Rotating black hole orbit functionals in the frequency domain

In many astrophysical problems, it is important to understand the behavior of functions that come from rotating (Kerr) black hole orbits. It can be particularly useful to work with the frequency domain representation of those functions, in order to bring out their harmonic dependence upon the fundamental orbital frequencies of Kerr black holes. Although, as has recently been shown by W. Schmidt, such a frequency domain representation must exist, the coupled nature of a black hole orbit's $r$ and $\theta$ motions makes it difficult to construct such a representation in practice. Combining Schmidt's description with a clever choice of timelike coordinate suggested by Y. Mino, we have developed a simple procedure that sidesteps this difficulty. One first Fourier expands all quantities using Mino's time coordinate $\lambda$. In particular, the observer's time $t$ is decomposed with $\lambda$. The frequency domain description is then built from the $\lambda$-Fourier expansion and the expansion of $t$. We have found this procedure to be quite simple to implement, and to be applicable to a wide class of functionals. We test the procedure using a simple test function, and then apply it in a particularly interesting case, the Weyl curvature scalar $\psi_4$ used in black hole perturbation theory.Comment: 16 pages, 2 figures. Submitted to Phys Rev D. New version gives a vastly improved algorithm due to Drasco for computing the Fourier transforms. Drasco has been added as an author. Also fixed some references and exterminated a small herd of typos; final published versio

Towards a formalism for mapping the spacetimes of massive compact objects: Bumpy black holes and their orbits

Observations have established that extremely compact, massive objects are common in the universe. It is generally accepted that these objects are black holes. As observations improve, it becomes possible to test this hypothesis in ever greater detail. In particular, it is or will be possible to measure the properties of orbits deep in the strong field of a black hole candidate (using x-ray timing or with gravitational-waves) and to test whether they have the characteristics of black hole orbits in general relativity. Such measurements can be used to map the spacetime of a massive compact object, testing whether the object's multipoles satisfy the strict constraints of the black hole hypothesis. Such a test requires that we compare against objects with the ``wrong'' multipole structure. In this paper, we present tools for constructing bumpy black holes: objects that are almost black holes, but that have some multipoles with the wrong value. The spacetimes which we present are good deep into the strong field of the object -- we do not use a large r expansion, except to make contact with weak field intuition. Also, our spacetimes reduce to the black hole spacetimes of general relativity when the ``bumpiness'' is set to zero. We propose bumpy black holes as the foundation for a null experiment: if black hole candidates are the black holes of general relativity, their bumpiness should be zero. By comparing orbits in a bumpy spacetime with those of an astrophysical source, observations should be able to test this hypothesis, stringently testing whether they are the black holes of general relativity. (Abridged)Comment: 16 pages + 2 appendices + 3 figures. Submitted to PR

N-body simulations of gravitational dynamics

We describe the astrophysical and numerical basis of N-body simulations, both of collisional stellar systems (dense star clusters and galactic centres) and collisionless stellar dynamics (galaxies and large-scale structure). We explain and discuss the state-of-the-art algorithms used for these quite different regimes, attempt to give a fair critique, and point out possible directions of future improvement and development. We briefly touch upon the history of N-body simulations and their most important results.Comment: invited review (28 pages), to appear in European Physics Journal Plu

Optical/IR Confirmation of Outburst of Aql X-1

Ongoing optical/IR monitoring of Aql X-1 with the SMARTS 1.3m telescope and ANDICAM instrument at CTIO show a steady rise over the past week

Increased radio and optical activity in GX 339-4

Recent optical and radio observations of the black hole X-ray binary jet source GX 339-4 reveal it to be in a brighter optical and radio state since observations were last made in 2003. On UT 2004 Feb 05.34, observations made with the 1.3m CTIO telescope using ANDICAM yielded V=17.7, compared to UT 2003 Oct 27.00 when V=19.5. Radio observations with the Australia Telescope Compact Array measure a flux density at 8.2 GHz of ~0.6 mJy on 2004 Feb 4/5, compared to an upper limit of <0.1 mJy on 2003 Dec 22 (see also ATEL #196)

X-ray Nova XTE J1550-564: optical observations

We report the identification of the optical counterpart of the X-ray transient XTE J1550-564 described in two companion papers by Sobczak et al (1999) and Remillard et al (1999). We find that the optical source brightened by approximately 4 magnitudes over the quiescent counterpart seen at B~22 on a SERC survey plate, and then decayed by approximately 1.5 magnitudes over the 7 week long observation period. There was an optical response to the large X-ray flare described by Sobczak et al (1999), but it was much smaller and delayed by roughly 1 day.Comment: 9 pages, 2 figures. Accepted for publication in ApJ Letter

Optical monitoring of GM Sgr and discovery of a Mira and a short-period pulsator

This Note is the third in a series of Notes where we report new variable stars discovered during our extensive CCD monitoring program of the black hole binary V 4641 Sgr (SAX J1819.3-2525, Orosz et al. 2001). In this Note we report on the discovery of a Mira and a short-period pulsating star. We also give the light curves of GM Sgr, a previously known Mira (Orosz 2000, Kato et al. 2001)

Optical and Near-IR Observations of the Current Outburst of Aql X-1

As part of a monitoring campaign of quiescent and outbursting low-mass X-ray binaries with the CTIO/SMARTS telescopes and the Faulkes telescopes , we report observations of Aql X-1 and confirm the optical counterpart of the new X-ray outburst (ATel #1216). On 2007 Aug 30 (MJD 54342.26) we performed optical observations of Aql X-1 for 100 sec each in V and i' filters using the Faulkes Telescope North in V and i'-bands and detected the source with a confidence of 20 and 63 sigma in V and i', respectively

NGC 300 ULX1: spin evolution, super-Eddington accretion and outflows

International audienceNGC 300 ULX1 is an ultraluminous X-ray pulsar, showing an unprecedented spin evolution, from about 126 s to less than 20 s in only 4 yr, consistent with steady mass accretion rate. Following its discovery we have been monitoring the system with Swift and NICER to further study its properties. We found that even though the observed flux of the system dropped by a factor of ≳20, the spin-up rate remained almost constant. A possible explanation is that the decrease in the observed flux is a result of increased absorption of obscuring material due to outflows or a precessing accretion disc

An improvement of the position of IGR J17191-2821

We observed the transient IGR J17191-2821 (Atels #1021,#1022,#1025,#1065,#1068,#1069) again for 1.8 ksec on May 07 (18:30 UTC) with Swift/XRT in the imaging mode (Photon-Counting mode). The source is heavily piled-up but a position could be obtained: (RA, DEC)=(259.81306, -28.29919) (J2000) with an error of ~4 arcsec. This improved position is offset from the original Integral position (Atel #1021) by about 3.8 arcminutes, but is consistent with the variable faint source detected in our Swift/XRT observation taken on March 8 (Atel #1025)