Calibration of multibeam echo sounders: a comparison between two methodologies

Multibeam echo sounders (MBES) are widely used in applications like seafloor imaging, fisheries, and habitat mapping. Calibration of acoustic backscatter is an important aspect of understanding and validating the performance of a MBES. Combined transmit/receive beampattern calibrations were performed on a 200 kHz Reson Seabat 7125 MBES in the acoustic tank of the University of New Hampshire utilizing two different methodologies. The first methodology employs fixed standard target spheres and a high accuracy/high resolution rotation mechanism. This method, similar to that proposed by Foote et al [ Protocols forcalibrating multibeam sonar , J. Acoust. Soc. Am. 117(4), 2005], is designed for a calibrationtank and provides accurate results but requires a large amount of operation time and cannot be performed in situ. The second methodology has been designed for field calibration of MBES. It employs a standard target sphere and a 200 kHz Simrad EK60 split-beam sonar system to provide athwartship and alongship angular information of the target sphere position. This method offers the possibility of field calibration for vessel mounted systems and a significantly reduced operation time, but has a potential reduction in accuracy. In this paper, results from these two methods applied to the same MBES are compared

A Method for Field Calibration of a Multibeam Echo Sounder

The use of multibeam echo sounders (MBES) has grown more frequent in applications like seafloor imaging, fisheries, and habitat mapping. Calibration of these instruments is important for understanding and validating the performance of MBES. For echo sounders in general, different calibration methodologies have been developed in controlled environments such as a fresh water tank and in the actual field of operation. While calibration in an indoor tank facility can bring excellent results in terms of accuracy, the amount of time required for a complete calibration can become prohibitively large. A field calibration can reveal the actual radiation beam pattern for shipmounted sonar systems, accounting for acoustic interferences which may be caused by objects around the installed transducers. The standard target method is a common practice for field calibration of split-beam echo sounders. However, when applied to a Mills Cross MBES, this method does not provide means to determine the alongship angle of the target, since the receiver transducer is a line array. A method to determine the combined transmit/receive radiation beam pattern for a ship-mounted multibeam system was developed and tested for a Reson Seabat 7125 MBES inside the fresh water calibration tank of the University of New Hampshire. This calibration methodology employs a tungsten carbide sphere of 38.1 mm diameter as target and a Simrad EK60 split-beam sonar system to provide athwartship and alongship angular information of the target sphere position. The multibeam sonar system was configured for 256 beams equi-angle mode at an operating frequency of 200 kHz; the split-beam system was set to work passively at the same frequency. A combined transmit/receive beam pattern was computed for an athwartship angular range between –6o and +6o and an alongship angular range between –1o and +3o . The limited angular range of the measurements is due to the –3 dB beamwidth of 7.1o in the athwartship and alongship directions of the split-beam sonar system coupled with the alongship offset of 1.6o between the maximum response axes (MRA) of the two sonar systems. Possible acoustic interferences caused by the monofilament line used to suspend the target sphere in the water column were found in the measurements for alongship angle values less than –1o . Beam pattern measurements for the combined transmit/receive beam pattern at a distance of 8 m show a –3 dB beamwidth of 1.1o in the athwartship direction and a –3 dB beamwidth of 2.0o in the alongship direction for the most inner beams. The dynamic range for the measurements was approximately of –40 dB

