James Rodi

When A Standard Candle Flickers

The Crab Nebula is the only hard X-ray source in the sky that is both bright enough and steady enough to be easily used as a standard candle. As a result, it has been used as a normalization standard by most X-ray/gamma-ray telescopes. Although small-scale variations in the nebula are well known, since the start of science operations of the Fermi Gamma-ray Burst Monitor (GBM) in 2008 August, a ~7% (70 mCrab) decline has been observed in the overall Crab Nebula flux in the 15-50 keV band, measured with the Earth occultation technique. This decline is independently confirmed in the ~15-50 keV band with three other instruments: the Swift Burst Alert Telescope (Swift/BAT), the Rossi X-ray Timing Explorer Proportional Counter Array (RXTE/PCA), and the Imager on-Board the INTEGRAL Satellite (IBIS). A similar decline is also observed in the ~3-15 keV data from the RXTE/PCA and in the 50-100 keV band with GBM, Swift/BAT, and INTEGRAL/IBIS. The pulsed flux measured with RXTE/PCA since 1999 is consistent with the pulsar spin-down, indicating that the observed changes are nebular. Correlated variations in the Crab Nebula flux on a ~3 year timescale are also seen independently with the PCA, BAT, and IBIS from 2005 to 2008, with a flux minimum in 2007 April. As of 2010 August, the current flux has declined below the 2007 minimum.

THREE YEARS OF FERMI GBM EARTH OCCULTATION MONITORING: OBSERVATIONS OF HARD X-RAY/SOFT GAMMA-RAY SOURCES

The Gamma-ray Burst Monitor (GBM) on board Fermi has been providing continuous data to the astronomical community since 2008 August 12. In this paper, we present the results of the analysis of the first three years of these continuous data using the Earth occultation technique to monitor a catalog of 209 sources. From this catalog, we detect 99 sources, including 40 low-mass X-ray binary/neutron star systems, 31 high-mass X-ray binary/neutron star systems, 12 black hole binaries, 12 active galaxies, and 2 other sources, plus the Crab Nebula, and the Sun. Nine of these sources are detected in the 100-300 keV band, including seven black hole binaries, the active galaxy Cen A, and the Crab. The Crab and Cyg X-1 are also detected in the 300-500 keV band. GBM provides complementary data to other sky-monitors below 100 keV and is the only all-sky monitor above 100 keV. Up-to-date light curves for all of the catalog sources can be found online.

FIRST RESULTS FROM FERMI GAMMA-RAY BURST MONITOR EARTH OCCULTATION MONITORING: OBSERVATIONS OF SOFT GAMMA-RAY SOURCES ABOVE 100 keV

The NaI and BGO detectors on the Gamma-ray Burst Monitor (GBM) on Fermi are now being used for long-term monitoring of the hard X-ray/low-energy gamma-ray sky. Using the Earth occultation technique as demonstrated previously by the BATSE instrument on the Compton Gamma-Ray Observatory, GBM can be used to produce multiband light curves and spectra for known sources and transient outbursts in the 8 keV to 1 MeV energy range with its NaI detectors and up to 40 MeV with its BGO detectors. Over 85% of the sky is viewed every orbit, and the precession of the Fermi orbit allows the entire sky to be viewed every ~26 days with sensitivity exceeding that of BATSE at energies below ~25 keV and above ~1.5 MeV. We briefly describe the technique and present preliminary results using the NaI detectors after the first two years of observations at energies above 100 keV. Eight sources are detected with a significance greater than 7σ: the Crab, Cyg X-1, SWIFT J1753.5–0127, 1E 1740-29, Cen A, GRS 1915+105, and the transient sources XTE J1752–223 and GX 339-4. Two of the sources, the Crab and Cyg X-1, have also been detected above 300 keV.

