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Bennett, E.F.; Maxson, D.R.

doi:10.1103/physrev.116.131

Cited by 19 publications

(12 citation statements)

References 8 publications

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“…Thus, we have some independent knowledge of the location of the lenses in both cases of observed microlensing events towards the SMC, and they both are due to self-lensing within the SMC. Towards the LMC, there is only one binary event, MACHO-LMC-9, for which we have independent knowledge of the location of the lens (Bennett et al 1996) where the lens is inferred to be within the LMC. Thus, out of all the events observed so far towards the LMC and the SMC, there are a total of 3 events for which we have some independent knowledge of the location of the lenses: 2 towards the SMC as stated above, and one towards the LMC (MACHO-LMC-9).…”

Section: Discussionmentioning

confidence: 99%

Spectroscopy of MACHO 97-SMC-1: Self-Lensing within the Small Magellanic Cloud

Sahu

1998

The Astrophysical Journal

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More than a dozen microlensing events have been detected so far toward the LMC, and two have been detected toward the SMC. If all of the lenses are in the Galactic halo, both the LMC and the SMC events are expected to have similar timescales. However, the first event toward the SMC, MACHO 97-SMC-1, had a timescale of 123 days, which is much larger than the typical timescale for the LMC events. Since the observed timescale of the SMC event would need the mass of the halo lens to be ∼3 M , , it has been argued earlier that the lens must be within the SMC, which we spectroscopically confirm in this Letter. From optical depth estimates, we first show that the stars within the SMC play a dominant role as gravitational lenses and can fully account for the observed microlensing events, mainly due to its large physical depth. We also show that if the lenses are within the Magellanic Clouds, then the SMC events should be longer in duration than the LMC events, a fact that is consistent with the observations. The timescale of the event implies that the mass of the lens is տ2 M , if it is in the Milky Way disk or halo, in which case the lens, if it is a normal star, is expected to be bright and should reveal itself in the spectrum. Here, we present an optical spectrum of MACHO 97-SMC-1 obtained in 1997 May that shows that the source is a main-sequence B star. There is no trace of any contribution from the lens, which suggests that the lens is not in the Milky Way disk or halo but is a low-mass star within the SMC. The other alternative, that the lens could be a black hole in the Galactic halo, cannot be ruled out from the spectrum alone, but this is disfavored by the timescales of the LMC events. It is worth noting here that MACHO SMC-98-1 is the only other observed event toward the SMC. This was a binary lens event for which the caustic crossing timescale as observed by the PLANET, MACHO, EROS, and OGLE collaborations suggests that the lens is within the SMC. Furthermore, the only LMC event for which we have independent information on the location of the lens is the binary lens event MACHO LMC-9, where the caustic crossing timescale suggests the lens to be within the LMC. Thus, our current knowledge of the events indicates that all three microlensing events toward the Magellanic Clouds for which we have independent knowledge of the location of the lenses are due to selflensing within the Magellanic Clouds.

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Section: Discussionmentioning

confidence: 99%

Spectroscopy of MACHO 97-SMC-1: Self-Lensing within the Small Magellanic Cloud

Sahu

1998

The Astrophysical Journal

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“…This is due, in part, to SoDOPHOT's error flags, which allow the removal of suspect data points (Alcock et al 1999), but the scatter in DOPHOT photometry is often increased by the user's choice of point-spread function (PSF) fitting parameters. Contrary to expectations, allowing the PSF fit box size to scale with the seeing generally causes increased photometric scatter (Bennett et al 1993). The photometric errors reported by SoDOPHOT are modified by adding of 1.4% and 1.0% in quadrature to the MACHO and MPS data, respectively, to account for normalization and flat-fielding errors.…”

Section: The Data Setmentioning

confidence: 99%

“…The MACHO and MPS data were reduced with slightly different versions of the SoDOPHOT photometry code (Bennett et al 1993;Alcock et al 1999). SoDOPHOT is quite similar to the DOPHOT photometry code (Schechter, Mateo, & Saha 1993) that it was derived from, but SoDO-PHOT photometry generally exhibits smaller photometric scatter than DOPHOT photometry.…”