High Resolution Calibration of a Multibeam Echo Sounder

Calibration can greatly increase the utility of collecting seafloor backscattering strength with multibeam echo sounders (MBES). A calibration procedure to determine high resolution, three dimensional transmit and receive beam patterns of a Reson SeaBat 7125 MBES was performed using the fresh water calibration tank at the University of New Hampshire. The measurements employed an omni-directional Reson TC4034 projector/hydrophone as a reference transducer at an acoustic distance of 13 m. The multibeam sonar system was configured for 256 beams equi-angle mode and operating frequency of 396 kHz. The transmit beam pattern was computed for across-track angular range between +/–90o in 0.1o increments and along-track angular range between +/–2.55o in 0.09o increments. Receive directional characteristics were measured for the across-track range of +/–120o in 0.1o increments with along-track angular range of +/–1.23o in 0.18o increments. With these measurements, a three-dimensional plot of the transmit beam pattern was computed for the entire range of angles, although for the receive beam pattern it was only determined for the 0o along-track angle. The reduced angular coverage of the receive beam pattern was due to discrepancies in the off-axis results that are thought to be related to receiver saturation. Measurements for the transmit beam pattern show a –3 dB beamwidth of 0.99o for across-track angle of 0o with side lobes below –17 dB. The results show an unexplained curvature in the transmit beam pattern that would have gone un-noticed during a two dimensional beam pattern measurement. An interference effect causing ripples and attenuation of the transmit beam pattern was also observed. This effect is thought to be caused by a second 200 kHz projector mounted parallel to the 396 kHz projector. This interference occurs at an across-track angular region between +30o and +75o with gradual decrease in amplitude of about 3 dB. The receive beam pattern measurements show a –3 dB beamwidth of approximately 0.62o for the most inner beams (beams 128 and 129) and of approximately 1.40o for the most outer beams (beams 1 and 256) at along-track angular position of 0o with side-lobes below –26 dB for the most inner beams and below –17 dB for the most outer beams

Constraining the true nature of an exotic binary in the core of NGC 6624

We report on the identification of the optical counterpart to Star1, the exotic object serendipitously discovered by Deutsch et al. in the core of the Galactic globular cluster NGC 6624. Star1 has been classified by Deutsch et al. as either a quiescent Cataclysmic Variable or a low-mass X-ray binary. Deutsch et al. proposed StarA as possible optical counterpart to this object. We used high-resolution images obtained with the Hubble Space Telescope to perform a variability analysis of the stars close to the nominal position of Star1. While no variability was detected for StarA, we found another star, here named COM_Star1, showing a clear sinusoidal light modulation with amplitude \Delta m_F435W~0.7 mag and orbital period of P_orb~98 min. The shape of the light curve is likely caused by strong irradiation by the primary heating one hemisphere of the companion, thus suggesting a quite hot primary.Comment: Accepted for publication by ApJ Letters; 6 pages, 5 figure

The optical counterpart to the X-ray transient IGR J18245-2452 in the globular cluster M28

We report on the identification of the optical counterpart to the recently detected INTEGRAL transient IGR J18245-2452 in the Galactic globular cluster M28. From the analysis of a multi epoch HST dataset we have identified a strongly variable star positionally coincident with the radio and Chandra X-ray sources associated to the INTEGRAL transient. The star has been detected during both a quiescent and an outburst state. In the former case it appears as a faint, unperturbed main sequence star, while in the latter state it is about two magnitudes brighter and slightly bluer than main sequence stars. We also detected Halpha excess during the outburst state, suggestive of active accretion processes by the neutron star.Comment: Accepted for publication by ApJ; 15 pages, 4 figures, 1 tabl

The Optical Counterpart to the Accreting Millisecond X-ray Pulsar SAX J1748.9-2021 in the Globular Cluster NGC 6440

We used a combination of deep optical and Halpha images of the Galactic globular cluster NGC 6440, acquired with the Hubble Space Telescope, to identify the optical counterpart to the accreting millisecond X-ray pulsar SAX J1748.9-2021during quiescence. A strong Halpha emission has been detected from a main sequence star (hereafter COM-SAX J1748.9-2021) located at only 0.15" from the nominal position of the X-ray source. The position of the star also agrees with the optical counterpart found by Verbunt et al. (2000) during an outburst. We propose this star as the most likely optical counterpart to the binary system. By direct comparison with isochrones, we estimated that COM-SAX J1748.9-2021 has a mass of 0.70 Msun - 0.83 Msun, a radius of 0.88 pm 0.02 Rsun and a superficial temperature of 5250pm80 K. These parameters combined with the orbital characteristics of the binary suggest that the system is observed at a very low inclination angle (~8 deg -14 deg) and that the star is filling or even overflowing its Roche Lobe. This, together with the equivalent width of the Halpha emission (~20 Ang), suggest possible on-going mass transfer. The possibile presence of such a on-going mass transfer during a quiescence state also suggests that the radio pulsar is not active yet and thus this system, despite its similarity with the class of redback millisecond pulsars, is not a transitional millisecond pulsar.Comment: 8 pages, 6 figures. Accepted for publication in Ap

Optical Identification of He White Dwarfs Orbiting Four Millisecond Pulsars in the Globular Cluster 47 Tucanae