Observations of Gamma-Ray Outbursts from Galactic Microquasars

The BATSE earth-occultation database provides nine years of coverage for 75 γ-ray sources in the energy range 35–1700 keV. For transient sources, this long time-base dataset makes it possible to study the repeated outbursts from individual objects. We have used the JPL data analysis package EBOP (Enhanced BATSE Occultation Package) to derive the light curves and the time evolution of the spectra for the black hole candidate and microquasar sources GRO J1655–40 and GRS 1915+105.We find that GRO J1655–40, during high-intensity flaring periods, is characterized by a single power-law spectrum up to 500 keV with a spectral index consistent with that observed by OSSE. During one flare observed contemporaneously with OSSE and HEXTE, the GRO J1655–40 spectrum was observed to steepen as the γ-ray intensity increased. For GRS 1915+105, the spectrum during high intensity flaring periods can be characterized by a broken power law with a time-varying high-energy component. The spectra of these microquasars differ from the black hole candidates Cygnus X-1, GRO J0422+32, and GRO J1719–24, which have thermal contributions to their spectra when in high γ-ray states. This suggests that there may be two different classes of Galactic black hole candidates.

Hard X-ray Tail Discovered in the Clocked Burster gs 1826–238

The low-mass X-ray binary (LMXB) neutron star (NS) GS 1826–238 was discovered by Ginga in 1988 September. Due to the presence of quasi-periodicity in the type I X-ray burst rate, the source has been a frequent target of X-ray observations for almost 30 years. Though the bursts were too soft to be detected by INTEGRAL/SPI, the persistent emission from GS 1826–238 was detected over 150 keV during the ~10 years of observations. Spectral analysis found a significant high-energy excess above a Comptonization model that is well fit by a power law, indicating an additional spectral component. Most previously reported spectra with hard tails in LMXB NS have had an electron temperature of a few keV and a hard tail dominating above ~50 keV with an index of Γ ~ 2–3. GS 1826–238 was found to have a markedly different spectrum with kTe ~ 20 keV and a hard tail dominating above ~150 keV with an index of Γ ~ 1.8, more similar to black hole X-ray binaries. We report on our search for long-term spectral variability over the 25–370 keV energy range and on a comparison of the GS 1826–238 average spectrum to the spectra of other LMXB NSs with hard tails.

INTEGRAL Observations of Gravitational-Wave Counterparts & Future Perspectives: Searching for GBM Un-Triggered SGRB with PICsIT

The X-ray/gamma-ray mission \emph{INTEGRAL} detected the short GRB170817A and demonstrated its association to a gravitational wave trigger, GW170817. This marks the first time a binary neutron star merger was detected by the LIGO-Virgo collaboration and that an electromagnetic counterpart to a gravitational wave event has been observed. GRB170817A was detected by the SPI-ACS on-board \emph{INTEGRAL} and the \emph{Fermi}/GBM instruments \( \sim 1.7\) s after the GW event. Following the prompt emission, \emph{INTEGRAL} performed pointed observations for 5.4 days. During this time the instruments provided stringent upper limits on any electromagnetic signal in the 3 keV to 8 MeV range. Interestingly, the GRB was found to be extremely subluminous.In light of these results from GRB170817A, we have begun analysis of soft gamma-ray data (\(200 \textrm{ keV } - 2.6 \textrm{ MeV }\)) from \emph{INTEGRAL}/PICsIT. With this wide field-of-view instrument, we have begun searching for untriggered SGRBs reported by \emph{Fermi}/GBM as well as preparing for real-time analysis during future LIGO-Virgo observing runs.

