First Results from Fermi GBM Earth Occultation Monitoring: Observations of Soft Gamma-Ray Sources Above 100 keV
Gary L. Case, Michael L. Cherry, James C. Rodi, Peter Jenke, Colleen A. Wilson-Hodge, Mark H. Finger, Charles A. Meegan, Ascencion Camero-Arranz, Elif Beklen, P. Narayan Bhat, Michael S. Briggs, Vandiver Chaplin, Valerie Connaughton, William S. Paciesas, Robert Preece, R. Marc Kippen, Andreas von Kienlin, Jochen Griener
aa r X i v : . [ a s t r o - ph . H E ] S e p First Results from
Fermi
GBM Earth Occultation Monitoring:Observations of Soft Gamma-Ray Sources Above 100 keV
G. L. Case, M. L. Cherry, J. C. Rodi
Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803, USA [email protected]
P. Jenke , C. A. Wilson-Hodge Marshall Space Flight Center, Huntsville, AL 35182, USA
M. H. Finger, C. A. Meegan
Universities Space Research Association, Huntsville, AL 35805, USA
A. Camero-Arranz
National Space Science and Technology Center, Huntsville, AL 35805, USA
E. Beklen
Physics Department, Middle East Technical University, 06531 Ankara, TurkeyPhysics Department, S¨uleyman Demirel University, 32260 Isparta, Turkey
P. N. Bhat, M. S. Briggs, V. Chaplin, V. Connaughton, W. S. Paciesas, R. Preece
University of Alabama in Huntsville, Huntsville, AL 35899, USA
R. M. Kippen
Los Alamos National Laboratory, Los Alamos, NM 87545 and
A. von Kienlin, J. Greiner
Max-Planck Institut f¨ur Extraterrestische Physik, 85748 Garching, Germany
ABSTRACT
The NaI and BGO detectors on the Gamma-ray Burst Monitor (GBM) on
Fermi are now beingused for long-term monitoring of the hard X-ray/low energy gamma-ray sky. Using the Earthoccultation technique as demonstrated previously by the BATSE instrument on the
Compton NASA Postdoctoral Program Fellow amma-Ray Observatory , GBM can be used to produce multiband light curves and spectra forknown sources and transient outbursts in the 8 keV to 1 MeV energy range with its NaI detectorsand up to 40 MeV with its BGO detectors. Over 85% of the sky is viewed every orbit, and theprecession of the Fermi orbit allows the entire sky to be viewed every ∼
26 days with sensitivityexceeding that of BATSE at energies below ∼
25 keV and above ∼ . σ : the Crab, Cyg X-1, SWIFT J1753.5-0127, 1E 1740-29, Cen A, GRS 1915+105, and thetransient 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. Subject headings: gamma rays: individual (1E 1740-29, Cen A, Crab, Cyg X-1, GRS 1915+105, GX339-4, Swift J1753.5-0127, XTE J1752-223) — gamma rays: observations
1. Introduction
The ability to monitor the gamma-ray sky con-tinuously is extremely important. The majority ofgamma-ray sources are variable, exhibiting flaresand transient outbursts on time scales from sec-onds to years. While there are currently sev-eral all-sky monitors in the hard X-ray energyrange providing daily light curves, e.g. the All-Sky Monitor (ASM) on the
Rossi X-ray TimingExplorer ( RXTE ) from 2–10 keV (Levine et al.1996), the Gas Slit Camera (GSC) on the
Mon-itor of All-sky X-ray Image ( MAXI ) from 1.5–20 keV (Matsuoka et al. 2009), and the BurstAlert Telescope (BAT) on
Swift from 15–50 keV(Gehrels et al. 2004), there has not been an all-sky monitor in the low energy gamma-ray regionsince the Burst and Transient Source Experiment(BATSE) instrument on the
Compton Gamma-Ray Observatory ( CGRO ), which was sensitivefrom 20–1800 keV (Fishman et al. 1989).The gamma-ray satellite
Fermi was launched on2008 June 11 and commenced science operationson 2008 August 12.
