known X-ray transient, although regular outbursts were not visible in the bulge scans before early 2009. Fast, low-luminosity transients like NGC 6440 X-2 may be easily missed by current X-ray monitoring.'
author:
- 'C. O. Heinke, D. Altamirano, H. N. Cohn, P. M. Lugger, S. A. Budac, M. Servillat, M. Linares, T. E. Strohmayer, C. B. Markwardt, R. Wijnands, J. H. Swank, C. Knigge, C. Bailyn, J. E. Grindlay'
bibliography:
- 'src\_ref\_list.bib'
title: 'Discovery of a Second Transient Low-Mass X-ray Binary in the Globular Cluster NGC 6440'
---

Introduction {#s:intro}
============

The dense cores of globular clusters are known to be efficient factories for dynamically producing tight binaries containing heavy stars, and thus X-ray binaries [@Clark75; @Hut91; @Pooley03]. Thirteen luminous ($L_X>10^{35}$ ) low-mass X-ray binaries (LMXBs) have previously been identified in Galactic globular clusters [see @Verbunt04], concentrated in the densest and most massive clusters [@Verbunt87; @Verbunt02]. Luminous LMXBs in globular clusters of other galaxies are clearly concentrated in the most massive, most metal-rich, and densest globular clusters [@Kundu02; @Sarazin03; @Jordan04]. A critical question for studies of globular cluster LMXBs is whether X-ray emission from a globular cluster is due to one LMXB or multiple LMXBs. This affects the inferred nature 