INTEGRAL follow-up of the gravitational wave events

We use observations of the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) to search for gamma-ray and hard X-ray emission associated with the gravitational wave events discovered during the first and the second scientific runs of Advanced LIGO and Advanced Virgo. The highly eccentric orbit of INTEGRAL ensures high duty cycle, long-term stable background, and unobstructed view of nearly the entire sky. This enables us to use a combination of INTEGRAL instruments (SPectrometer onboard INTEGRAL - Anti-Coincidence Shield (SPI-ACS), Imager on Board the INTEGRAL Satellite (IBIS), and IBIS/Veto) to search for a hard X-ray electromagnetic signal in the full high-probability sky region for almost every single LIGO trigger. INTEGRAL observations of the binary black hole (BBH) mergers GW150914, LVT151012, GW170104, and GW170814 allowes to constrain the fraction of the energy promptly released in gamma-rays in 75 keV - 2 MeV energy range in the direction of the observer down to as little as one millionth of the gravitational wave energy, in the majority of the localization region. Moreover, in the case of LVT151012 INTEGRAL high-energy imaging instruments, IBIS, SPectrometer onboard INTEGRAL (SPI), and Joint European X-Ray Monitor (JEM-X), provided the unique opportunity to search also for long-lasting electromagnetic counterparts of this event over 3 decades in energy, from 5 keV to 8 MeV. Finally, we discuss the INTEGRAL detection of the short gamma-ray burst GRB 170817A (discovered by Fermi-Gamma-ray Burst Monitor (GBM)) with a signal-to-noise ratio of 4.6, and, for the first time, its association with the gravitational waves (GWs) from binary neutron star (BNS) merging event GW170817 detected by the LIGO and Virgo observatories. The significance of association between the gamma-ray burst observed by INTEGRAL and GW170817 is 3.2 σ, while the association between the Fermi-GBM and INTEGRAL detections is 4.2 σ. GRB 170817A was detected by the SPI-ACS instrument about 2 s after the end of the gravitational wave event. We measure a fluence of 1.4±0.4±0.6×10−7 erg cm−2 (75–2000 keV), where, respectively, the statistical error is given at the 1 σ confidence level, and the systematic error corresponds to the uncertainty in the spectral model and instrument response.

Observations of V0332+53 during the 2015 outburst using Fermi/GBM, MAXI, Swift and INTEGRAL

We present the lightcurves, spectra, and hardness-intensity diagram (HID) of the high mass X-ray binary V0332+53 using Fermi/GBM, MAXI, Swift/BAT, and INTEGRAL through its 2015 Type II outburst. We observe characteristic features in the X-ray emission (2-50 keV) due to periastron passages, the dynamical timescale of the accretion disc, and changes within the accretion column between a radiation-dominated flow and a flow dominated by Coulomb interactions. Based on the HID and the light curves, the critical luminosity is observed to decrease by ~5-7 percent during the outburst, signaling a decrease in the magnetic field.

LONG-TERM MONITORING OF THE BLACK HOLE CANDIDATE SWIFT J1753.5–0127 WITH INTEGRAL/SPI

The black hole candidate (BHC) Swift J1753.5–0127 went into outburst in 2005 June. Rather than fade into quiescence as most BHC transients do, it has remained in a low hard spectral state for most of the ~9 years after its outburst. Its persistent emission during the hard state is reminiscent of the black hole Cyg X-1 and the BHCs 1E 1740.7–2942 and GRS 1758–258. Thus far, hard X-ray/soft gamma-ray results have mainly focused on the 2005 flare, with a few additional observations during 2007. Here, we present results from INTEGRAL/SPI observations from 2005–2010 spanning the 22–650 keV energy range. Spectral analysis shows a weak high-energy excess () above a cutoff power-law model that is well fit by a power law, suggesting an additional spectral component. Observations of Cyg X-1, 1E 1740.7–2942, and GRS 1758–258 have shown similar spectra requiring an additional high-energy component. The SPI results are compared to previously reported results for Swift J1753.5–0127 as well as observations of other sources.

Earth occultation imaging of the low energy gamma-ray sky with GBM

The Earth Occultation Technique (EOT) has been applied to Fermis Gamma-ray Burst Monitor (GBM) to perform all-sky monitoring for a predetermined catalog of hard X-ray/soft gamma-ray sources. In order to search for sources not in the catalog, thus completing the catalog and reducing a source of systematic error in EOT, an imaging method has been developed -- Imaging with a Differential filter using the Earth Occultation Method (IDEOM). IDEOM is a tomographic imaging method that takes advantage of the orbital precession of the Fermi satellite. Using IDEOM, all-sky reconstructions have been generated for ~sim 4 years of GBM data in the 12-50 keV, 50-100 keV and 100-300 keV energy bands in search of sources otherwise unmodeled by the GBM occultation analysis. IDEOM analysis resulted in the detection of 57 sources in the 12-50 keV energy band, 23 sources in the 50-100 keV energy band, and 7 sources in the 100-300 keV energy band. Seventeen sources were not present in the original GBM-EOT catalog and have now been added. We also present the first joined averaged spectra for four persistent sources detected by GBM using EOT and by the Large Area Telescope (LAT) on Fermi: NGC 1275, 3C 273, Cen A, and the Crab.