Fermi contains two instru-ments: the Large Area Telescope (LAT), sensitiveto gamma rays from ∼
20 MeV to ∼
300 GeV(Atwood et al. 2009); and the Gamma-ray BurstMonitor (GBM), which is sensitive to X-rays andgamma rays from 8 keV to 40 MeV (Meegan et al.2009). With its wide field of view, GBM can beused to provide nearly continuous full-sky coveragein the hard X-ray/soft gamma-ray energy range.It is the only instrument currently in orbit that canperform all-sky monitoring above 100 keV (but be-low the 30 MeV threshold of the
Fermi
LAT) withuseable sensitivity. The
Swift /BAT sensitivitydrops off rapidly above 100 keV, and its energy range effectively ends at 195 keV.
INTEGRAL ,which has a relatively narrow field of view, cannotmake continuous observations of a large number ofindividual sources. Also, GBM is not limited bysolar pointing constraints, as are most other in-struments, which allows the monitoring of sourcesat times during which other instruments cannot.The Earth occultation technique, used very suc-cessfully with BATSE, has been adapted to GBMto obtain fluxes for an input catalog of knownor potential sources. A catalog of 82 sources iscurrently being monitored and regularly updated.This catalog contains predominantly Galactic x-ray binaries, but also includes the Crab, the Sun,two magnetars, five active galactic nulcei (AGN),and two cataclysmic variables. At energies above100 keV, six persistent sources (the Crab, CygX-1, SWIFT J1753.5-0127, 1E 1740-29, Cen A,GRS 1915+105) and two transient sources (GX339-4 and XTE J1752-223) have been detected inthe first two years of observations. In section 2,we briefly describe the GBM instrument and out-line the Earth occultation technique as applied toGBM, in section 3 we present the light curves forthe eight sources, and in section 4 we discuss theresults, the GBM capabilities and future work.
2. GBM and the Earth Occultation Tech-nique
GBM consists of 14 detectors: 12 NaI detec-tors, each 12.7 cm in diameter and 1.27 cm thick;and two BGO detectors, 12.7 cm in diameter and12.7 cm thick. The NaI detectors are located onthe corners of the spacecraft, with six detectorsoriented such that the normals to their faces areperpendicular to the z-axis of the spacecraft (the2ig. 1.— Single Crab occultation step seen in theCTIME raw counts data of a single GBM NaI de-tector (NaI 2) in the 12–25 keV band with 2.048-second time bins. The Crab was 4 . ◦ from thenormal to the detector. The time window is cen-tered on the calculated occultation time for 100keV.LAT is pointed in the + z -direction), four detec-tors pointed at 45 ◦ from the z-axis, and 2 detec-tors pointed 20 ◦ off the z-axis. Together, these 12detectors provide nearly uniform coverage of theunocculted sky in the energy range from 8 keV to1 MeV. The two BGO detectors are located on op-posite sides of the spacecraft and view a large partof the sky in the energy range ∼
150 keV to ∼ β , is defined as the elevation angle of thesource being occulted with respect to the planeof the Fermi orbit. The transmission through the atmosphere as a function of time is modeledas T ( t ) = exp[ − µ ( E ) A ( h )], where µ ( E ) is themass attenuation coefficient of gamma rays at en-ergy E in air and A ( h ) is the air mass along theline of sight at a given altitude h ( t ) based on theUS Standard Atmosphere (1976). This requiresinstantaneous knowledge of the spacecraft posi-tion, the direction to the source of interest as seenfrom the spacecraft, and a model of the Earththat includes its oblateness. Fermi was launchedinto a i = 25 . ◦ inclination orbit at an altitudeof 555 km. The orbital period is 96 minutes, andindividual occultation steps last for ∼ / cos β sec-onds. Figure 1 shows a single step due to a Craboccultation in the count rate in the 12–25 keVband of a single GBM NaI detector observing theoccultation nearly face-on.The diameter of the Earth as seen from Fermi is ≈ ◦ , so roughly 30% of the sky is occultedby the Earth at any one time. One complete orbitof the spacecraft allows over 85% of the sky to beobserved. The precession of the orbital plane al-lows the entire sky to be occulted every ∼
26 days(half the precession period for the
Fermi orbit),though the exposure is not uniform.The Earth occultation technique was devel-oped for BATSE using two separate approaches,one at the NASA Marshall Space Flight Center(Harmon et al. 2002) and the other at the NASAJet Propulsion Laboratory (Ling et al. 2000). ForGBM, we follow the Harmon et al. (2002) ap-proach.The primary difference in the implementationof the occultation technique between GBM andBATSE arises from the different pointing schemesof the respective missions.