Section: The Data Setmentioning

confidence: 99%

“…Gravitational microlensing surveys (Liebes 1964;Paczyń ski 1986;Alcock et al 1993;Aubourg et al 1993; Udalski et al 1993;Bond et al 2001) have the potential to detect completely dark stellar remnants, but for most microlensing events, the mass can only be estimated very crudely based on the observed Einstein ring diameter crossing timet t. For an individual microlensing event, the mass can only be estimated so crudely that a 7 M black hole cannot be distinguished from a 0.5 M star. However, for some microlensing events, it is possible to measure other parameters besidest t that allow tighter constraints on the lens mass (Refsdal 1966;Gould 1992;Nemiroff & Wickramasinghe 1994;Alcock et al 1995Alcock et al , 1997cAlcock et al , 2001aBennett et al 1996;Han & Gould 1997;Afonso et al 2000). For longtimescale microlensing events, which are often due to massive lenses, it is frequently possible to measure the microlensing parallax effect (Refsdal 1966;Gould 1992;Alcock et al 1995), which is an observable deviation in the microlensing light curve due to the orbital motion of the Earth.…”

Section: Introductionmentioning

confidence: 99%

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Gravitational Microlensing Events Due to Stellar‐Mass Black Holes

Bennett

Becker

Quinn

et al. 2002

ApJ

135

134

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We present an analysis of the longest timescale microlensing events discovered by the MACHO Collaboration during a 7 year survey of the Galactic bulge. We find six events that exhibit very strong microlensing parallax signals due, in part, to accurate photometric data from the GMAN and MPS collaborations. The microlensing parallax fit parameters are used in a likelihood analysis, which is able to estimate the distances and masses of the lens objects based on a standard model of the Galactic velocity distribution. This analysis indicates that the most likely masses of five of the six lenses are greater than 1 M , which suggests that a substantial fraction of the Galactic lenses may be massive stellar remnants. This could explain the observed excess of long-timescale microlensing events. The lenses for events MACHO-96-BLG-5 and MACHO-98-BLG-6 are the most massive, with mass estimates of M=M ¼ 6 þ10 À3 and M=M ¼ 6 þ7 À3 , respectively. The observed upper limits on the absolute brightness of main-sequence stars for these lenses are less than 1 L , so both lenses are black hole candidates. The black hole interpretation is also favored by a likelihood analysis with a Bayesian prior using a conventional model for the lens mass function. We consider the possibility that the source stars for some of these six events may lie in the foreground Galactic disk or in the Sagittarius (Sgr) dwarf galaxy behind the bulge, but we find that bulge sources are likely to dominate our microlensing parallax event sample. Future Hubble Space Telescope observations of these events can either confirm the black hole lens hypothesis or detect the lens stars and provide a direct measurement of their masses. Future observations of similar events by the Space Interferometry Mission or the Keck or VLT interferometers, as explained by Delplancke, Gó rski, & Richichi, will allow direct measurements of the lens masses for stellar remnant lenses as well.

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“…Circumprimary systems have been examined by Lee et al (2008), but notably Poleski et al (2014) and Gould et al (2014) have recently found examples of such planets. 4 To date, only one microlensing event has been claimed to be caused by a circumbinary planet, MACHO-97-BLG-41L (Bennett et al 1999), but this was later shown to be caused by binary stars with orbital motion (Albrow et al 2000;Jung et al 2013;Han et al 2016). There have been no other microlensing circumbinary planet detections published, and relatively little theoretical work on microlensing by circumbinary planets, with only a brief paper by Han (2008b) addressing them directly.…”

Section: Introductionmentioning

confidence: 99%

Caustic Structures and Detectability of Circumbinary Planets in Microlensing

Luhn

Penny

Gaudi

2016

ApJ

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Recent discoveries of circumbinary planets in Kepler data show that there is a viable channel of planet formation around binary main sequence stars. Motivated by these discoveries, we have investigated the caustic structures and detectability of circumbinary planets in microlensing events. We have produced a suite of animations of caustics as a function of the projected separation and angle of the binary host to efficiently explore caustic structures over the entire circumbinary parameter space. Aided by these animations, we have derived a semi-empirical analytic expression for the location of planetary caustics, which are displaced in circumbinary lenses relative to those of planets with a single host. We have used this expression to show that the dominant source of caustic motion will be due to the planet's orbital motion and not that of the binary star. Finally, we estimate the fraction of circumbinary microlensing events that are recognizable as such to be significant (5-50 percent) for binary projected separations in the range 0.1-0.5 in units of Einstein radii.

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Angular Distributions of Deuterons fromLi7(p, d)Li6

Cited by 19 publications

References 8 publications

Spectroscopy of MACHO 97-SMC-1: Self-Lensing within the Small Magellanic Cloud

Spectroscopy of MACHO 97-SMC-1: Self-Lensing within the Small Magellanic Cloud

Gravitational Microlensing Events Due to Stellar‐Mass Black Holes

Caustic Structures and Detectability of Circumbinary Planets in Microlensing

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