...the LAT show that gamma-ray emission from the system is modulated at a period of 13.7135 0.0019 days, with the presence of two maxima per orbit with different spectral properties. X-ray observations using the Neil Gehrels Swift Observatory X-ray Telescope show that X-ray emission is also modulated at this period, but with a single maximum that is closer to the secondary lower-energy gamma-ray maximum. A radio source, coincident with the X-ray source is also found from Australia Telescope Compact Array observations, and the radio emission is modulated on the gamma-ray period with similar phasing to the X-ray emission. A large degree of interstellar obscuration severely hampers optical observations, but a near-infrared counterpart is found. Near-infrared spectroscopy indicates an O6 III spectral classification. This is the third gamma-ray binary to be discovered with the Fermi-LAT from periodic modulation of the gamma-ray emission, the other two sources also have early O star, rather than Be star, counterparts. We consider at what distances we can detect such modulated gamma-ray emission with the LAT, and examine constraints on the gamma-ray binary population of the Milky Way.
Three periastron passages of the PSR B1259−63/LS 2883 binary system, consisting of a 48 ms rotation-powered pulsar and a Be star, have been observed by the Large Area Telescope (LAT) on board the ...Fermi Gamma-ray Space Telescope, in 2010, 2014, and 2017. During the most recent periastron passage, sustained low-level gamma-ray emission was observed over a ∼3-week-long interval immediately after periastron, which was followed by an interval of no emission. Sporadic flares were detected starting 40 days post-periastron and lasted approximately 50 days, during which the emission displayed significant spectral curvature, variability on timescales as short as 1.5 minutes, and peak flux levels well in excess of the pulsar spin-down power. By contrast, during the 2010 and 2014 periastron passages, significant gamma-ray emission was not observed with the LAT until 30 and 32 days post-periastron, respectively. The previous flares did not exhibit spectral curvature, showed no short term variability, and did not exceed the pulsar spin-down power. The high flux and short timescales observed in 2017 suggest significant beaming of the emission is required and constrain the size of the emission region. The flares occur long enough after periastron that the neutron star should already have passed through the extended disk-like outflow, thus constraining options for target material and seed photon sources for inverse Compton models.
Supermassive black holes have powerful gravitational fields with strong gradients that can destroy stars that get too close, producing a bright flare in ultraviolet and X-ray spectral regions from ...stellar debris that forms an accretion disk around the black hole. The aftermath of this process may have been seen several times over the past two decades in the form of sparsely sampled, slowly fading emission from distant galaxies, but the onset of the stellar disruption event has not hitherto been observed. Here we report observations of a bright X-ray flare from the extragalactic transient Swift J164449.3+573451. This source increased in brightness in the X-ray band by a factor of at least 10,000 since 1990 and by a factor of at least 100 since early 2010. We conclude that we have captured the onset of relativistic jet activity from a supermassive black hole. A companion paper comes to similar conclusions on the basis of radio observations. This event is probably due to the tidal disruption of a star falling into a supermassive black hole, but the detailed behaviour differs from current theoretical models of such events.
Abstract
Observations with the Fermi Large Area Telescope (LAT) of the gamma-ray source 4FGL J1702.7−5655, previously classified as a candidate millisecond pulsar, show highly significant modulation ...at a period of 0.2438033 days (∼5.85 hr). Further examination of the folded light curve indicates the presence of narrow eclipses, suggesting that this is a redback binary system. An examination of the long-term properties of the modulation over 13 years of LAT observations indicates that the orbital modulation of the gamma rays changed from a simple eclipse before early 2013 to a broader, more easily detected quasi-sinusoidal modulation. In addition, the time of the eclipse shifts to ∼0.05 later in phase. This change in the orbital modulation properties, however, is not accompanied by a significant overall change in gamma-ray flux or spectrum. The quasi-sinusoidal component peaks ∼0.5 out of phase with the eclipse, which would indicate inferior conjunction of the compact object in the system. Swift X-ray Telescope observations reveal a possible X-ray counterpart within the LAT error ellipse. However, radio observations obtained with the Australia Telescope Compact Array do not detect a source in the region. 4FGL J1702.7−5655 appears to have changed its state in 2013, perhaps related to changes in the intrabinary shock in the system. We compare the properties of 4FGL J1702.7−5655 to those of other binary millisecond pulsars that have exhibited orbital modulation in gamma-rays.
ABSTRACT
We present the first Swift/XRT long-term monitoring of 2S 0114+650, a wind-fed supergiant X-ray binary for which both orbital and superorbital periods are known (Porb ∼ 11.6 d and Psup ∼ ...30.8 d). Our campaign, summing up to ∼79 ks, is the most intense and complete sampling of the X-ray light curve of this source with a sensitive pointed X-ray instrument, and covers 17 orbital, and 6 superorbital cycles. The combination of flexibility, sensitivity, and soft X-ray coverage of the X-ray telescope (XRT) allowed us to confirm previously reported spectral changes along the orbital cycle of the source and unveil the variability in its spectral parameters as a function of the superorbital phase. For completeness, we also report on a similar analysis carried out by exploiting XRT archival data on three additional wind-fed supergiant X-ray binaries IGR J16418−4532, IGR J16479−4514, and IGR J16493−4348. For these sources, the archival data provided coverage along several superorbital cycles but our analysis could not reveal any significant spectral variability.
ABSTRACT
4U 1210−64 is a peculiar X-ray binary that was first discovered in 1978 by the Uhuru satellite. The analysis of the X-ray data revealed a 6.7-d orbital period and an additional long-term ...modulation that is manifested as low- and high-flux states. Based on the previous classification of the donor star from the analysis of its optical spectra, the system has been suggested to be a high-mass X-ray binary. We revisit the optical classification where, based on the spectra from the Southern African Large Telescope (SALT), we conclude that the donor star is of spectral class A8 III–IV, making it a member of the rare intermediate-mass X-ray binaries. We perform radial velocity analysis using the SALT spectra where we consider circular and eccentric orbits. From the mass function derived and the mass constraints of the donor star, we demonstrate that a neutron star is favoured as the compact object in the binary system. We show, for the first time, the folded optical light curves, whose shape is interpreted to be due to a combination of ellipsoidal variations, irradiation of the donor star, and mutual eclipses of the star and accretion disc.
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