CGRO was three-axisstabilized for each viewing period, which typicallylasted for two weeks. This meant that a sourceremained at a fixed orientation with respect tothe detectors through an entire viewing period. Incontrast,
Fermi scans the sky by pointing in a di-rection 35 ◦ (August 2008–September 2009) or 50 ◦ (October 2009–present) north of the zenith for oneorbit; it then rocks to 35 ◦ or 50 ◦ south for the nextorbit, continuing to alternate every orbit unlessthe spacecraft goes into a pointed mode (which oc-curs rarely). In addition, the spacecraft performsa roll about the z-axis as it orbits. Because theorientation of a source with respect to the GBMdetectors varies as a function of time, the detec-3or response as a function of angle must be ac-counted for. A detailed instrument modeling andmeasurement program has been used to developthe GBM instrument response as a function of di-rection (Hoover et al. 2008; Bissaldi et al. 2009),which is incorporated into the occultation analy-sis. It should be noted that the GBM occultationsensitivity exceeds that of BATSE at energies be-low ∼
25 keV and above ∼ . T ( t ). Foreach occultation step, a 4-minute window is de-fined that is centered on the 100 keV occultationtime. For each energy band, the count rates inthe window are fit separately for each detectorviewing the source of interest. In each of these de-tectors, the count rates are fitted with a quadraticbackground plus source models for the source ofinterest and each interfering source that appearsin the window. The source models consist of T ( t )and a time-dependent model count rate, derived from the time-dependent detector response con-volved with an assumed source spectrum. Eachsource model is multiplied by a scaling factor, andthe source flux is then computed by a joint fit tothe scaling factors across all detectors in the fit.The best-fit scaling factor is then multiplied bythe assumed source flux model integrated over theenergy band to obtain the photon flux.Up to 31 occultation steps are possible for agiven source in a day, and these steps are summedto get a single daily average flux. This techniquecan be used with either the NaI or BGO detectors,though the analysis presented here uses only theNaI detectors. A more complete description of theGBM implementation of the occultation techniquewill be given in Wilson-Hodge et al. (2010).
3. Results
In Wilson-Hodge et al. (2009a), the measuredGBM 12–50 keV light curves are compared tothe
Swift
BAT 15–50 keV light curves for severalsources over the same time intervals, and it is seenthat the fluxes measured by the two instrumentscompare well. At energies above the ∼
195 keVupper energy limit of the
Swift σ after two yearsof observations, as well as two transient sources.Table 1 gives the fluxes averaged over all 730days from 2008 August 12 (MJD 54690, the be-ginning of science operations) to 2010 August11 (MJD 55419) for the persistent sources, andover all of the days of the flares for the transientsources. Also given are the significances for eachenergy band. The errors are statistical only. Thesources are sorted by their detection significancein the 100–300 keV band. The six persistent sources Crab, Cyg X-1, CenA, GRS 1915+105, 1E 1740-29, and Swift J1753.5-0127 are detected by GBM at energies above 100keV. In Figures 2–7 we show light curves for thesesources generated from the GBM data in sev-eral broad energy bands with five-day resolution.4 able 1Fluxes and Significances in GBM Broad High Energy Bands σ ) (mCrab) (mCrab) ( σ ) (mCrab) (mCrab) ( σ )Cyg X-1 1151.0 3.7 312 1130.7 6.9 163 529.0 49.5 10.7Crab 1000.0 3.3 307 1000.0 6.3 158 1000.0 48.0 20.9XTE J1752-223 a < . c . . . . . .SWIFT 1753.5-0127 121.0 4.4 28 126.8 8.2 15 104.5 62.0 1.71E 1740-29 116.3 4.7 25 92.3 8.8 11 126.8 65.0 2.0GRS 1915+105 128.1 3.6 35 54.9 6.8 8.0 < . c . . . . . .GX 339-4 b < . c . . . . . . a Fluxes are given for MJD 55129-55218 when XTE J1752-223 was flaring. b Fluxes are given for MJD 55244-55289 when GX 339-4 was flaring. c σ upper limit. These persistent sources demonstrate the capabil-ities of the GBM Earth occultation monitoring.
