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Circumstantial evidence for a blue supergiant companion of IGR J16465-4507

ATel #338; D. M. Smith (Physics Department and Santa Cruz Institute for Particle Physics, U. C. Santa Cruz)
on 5 Oct 2004; 22:41 UT
Credential Certification: David M. Smith (dsmith@ssl.berkeley.edu)

Subjects: Infra-Red, Optical, X-ray, Binary, Transient

Referred to by ATel #: 429

The 2MASS counterpart to IGR J16465-4507 (ATEL #329) identified by Heras and Walter (ATEL #336) is also a bright star in the USNO B1.0 catalog, 0448-0520455, with the magnitudes shown in the table below. Its short outburst and high duty cycle of quiescence suggest a similarity with the systems XTE J1739-302 (ATEL #181,#182,#184,#186,#218) and IGR J17544-2619/1RXS J175428.3-262035 (ATEL #190,#191,#192,#194,#252, IAUC # 8202 ). The former is known from optical and infrared spectroscopy to have a supergiant companion of spectral type O7.5Iaf or O8Iaf (I. Negueruela et al., in preparation), and the latter also has a blue supergiant companion (S. Chaty et al., in preparation). The photometry of all three sources from USNO B1.0 and 2MASS also suggests they may all be similar. Taking canonical values of R, B, and K for an O9I supergiant and the relations between interstellar absorption in these bands from Cox (Allen's Astrophysical Quantities, 4th Edition, AIP Press, 2000), I find the following parameters for each system:

Name USNO B1.0USNO B1.02MASSB ExtinctionDistanceK Excess
B2 R2 K (Ab, mag.) (kpc) (K mag.)
XTE J1739-302 17.0 12.9 7.43 11.1 3.3 0.5
IGR J17544-2619 13.9 11.9 8.02 6.0 8.5 1.5
IGR J16465-4507 15.2 13.0 9.84 6.5 12.5 0.5

All three systems show an infrared excess beyond the expectation for this spectral type, which may be related to the local material which creates high x-ray absorption in XTE J1739-302 and IGR J17544-2619 at least. The derived columns are somewhat sensitive to the exact spectral class assumed and to differences between magnitudes from the first and second epochs in the USNO B1.0 catalog (I have used the second epoch throughout for consistency). More precise values will await higher-quality photometry as well as detailed spectroscopy on all systems. The large distance to IGR J16465-4507 and an early-type companion are consistent with its small Galactic latitude (0.13 degrees).

It is interesting to note that two very bright, persistent binaries with blue supergiant companions also show extremely fast outbursts lasting on the order of hours: Cyg X-1, a black-hole binary with a companion of type O9.7Iab (Golenetskii et al. 2003, ApJ 596, 1113), and Vela X-1, an x-ray pulsar with a companion of type B0.5Iab (Krivonos et al. 2003, ATEL #211). Fast transient outbursts may be a hallmark of systems with blue supergiant secondaries, perhaps due either to the short viscous timescale in small accretion disks associated with wind accretion, or to some kind of brief ejection intrinsic to the secondaries.

I am grateful to I. Negueruela and S. Chaty for permission to mention their upcoming results on the other two systems.