We used ultra-deep UV observations obtained with the Hubble Space Telescope to search for optical companions to binary millisecond pulsars (MSPs) in the globular cluster 47 Tucanae. We identified four new counterparts (to MSPs 47TucQ, 47TucS, 47TucT and 47TucY) and confirmed those already known (to MSPs 47TucU and 47TucW). In the color magnitude diagram, the detected companions are located in a region between the main sequence and the CO white dwarf cooling sequences, consistent with the cooling tracks of He white dwarfs of mass between 0.15 Msun and 0.20 Msun. For each identified companion, mass, cooling age, temperature and pulsar mass (as a function of the inclination angle) have been derived and discussed. For 47TucU we also found that the past accretion history likely proceeded in a sub-Eddington rate. The companion to the redback 47TucW is confirmed to be a non degenerate star, with properties particularly similar to those observed for black widow systems. Two stars have been identified within the 2-sigma astrometric uncertainty from the radio positions of 47TucH and 47TucI, but the available data prevent us from firmly assessing whether they are the true companions of these two MSPs.Comment: 27 pages, 7 figures, Accepted for publication by Ap

Probing the MSP prenatal stage: the optical identification of the X-ray burster EXO 1745-248 in Terzan 5

We report on the optical identification of the neutron star burster EXO 1745-248 in Terzan 5. The identification was performed by exploiting HST/ACS images acquired in Director's Discretionary Time shortly after (approximately 1 month) the Swift detection of the X-ray burst. The comparison between these images and previous archival data revealed the presence of a star that currently brightened by ~3 magnitudes, consistent with expectations during an X-ray outburst. The centroid of this object well agrees with the position, in the archival images, of a star located in the Turn-Off/Sub Giant Branch region of Terzan 5. This supports the scenario that the companion should has recently filled its Roche Lobe. Such a system represents the pre-natal stage of a millisecond pulsar, an evolutionary phase during which heavy mass accretion on the compact object occurs, thus producing X-ray outbursts and re-accelerating the neutron star.Comment: ApJ Letter, in pres

The optical companion to the intermediate mass millisecond pulsar J1439-5501 in the Galactic field

We present the identification of the companion star to the intermediate mass binary pulsar J1439-5501 obtained by means of ground-based deep images in the B, V and I bands, acquired with FORS2 mounted at the ESO-VLT. The companion is a massive white dwarf (WD) with B=23.57+-0.02, V=23.21+-0.01 and I=22.96+-0.01, located at only ~0.05" from the pulsar radio position. Comparing the WD location in the (B, B-V) and (V, V-I) Color-Magnitude diagrams with theoretical cooling sequences we derived a range of plausible combinations of companion masses (1<~Mcom<~1.3 Msun), distances (d<~1200 pc), radii (<~7.8 10^3 Rsun) and temperatures (T=31350^{+21500}_{-7400}). From the PSR mass function and the estimated mass range we also constrained the inclination angle i >~ 55 degrees and the pulsar mass (Mpsr <~2.2 Msun). The comparison between the WD cooling age and the spin down age suggests that the latter is overestimated by a factor of about ten.Comment: Accepted for publication by ApJ; 19 pages, 5 figures, 1 tabl

Radio timing and optical photometry of the black widow system PSR J1518+0204C in the globular cluster M5

We report on the determination of astrometric, spin and orbital parameters for PSR J1518+0204C, a "black widow" binary millisecond pulsar in the globular cluster M5. The accurate position and orbital parameters obtained from radio timing allowed us to search for the optical companion. By using WFC3/HST images we identified a very faint variable star (m_F390W > 24.8, m_F606W > 24.3, m_F814W > 23.1) located at only 0.25" from the pulsar's timing position. Due to its strong variability, this star is visible only in a sub-sample of images. However, the light curve obtained folding the available data with the orbital parameters of the pulsar shows a maximum at the pulsar inferior conjunction and a possible minimum at the pulsar superior conjunction. Furthermore, the shape of the optical modulation indicates a heating process possibly due to the pulsar wind. This is the first identification of an optical companion to a black widow pulsar in the dense stellar environment of a globular cluster.Comment: Accepted for publication by ApJ; 24 Pages, 5 Figures, 1 Tabl