The Crab emission in the hard X-ray/low en-ergy gamma-ray regime contains a combinationof pulsar and pulsar wind nebula contributions.Figure 2 shows the light curves measured byGBM in four broad energy bands from 12 keVup to 500 keV. The spectrum in this regimehas been shown by analysis of BATSE occul-tation data (Much et al. 1996; Ling & Wheaton2003b) and data from SPI on board
INTEGRAL (Jourdain & Roques 2009) to agree with the spec-trum measured with other instruments at lowerX-ray energies, and then to steepen near 100 keV.Results of the BATSE analysis can be describedby a broken power law, while results of the SPIanalysis suggest a smoothly steepening spectrum.The BATSE analysis further noted a distinct hard-ening of the spectrum near 650 keV, although thishas not been confirmed by
INTEGRAL or theCOMPTEL instrument on
CGRO (Kuiper et al.2001).The
INTEGRAL spectral measurements areconsistent with either a smoothly steepening spec-trum or a spectrum of the form F = 6 . × − ( E/
100 keV) − α photons cm − s − keV − ,where α = 2 . ± .
01 for
E <
100 keV and α =2 . ± .
02 for
E >
100 keV (Jourdain & Roques 2009). This corresponds to a (50–100 keV)/(12–50 keV) flux ratio of R = 0 .
145 for
INTEGRAL compared to 0 . ± .
001 for GBM. The (100–300keV)/(12–50 keV) flux ratio R = 0 .
041 corre-sponding to the
INTEGRAL spectrum comparesto the GBM value of 0 . ± . R = 0 . INTEGRAL corresponds to the GBM value of0 . ± . INTEGRAL , particularly above 100 keV, and arebest described by a spectrum with α ∼ . − . Cygnus X-1 is a high-mass X-ray binary andwas one of the first systems determined to con-tain a black hole (Bolton 1972; Paczynski 1974).The X-ray emission is bimodal, with the >
10 keVemission anticorrelated with the <
10 keV emis-sion (Dolan et al. 1977). It has been observed toemit significant emission above 100 keV includinga power law tail extending out to greater than1 MeV (McConnell et al. 2000; Ling & Wheaton5ig. 2.— GBM light curve for the Crab. The hori-zontal scale is in modified Julian days over the 730day GBM exposure period, and has been binned 5days per data point. The dashed horizontal linesshow the average flux in each of four energy bandsincreasing from top to bottom. In the bottom plot,the solid line marks the zero flux level. Note thatthe apparent “flare” near MJD 55180 is due toa giant outburst in the nearby accreting pulsarA0535+262.2005b). Based on BATSE occultation analysis,Ling & Wheaton (2005b) have shown that in thehigh gamma-ray intensity (hard) state, the spec-trum consists of a Comptonized shape below 200–300 keV with a soft (Γ >
3) power-law tail ex-tending to at least 1 MeV. In the low-intensity(soft) state, however, the spectrum takes on a dif-ferent shape: In this case, the entire spectrumfrom 30 keV to 1 MeV is characterized by a sin-gle power law with a harder index Γ ∼ − . ≈
150 mCrab,consistent with the γ state of Ling et al. (1997).We will continue to monitor Cyg X-1 during thethermally-dominated state and follow its transi-tion back to the low/hard state.The GBM light curves (Fig. 3) reveal significantemission above 300 keV, consistent with the powerlaw tail observed when Cyg X-1 is in its low/hardstate. The 50–100 keV flux level observed by GBMover the full observation period (Table 1) is 1.15Crab, consistent with the BATSE high gamma-ray state ( γ of Ling et al. (1987, 1997)). The ob-served GBM flux ratios are R = 0 . ± . R = 0 . ± . R = 0 . ± . The relatively nearby radio galaxy Cen A is aSeyfert 2 galaxy that is the brightest AGN in hardX-rays/low energy gamma rays. It has powerfuljets aligned at approximately 70 ◦ from the line ofsight and is seen to vary on time scales of tens ofdays to years. It has been observed at hard X-ray energies by OSSE (Kinzer et al. 1995),
INTE-GRAL and
RXTE (Rothschild et al. 2006), and atenergies > ∼ . − .
