34,600 research outputs found
Witnessing the Birth of a Quasar
The coalescence of a supermassive black hole binary (SMBHB) is thought to be
accompanied by an electromagnetic (EM) afterglow, produced by the viscous
infall of the surrounding circumbinary gas disk after the merger. It has been
proposed that once the merger has been detected in gravitational waves (GWs) by
LISA, follow-up EM searches for this afterglow can help identify the EM
counterpart of the LISA source. Here we study whether the afterglows may be
sufficiently bright and numerous to be detectable in EM surveys alone. The
viscous afterglow, which lasts for years to decades for SMBHBs in LISA's
sensitivity window, is characterized by rapid increases in both the bolometric
luminosity and in the spectral hardness of the source. If quasar activity is
triggered by the same major galaxy mergers that produce SMBHBs, then the
afterglow could be interpreted as a signature of the birth of a quasar. Using
an idealized model for the post-merger viscous spreading of the circumbinary
disk and the resulting light curve, and using the observed luminosity function
of quasars as a proxy for the SMBHB merger rate, we delineate the survey
requirements for identifying such birthing quasars. If circumbinary disks have
a high disk surface density and viscosity, an all-sky soft X-ray survey with a
sensitivity of ~<3x10^-14 erg s^-1 cm^-2 and a time resolution of ~months could
identify dozens of birthing quasars with sustained brightening rates of
>10%/yr. If >1% of the X-ray emission is reprocessed into optical frequencies,
birthing quasars could also be identified in optical transient surveys such as
the LSST. Distinguishing a birthing quasar from other variable sources may be
facilitated by the monotonic hardening of its spectrum, but will likely remain
challenging. This reinforces the notion that joint EM-plus-GW observations
offer the best prospects for identifying the EM signatures of SMBHB mergers.Comment: 27 pages, 5 figures. Updated references. Accepted to the Astronomical
Journal, 140 (2010) 642-65
A secure archive for Voice-over-IP conversations
An efficient archive securing the integrity of VoIP-based two-party
conversations is presented. The solution is based on chains of hashes and
continuously chained electronic signatures. Security is concentrated in a
single, efficient component, allowing for a detailed analysis.Comment: 9 pages, 2 figures. (C) ACM, (2006). This is the author's version of
the work. It is posted here by permission of ACM for your personal use. Not
for redistribution. The definitive version was published in Proceedings of
VSW06, June, 2006, Berlin, German
Voters as a hard budget constraint: On the determination of intergovernmental grants
Recent empirical literature has shown that the determination of intergovernmental grants is highly influenced by the political bargaining power of the recipient states. In these models federal politicians are assumed to buy the support of state voters, state politicians and state interest groups by providing grants. In this paper we provide evidence that the fiscal referendum reduces the reliance of states on matching grants received from the central government and thus the possibility of state interest groups and state bureaucrats to obtain more grants. If referendums are available, voters serve as a hard budget constraint.Budget Referendums, Intergovernmental Grants, Interest Group Influence.
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