9. The combined OSSE andCOMPTEL data are consistent with a steepeningof the spectrum at 150 keV to Γ ∼ .
3, with the spectrum then extending unbroken to beyond 10MeV.The GBM light curve for Cen A is shown inFig. 4. Because Cen A is relatively far below theequatorial plane, with a declination δ = − ◦ , itsbeta angle (which ranges between δ ± i ) can belarger than the half-angle size of the Earth as seenfrom Fermi ( β earth ≈ ± ◦ ). When this happens,Cen A is not occulted. This causes periodic gapsin the light curve, with the period of the gaps equalto the precession period of the orbit.The fluxes as measured by GBM and given inTable 1 are consistent with the hard spectrummeasured by previous instruments. The flux ra-tios measured by GBM are R = 0 . ± . R = 0 . ± .
008 respectively. An unbro-ken Γ = 1 . . R = 0 . ± . The galactic microquasar GRS 1915+105 isa LMXB with the compact object being a mas-sive black hole (Greiner, Cuby, & McCaughrean2001). It was highly variable over the 9-year obser-vation period of the
CGRO mission (Paciesas et al.1995; Case et al. 2005) with significant emissionobserved out to ∼ ∼ . ∼ . IN-TEGRAL /SPI in the 20–500 keV energy range(Droulans & Jourdain 2009) showed evidence fora time-variable thermal Comptonization compo-nent below ∼
100 keV along with a relativelysteady, hard power law at higher energies, indi-cating that different emission regions are likelyresponsible for the soft and hard emission.7ig. 5.— GRS 1915+105 light curve. The lightcurve has been binned 5 days per data point. Thefluxes are in Crab units, The fluxes are in Crabunits, and the dashed and solid lines mark theaverage flux and zero flux levels, respectively.The GBM daily fluxes integrated over 730 days(Table 1) show significant emission above 100 keV,consistent with the relatively hard power law spec-trum seen in BATSE and SPI data. The GBMlight curve (Fig. 5) shows distinct variability be-low 100 keV, with statistics above 100 keV in-sufficient to determine the level of variability ofthe emission. The flux ratios observed by GBM( R = 0 . ± .
001 and R = 0 . ± . ∼ R = 0 .
046 and R = 0 . The black hole candidate 1E 1740-29 (alsoknown as 1E 1740.7-2942) is a LMXB very nearthe Galactic Center. With a large double-endedradio jet, it was the first source identified as amicroquasar, and spends most of its time in thelow/hard state (Mirabel et al. 1992).
INTEGRAL observations indicate the presence of significantemission up to at least 500 keV with a steepen-ing of the spectrum near 140 keV (Bouchet et al.2009). The spectrum can be modeled either with a Fig. 6.— The GBM light curve for 1E 1740-29. The light curve has been binned 5 days perdata point. The fluxes are in Crab units, and thedashed and solid lines mark the average flux andzero flux levels, respectively.thermalized Compton spectrum and a high energypower law tail, or with two superimposed thermalCompton components. Evidence for a broad 511keV line observed by SIGMA (Bouchet et al. 1991;Sunyaev et al. 1991) suggests that 1E 1740-29 maybe a source of positrons.The GBM results (Fig. 6) are consistent withthe high energy component observed when 1E1740-29 is in the low/hard state. Below 100keV and above 300 keV, GBM sees approximately20 −
50% higher flux than
INTEGRAL , while inthe 100–300 keV band, GBM observes approxi-mately 90% of the level reported by
INTEGRAL . The X-ray nova SWIFT J1753.5-0127 (Fig. 7)is a LMXB with the compact object likely be-ing a black hole (Miller, Homan, & Miniutti 2006;Cadolle Bel et al. 2007).
Swift discovered thissource when it observed a large flare in 2005July (Palmer et al. 2005). The source did not re-turn to quiescence but settled into a low intensityhard state (Miller, Homan, & Miniutti 2006).
IN-TEGRAL observations (Cadolle Bel et al. 2007)8ig. 7.— The GBM light curve for SWIFTJ1753.5-0127. The light curve has been binned 5days per data point. The fluxes are in Crab units,with the average flux (dashed lines) and zero flux(solid lines) levels shown.showing emission up to ∼
600 keV were compati-ble with thermal Comptonization modified by re-flection, with evidence for separate contributionsfrom a jet, disk, and corona. BATSE occulta-tion measurements from 1991–2000 showed no sig-nificant emission from this source above 25 keV(Case et al. 2010).The GBM results are consistent with this sourcestill remaining in a hard state, with significantemission in excess of 100 mCrab above 100 keV.The light curves show that the emission fromthe higher energy bands declined beginning aboutMJD 55200, and that it increased again beginningabout MJD 55325 and is currently at or just belowits two-year average. The spectrum is inconsistentwith a single power law, and future work using theGBM CSPEC data will allow a more detailed anal-ysis of the spectrum. We will continue to monitorthis source while it is in the low/hard state. Fig. 8.— The GBM light curve for XTE J1752-223. The light curve has been binned 5 days perdata point. The vertical dashed lines at MJD55129 and MJD 55218 mark the flaring region usedto derive the average fluxes in Table 1. The fluxesare in Crab units, and the horizonial solid linesmark the zero flux levels.
The new transient black hole candidate XTEJ1752-223, discovered by
RXTE (Markwardt et al.2009b), was observed by GBM to rise from un-detectable on 2009 October 24 (MJD 55128) to511 ±
50 mCrab (12–25 keV), 570 ±
70 mCrab(25–50 keV), 970 ±
100 mCrab (50–100 keV), and330 ±
100 mCrab (100–300 keV) on 2009 Novem-ber 2 (Wilson-Hodge et al. 2009a,b). The lightcurve is variable, especially in the 12–25 keV band,where the flux initially rose to about 240 mCrab(25-28 Oct), suddenly dropped to non-detectableon October 29-30, then rose again during the pe-riod October 31 to November 2 (MJD 55135–55137). The flux remained relatively constantuntil November 25 (MJD 55160) when it began torise again, peaking in the high energies on 2009December 20 (MJD 55185). After an initial slowdecline, the high energy flux rapidly declined backto the pre-flare levels. The light curve for the9ntire mission to date, with 5-day resolution, isshown in Fig. 8. The fluxes for XTE J1752-223 inTable 1 are integrated over the days when XTEJ1752-223 was observed to be in a high gamma-rayintensity state, MJD 55129–55218.RXTE measurements indicate a black holelow/hard spectrum with a power law compo-nent (Γ ∼ .
4) superimposed on a weak blackbody (kT ∼ . RXTE /HEXTE analysis have shown evidencefor emission up to 200 keV (Mu˜noz-Darias et al.2010b), best fit with a broken power law with abreak energy near 130 keV, again markedly similarto Cyg X-1. The flux ratios measured with GBM( R = 0 . ± . , R = 0 . ± . , R =0 . ± . ∼ . ∼ . The highly variable LMXB and black hole can-didate GX 339-4 (Samimi et al. 1979; Doxsey et al.1979) is characterized by rapid time variabilityand low/hard X-ray states similar to those ofCyg X-1 (Harmon et al. 1994; Cowley et al. 2002).The results of analysis of both BATSE (Case et al.2008) and
INTEGRAL (Caballero-Garc´ıa et al.2009) data have indicated the presence of highenergy emission above 200 keV during previousoutbursts, with the
INTEGRAL spectrum fittedby a thermal Comptonization component togetherwith synchrotron or self-synchrotron emission pos-sibly originating at the base of a jet.GX 339-4 was observed by MAXI to begina large flare event starting on 2010 January 3(Yamaoka et al. 2010). The flux observed byGBM began to increase starting in early 2010 Jan-uary and continued to increase up to a level of ∼
400 mCrab (12–25 keV), ∼
650 mCrab (25–50 keV), ∼
800 mCrab (50–100 keV), and ∼ R =0 . ± .
012 and R = 0 . ± . ∼ . ∼ . ∼ σ ) withan average flux of 162 ±
31 mCrab.
4. Conclusions and Future Prospects
Using the Earth occultation technique, theGBM instrument on
Fermi has been monitor-ing the gamma-ray sky in the ∼ − Fermi mission, the Earth occulta-tion technique applied to the GBM CTIME datahas been used to detect six persistent sources andtwo transients at energies above 100 keV with asignificance greater than 7 σ , demonstrating thecapability of GBM to observe and monitor suchsources. Two of the sources, the Crab and CygX-1, were detected with high significance above300 keV.Light curves of all eight sources were presentedin four broad energy bands from 12–500 keV. Theoutbursts from the transient sources XTE J1752-223 and GX 339-4 were clearly visible in the12–50 keV, 50–100 keV, and 100–300 keV broadbands. XTE J1752-223 was a previously unknownsource, and the GBM light curves in the hard x-ray/low energy gamma-ray energy bands are con-sistent with the initial classification of this objectas a black hole candidate in a bright low/hardstate. The steep decline of the hard x-ray emis-sion starting around MJD 55215 correspondedto an increase in the soft x-ray flux (Homan2010; Negoro et al. 2010a), indicating the transi-tion from the low/hard state to a soft state. WhenXTE J1752-223 returned to the low/hard statearound MJD 55282 (Mu˜noz-Darias et al. 2010a),the hard x-ray emission was below the sensitivitylimit of GBM.The hard emission seen from GX 339-4 is con-sistent with the bright hard states seen in previousoutbursts from this object.Monitoring of Cyg X-1 at the onset of a recentstate transition showed a steady decrease in the100–300 keV flux that began about 19 days beforethe soft x-ray flux began to rise. As of MJD 55419,Cyg X-1 remains in a soft state, and we will con-tinued to monitor Cyg X-1 in anticipation of thetransition back to the canonical hard state.While the GBM CTIME data used here doesnot have enough spectral resolution to producedetailed spectra, the flux ratios between the 12–50 keV broad band and the 50–100, 100–300, and300–500 keV broad bands for the Crab are gener-ally consistent with those inferred from the mea-sured INTEGRAL spectrum, with the GBM re-sults suggesting a slightly harder spectrum. Theflux ratios observed for the transient sources XTEJ1752-223 and GX 339-4 are similar to Cyg X-1when it is in its canonical low/hard state, againconsistent with these transients being observed in bright low/hard states. Future work will use theGBM CSPEC data, with its finer energy binning,to examine the detailed spectra for all of thesesources, with particular emphasis on the low en-ergy gamma-ray energy range. Also, the BGO de-tectors, with their greater sensitivity at higher en-ergies, will be used to obtain additional measure-ments at energies above 150 keV. Several of thedetected sources have spectral breaks or cutoffs inthe 100–300 keV range, and we will look for thesefeatures and monitor their evolution over time.We will continue to add to the list of sourcesbeing monitored as appropriate. We have de-tected Cen A with GBM in all energy bands upto 300 keV. BATSE detected several BL Lac ob-jects known to exhibit flaring activity on the timescale of days (Connaughton et al. 1999). The flar-ing behavior of blazars is sometimes accompaniedby a shift upwards in the peak of the synchro-ton spectrum (e.g. above 100 keV in the 1997Mrk 501 flare (Petry et al. 2000)), making detec-tion of these sources possible with GBM. Whilenone have been detected so far, we will continueto expand our monitoring program to include theblazars with flares detected in other wavelengths.We plan to expand our monitoring program to in-clude other AGN as well.We continue to fine tune the algorithms andwork to reduce the systematic errors in the fluxdetermination. In the case of the BATSE Earthoccultation analysis, there was clear evidence forthe presence of sources in the data which werenot in the occultation input catalog and whichcaused uncertainty in the assignment of fluxes tothe sources that were in the input catalog. Wesee similar evidence for these uncataloged sourceswith GBM, especially below about 50 keV. To ad-dress this, our approach is two-fold: (1) We com-pare our GBM measurements with overlapping en-ergy bands from other operating missions and reg-ularly update our catalog as new transient out-bursts are observed with GBM or other instru-ments. Light curves are regenerated if needed af-ter catalog updates. (2) We are developing animaging technique for GBM to produce an all-skymap of hard X-ray/soft gamma-ray sources. Thismap will then be used to identify sources not cur-rently in the GBM occultation catalog, to expandthe catalog, and to reduce the uncertainties in themeasured fluxes.11his work is supported by the NASA FermiGuest Investigator program. At LSU, additionalsupport is provided by NASA/Louisiana Board ofRegents Cooperative Agreement NNX07AT62A.A.C.A. wishes to thank the Spanish Ministerio deCiencia e Innovaci´on for support through the 2008postdoctoral program MICINN/Fulbright undergrant 2008-0116.
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